A foundation is required to ensure loads from the building are sustained and safely transmitted to the ground. All load bearing elements, these include external walls, party walls, chimney breasts, piers and internal load bearing walls should sit on an adequate foundation.
- Foundation trenches should be excavated down to consistent and competent virgin ground which provides adequate bearing strength.
- In chalk soils foundation depths can be as little 500-750mm, but no less than 450mm to protect against frost action.
- In sand and clay soils the depth of foundations should be no less than 900mm deep and BS8103 recommends a depth of not less than 1.0m.
- In clay soils affected by seasonal moisture, foundations over 2.5m deep are not normally acceptable, in which case piles, raft or pad and beam foundations may be required. Additionally, some soils can be affected by particular species of trees and deeper foundations or special foundation types may be required (see building near trees below).
- When digging the trenches for the foundation, all existing services and nearby drains should be supported and protected.
- However, if the drain is no longer in use, it should be removed or opened up and filled with concrete
- Divert the drain if damage to the drain is possible & the drain is still in use.
- The design of the foundations should account for the effect of any nearby drainage trench on the newly excavated foundations. Any service trenches or other excavations should be above the 45 degree line extending down from the bottom of the foundation, as shown below. NB Reference should also be made to NHBC Chapter 5.3 'Drainage Below Ground'.
- Foundation trenches should be straight and even with horizontal bottoms and vertical sides.
- They should be compact and reasonably dry, rebottoming will be required if the trenches are allowed to crack or become water filled.
- The excavations should be taken below visible roots (especially in clay soils) and any loose material must be removed before the concrete is poured.
- The thickness of a strip foundation should be between 150mm and 500mm. 300mm thickness is used in most small domestic works.
- Strip foundations are usually at least 600mm wide as this tends to be the width of the digger bucket although on sand, silt or soft clay, it may be necessary to provide foundations as wide as 850mm.
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- The width of a trench foundation can be reduced to 450mm if ground conditions permit, although the brick layer may struggle to lay the bricks and blocks in a narrow trench. However there should always be 50mm projection from the brick outer face to the edge of the foundation concrete.
- Trench foundations can be dug deeper than strip, which means they are particularly practical where the water table is high, where soil is loose and unstable, and in areas with heavy clay soils.
- The trench sides may need to be lined with a slip membrane unless the soil is firm.
- The thickness of any trench filled foundation should be not less than 500mm and the foundation concrete should finish about 150mm to 100mm below ground level.
- NOTE: Trench foundation excavations in excess of 2.5m deep they should be designed by an engineer.
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- The foundation may require steel mesh reinforcement to provide additional strength.
- This needs to be adequately lapped and tied, clean and free from rust.
- The bars should be supported using proprietary spacers so that they are 75mm above the base of the foundation.
Refer to NHBC 'Foundations' Chapter 4.2
- In shrinkable clay soils, foundations can be affected by movement caused by ground heave. This means that the ground can swell or shrink when the moisture content of the soil changes.
- To allow for heave and to reduce the pressure on the foundation, a compressible clay board (low density pink expanded polystyrene boards) can be placed on the inside vertical face of the foundation (see below), 500mm above the bottom of the trench.
- Clay board should be used in clay soils over 1.5m deep as shown below. The board will compress under heave pressure.
- Steps in foundations can be used on sloping sites to minimise the amount of excavation and materials required by accommodating the change in levels.
- The height of a step should not exceed the thickness of the foundation (see below). In clay soils near trees steps should not exceed 0.5m.
And for Trench Fill:
It is advisable to pour the concrete for the foundation in one go. However, if this is not possible, a construction joint may be formed using one of the methods detailed below. Construction joints should be formed away from returns in the foundation.
Construction Joints with Reinforced Bars
Construction Joints with Expanded Metal Lath
Construction Joints with Corrugated Metal Former
Refer to NHBC Chapter 2.1 ‘Concrete and its Reinforcement’.
- The standard designated for ready mixed concrete for strip and trench foundations are known as GEN. A typical mix for non-aggressive soils would be a GEN1 or BS 8500.
- A site mixed ‘standard prescribed’ concrete (BS 8500), for non aggressive soils would be an ST2 mix, given in table below.
- If the foundation is reinforced or sulphates are present in the ground or there is a ground water problem, these mixes are not adequate and a stronger mix will be necessary.
Standard Concrete Mixes
St2 mix for strip footings*
- An St2 mix to give 1 m³ of concrete having 100-150mm slump:
- 285 kg Portland Cement
- 735 kg Concreting Sand
- 1105 kg Aggregate
St2 mix for trench fill foundations*
- An St2 mix to give 1 m³ of concrete having 160-210mm slump:
- 300 kg Portland Cement
- 725 kg Concreting Sand
- 1080 kg Coarse Aggregate
* Recommendations for 20mm maximum aggregate size (assumes cement of standard strength class 32.5).
'Standard Concrete Mixes' Table: BRE Good Building Guide GBG53, 'Foundations for low-rise building extensions'.
Refer to NHBC 'Foundations' Chapter 4.2
- Precautions should be taken when proposing to build near existing trees, especially in clay soils. A tree root system goes down to around 600mm in the ground and extends outwards, often further than the tree's equivalent height. These roots can affect a foundation, even when up to 30m away.
- Trees can cause shrinkage or heave which can cause damage to foundations in shrinkable soils which are subject to changes in volume as their moisture content is altered.
- Damage from trees can occur directly from physical contact with tree roots or indirectly from moisture shrinkage (often in long periods of dry weather), or heave which is often caused when high-water-demand trees, that would have had a drainage effect on the soil, have been removed or severely pruned.
- In order to determine the suitable foundation depth, it is important to identify the tree species to work out the water demand. The highest water demand trees are broad leaf trees such as oak, elm and poplar, as well as willow trees. The height of the trees and distance from the foundations must also be taken into account.
- The soil’s potential for shrinkage, must be determined and if not known should be assumed as high. With this information, refer to NHBC ‘Building Near Trees’, Chapter 4.2, to determine the suitable depth of the foundation.
- However, where there are trees within the specified distance, a special foundation design will need to be detailed by an engineer, for example bored piles and ground beams or a deep foundation incorporating clay boards (see above).
The construction of the ground floor can begin when the foundations have been laid, and all the trenches have been back-filled with properly compacted material and the loadbearing walls have been built up to the DPC.
A ground bearing solid concrete floor is by far the most common form of floor construction for extensions and small domestic works. However, the ground must be assessed in order to confirm that is suitable to support the floor and any other loads.
If the ground is formed of fill over 600mm, a suspended form of ground floor should be used.
A survey should be undertaken to ascertain whether sulphates or other hazardous materials are present in the ground. If so, special floor slab mixes, mortar, bricks, blocks and DPMs should be used which would require consultation with a specialist.
- Before the ground floor can be constructed, the ground must be prepared to ensure the slab has consistent support.
- Topsoil and any vegetable matter must be removed from the site. It is easily compressed and may sink causing the slab to settle and crack up.
- Pre-existing foundations must be appropriately dealt with.
- Precautions must be taken against ground contaminates, gases, landfill gases, radon, vapours etc.
- Avoid constructing ground bearing slabs on clay in summer and autumn unless NHBC is satisfied that the soil is not desiccated.
- Solid floors can also suffer from sulphate attack where they arch and bulge due to chemical reactions in the hardcore expanding the concrete.
- If the fill depth exceeds 600mm a suspended floor will be required.
- To provide suitable material for the floor slab a layer of clean hardcore at least 150mm thick but no more than 600mm is provided over the prepared oversite ground.
- The fill material used to make up the hardcore should contain nothing larger than about 100mm and be well graded inert fill without hazardous materials. It should contain a range of particles so that it can be firmly compacted such as clean broken bricks, roof tiles, concrete or crushed stone or ready-made loose granular material such as ‘type 1’ hardcore can be used.
- Fill should be mechanically compacted using a small vibrating plate or roller in layers no thicker than 225mm so that no air pockets are present and settlement is avoided.
- A layer of a minimum 20mm but can be up to 50mm of sand blinding should be provided over the hardcore before laying the concrete or DPM and will be essential to prevent sharp stones from puncturing the sheet DPM.
To prevent dampness getting through a ground bearing concrete floor should be protected by an impervious layer, usually a 1200 gauge (0.3mm) heavy duty polythene damp-proof membrane.
- The DPM can be positioned either on the sand blinding or on the concrete slab.
- Joints in a Polythene DPM should be welted or taped and should overlap by at least 300mm.
- The DPM must be linked with the DPC in the walls, to ensure that the entire interior of the building is protected from moisture by a continuous, impervious barrier.
- The DPM will need to be dressed up around service entry points.
Alternatives to Polythene DPM
- Bitumen Membrane
- Applied hot, about 3mm thick, on to the concrete floor slab.
- For cold-applied bitumen/rubber emulsions a minimum of 3 coats is required.
- Liquid Asphalt
- Applied hot, about 20mm thick.
- Usually a separate screed is not required.
- A typical concrete mix for a ground bearing slab is 1:2:4 ‘GEN 3’ mix. However, where there is a risk of sulphates of other harmful chemicals are in the ground a special concrete mix may be required.
- The floor slab is usually placed over the DPM
- The floor slab should not be less than 100mm thick.
- Ensure all the services and ducts running under the floor are installed and tested before pouring the slab.
- If temperatures are likely to go below zero, the concrete should not be poured.
- In cold conditions, hessian should be used to protect the concrete once poured.
- In hot weather, newly poured concrete will need protecting with polythene to prevent it drying too quickly.
- If required, the concrete slab may be reinforced with a layer of steel mesh, typically A142 mesh.
- Once the concrete slab has been poured, it can be temped with a heavy beam to remove air and surplus water and ensure a level surface.
- The concrete slab should be left to dry out for around two to three days or as required in BS 8203:1996.
- In order to provide the correct thickness of insulation to achieve a U value in compliance with the current building regulations (0.28 W/m²K) for a new solid ground floor it is necessary to calculate the p/A ratio (perimeter over area). This is done by dividing the exposed internal perimeter by the internal area.
- Around 70-80mm of a high performance rigid insulation board made from polyurethane for example, ie kingspan or celotex, will usually be more than adequate in most situations.
- The insulation is usually placed on top of the slab, although the boards can either be laid above or below the slab.
- The insulation boards should not be in direct contact with the hardcore base and it is recommended they be placed over the DPM.
- If placing the insulation on top of the slab, ensure the insulation boards are continually supported by laying the boards directly on a level and smooth concrete slab or use a thin layer of sand blinding.
- When laying the insulation tightly butt the boards together to maintain continuity and prevent cold bridging and lay in a staggered jointed pattern.
- A strip of insulation floor upstand should be positioned around the perimeter of the floor slab before pouring the concrete to in order to protect against cold bridging.
- Ensure the insulation inside the cavity walls is continuous with the insulation in the slab.
- Inject expanding foam around pipes passing through insulation boards.
Insulation under floor slab or screed finishes
- Use a sand /cement screed, with a minimum thickness of 65mm.
- If providing a screed finish or placing the insulation under the slab, it is advisable to provide a slip membrane of polythene with 150mm lap joints over the insulation to prevent the wet concrete from penetrating the joints in the boards and minimise the risk of condensation forming at the insulation/slab interface before pouring the screed or slab.
A 65mm deep sand/cement screed should be poured over the concrete slab or insulation boards and VLC.
- A typical screed mix is one part cement to three or four parts course sand.
- To avoid possible shrinkage, lay the mix fairly dry.
- If providing a board finish this can be laid onto the insulation, providing there is separating laying of polythene (VCL) over the insulation boards
- VCL should have 150mm lap joints and be continued up 100mm at room perimeters behind the skirting boards to minimise the risk of condensation forming at the insulation/slab interface, to prevent the screed penetrating the joints and to prevent moisture from the drying floor damaging the floor boards.
- Any chipboard used must be 18mm tongued and grooved flooring grade type C4 to BS 5669.
- Joints should be glued with a woodworking adhesive and laid staggered. These can then be sanded and stained, tiled or carpeted. Ensure a 10-12mm gap at all around the edges of the floor to allow for expansion.
- In all wet rooms, for example kitchens, utility rooms and bathrooms, the flooring board must be a minimum of 20mm moisture resistant grade in accordance with BS7331:1990.
- Identification marks on the boards must be laid upper most to allow easy identification.
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The masonry cavity wall is probably the most common form of construction for small modern domestic buildings. Bricks or blocks are laid in a stretcher bond with all the bricks positioned length ways.
A typical wall is comprised of a brick outer leaf and a block inner leaf. The inner leaf will usually carry the floor and roof loads. Each leaf will be separated by a clear cavity and tied together with wall ties.
The cavity prevents rain water from reaching the internal skin and the still air in the cavity is a good thermal insulator.
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- The concrete foundations should be allowed to cure for at least a couple of days before the external ground walls are constructed to DPC level.
- The walls, usually brick or block-work, should be built up in the centre of the strip foundations (It is possible to build off-centre when using trench fill, as the concrete is considerably thicker). At least 150mm projection of the concrete foundation is required at either side of the wall.
- Check that the bricks or blocks are suitable for underground use. The blocks used below the DPC should be specified to comply with BS 5628 part 3.
- Ensure mortar below DPC is suitable for underground use.
- The cavity in the walls below ground should be filled with a weak concrete mix (stopped 225mm below horizontal DPC in the walls or cavity tray provided), to prevent the leaves being pushed together when the trenches are back filled.
In Cold Weather:
- Do not lay brickwork or blockwork when the air temperature is 2°C and falling.
- If the air temperature should drop below 2°c once built, walls should be protected from frost.
The two skins of a cavity wall must be tied together at regular intervals with wall ties to ensure that the wall is structurally stable and strong.
- All wall ties should be stainless steel or non-ferrous and in accordance with BS EN 845.
- Wall ties should be long enough to ensure they can be embedded a minimum of 50mm in each masonry leaf.
There are a number of ties on the market, to suit particular cavity widths and wall thicknesses.
- Double triangle wall ties have taken over from the butterfly type as the most common in modern construction.
- Double triangle and butterfly type ties to BS 1243 are suitable for cavities up to 75mm.
- Vertical twist ties to BS DD 140 are suitable for wider cavities. Longer 250mm or 275mm ties can be used where cavities are over 100mm wide.
- Ties should be positioned in the wall at 750mm or 900mm apart horizontally and 450mm apart verically. This will ensure a spacing of at least 2.5 ties per square metre.
- Provide a row of ties every sixth brick course. In blockwork, this will be every second course.
- There must be additional ties at window and door openings and at either side of movement joints. They should be positioned within 225mm of the side of the opening, at not more 300mm centres vertically ensuring a tie every block course or every fourth brick course.
To provide structural stability the wall ties should be spaced at regular intervals and staggered where possible.
Wall ties and cavity insulation
- In a partial fill cavity wall, wall ties can be spaced more closely to match insulation board height.
- Ties should be spaced at 600mm centres horizontally using 2 ties to support the insulation to coincide with the horizontal joints of the 1200mm long boards. They do not need to be staggered vertically.
- Retaining devices clipped to the ties should be used to hold the partial cavity insulation against the inner leaf.
- Make a clean cut in the insulation at reveals where wall ties need to be closely spaced.
- To ensure the wall ties do not cause cracking in the event of thermal movement, ties should not be placed within 450mm of returns in a masonry wall.
Moisture can travel along the tie towards the inner skin if the wall ties are not correctly installed.
- Ties should slope at a slight gradient down towards the outer leaf, so that any moisture can find its way out.
- The drip of the wall tie should point downward and be placed in the centre of the unfilled cavity.
- Ties should be fully bedded at least of 50mm into mortar joints on each leaf of the cavity wall
- They should be pressed down into the mortar bed and should not be pushed into joints.
- Ties must be kept clean from any mortar droppings and debris which may bridge the cavity. A cavity batten can be used to prevent mortar dropping into the cavity or onto the insulation.
The standard brick size is are 215mm long x 102.5mm wide x 65mm deep, the majority of which are made from clay.
These have a high density giving them good acoustic properties. Their thermal mass enables them to retain heat and regulate temperature and humidity.
Bricks can be made in a variety of strengths by varying the quality and combinations of materials used and method of manufacture. Brick strength should be specified in compliance with BS EN 1996-1-1.
- Common bricks
- Common clay brick has a minimum compressive strength of 9N/mm2 and can be used for the construction of internal walls and buildings up to two storeys high.
- No particular regard is made to control their colour or appearance therefore the face of the brick will need to be covered by render or plaster. Their suitability must be checked for use below ground.
- Facing bricks
- Facing bricks most popular type of brick used in construction today and come in many different colours.
- Facing bricks have a consistent colour and texture, and give a building its aesthetic appearance. They are often chosen where the walls are to be left exposed.
- Engineering bricks
- Engineering bricks have a high compressive strength and low water absorption properties, used extensively for civil engineering purposes and often specified for DPCs, padstones or piers.
- They are rated as either class A or B, A being the strongest, and are typically red or blue in colour and smooth in texture.
- Calcium silicate bricks
- Calcium silicate bricks were developed about 100 years ago and made by mixing sand or crushed flint with hydrated lime. The materials are then mechanically pressed into shape and injected with superheated steam at high pressure.
- Calcium silicate bricks are suitable for most applications and have a good compressive strength. They are freeze /thaw resistant and come in a variety of colours and are regular in shape.
- Concrete bricks
- Early this century concrete bricks were developed. Modern concrete bricks have a strength Class of around 20N/mm2 which is adequate for most domestic construction.
- They are made from a combination of dense natural aggregate and a Portland cement binder and which has been compacted under pressure.
When specifying a brick, ensure it has the appropriate resistance to sulphate attack and the adverse effects of freeze/thaw and as detailed in BS EN 771.
‘M’ rated should be sufficiently frost resistant for most situations although severely exposed sites, parapets, copings and retaining walls may require an ‘F’ rated brick as well as and a low salt rating ‘L’.
All concrete blocks should comply with BS EN 1996-2. A Standard block is 440mm long x 215mm wide x 100mm deep.
Concrete Blocks come in a variety of grades and densities ranging from 3.6kn to 10kn. Blocks are made from combining cement, sand and crushed gravel and even aggregates such as expanded furnace slag, sintered ash and pumice.
The selection, combination and quality of materials will dictate the compressive strength.
Concrete blocks are cheap, quick to lay and are also good thermal insulators. They can be used as an infill for beam and block flooring, the internal leaf of cavity walls, internal dividing walls, and often for the external leaf if the external finish is to be cladding or render.
Most concrete blocks can now also be used below the Damp Proof Course. Concrete blocks have excellent fire protection properties providing at least 1 hour fire resistance and Class 'O' surface spread of flame.
The average standard block is 3.5N strength which is suitable for the construction of one and two storey dwellings (there may be other factors requiring a stronger block ie, sulphate resistance)
- Any building of 3 storeys or over will require dense blocks (heavyweight blocks) with a high 7.3N/mm2 strength. Their high strength means they are often used for foundations and load bearing walls.
- The high density provides good sound insulation ideal for use in party walls but also good heat conduction and therefore a low level of insulation.
- Lightweight blocks can have a compressive strength as low as 2.9N. These blocks are light and easy to handle on site.
- Made from a variety of lightweight aggregates, they are slightly more expensive than ordinary dense blocks but have better thermal insulating properties.
- Lightweight blocks are primarily used for the internal skins of cavity walls although some types are suitable for use in loadbearing walls and below the DPC and even as the infill for block and beams floors.
- Because of their low density most lightweight blocks will have a low compressive strength.
- Lightweight blocks are generally unsuitable for use in party walls due to their low mass which makes them poor sound insulators. They can be prone to shrinkage cracking during the during out process of the plastered internal walls.
- Aerated concrete blocks are light and easy to handle on site making them very popular for domestic buildings.
- Although not particularly strong, aerated blocks are extremely thermally efficient and are widely used for inner leaves and partition walls.
- Aerated blocks are made from cement, lime, sand, pulverised fuel ash (PFA) and aluminium powder and contain up to 80% recycled materials. Mixing aluminium filings with the concrete causes them to react with the lime producing hydrogen, creating tiny bubbles within the block.
- Due to their low mass, aerated blocks are generally unsuitable for party walls and are not usually suitable in situations where there are point loads or where high compressive strengths are required.
- Trench Blocks or Foundation blocks are lightweight and can provide a quicker build below ground.
- Commonly used in a range of thicknesses from 255mm upwards, these Blocks are highly resistant to the freeze-thaw conditions likely to occur below DPC level.
- All mortars used on site should be accordance with BS 5628.
- The strength of the mortar mix will depend upon the type of bricks and blocks use.
- Modern mortars use cement as the main binding agent.
- Adding a small amount of hydrated lime improves the mortar’s workability and its ability to cope with thermal movement. However, liquid plasticiser can be added instead of lime.
- A pre-mixed masonry cement could also be used. This has added chemicals to improve the workability of the mortar.
- Pre-mixed mortars should not be used underground or where a strong mix is required.
- A standard mortar mix for new brickwork would be 1:1:6 cement/lime/sand (1:4 Portland cement/sand)
- A stronger 1:3 mix would be more suitable in highly exposed areas such as parapets or underground work.
- In recent years, mortars which are retarded and ready to use have become more common place.
- Do not mix clay bricks and concrete blocks.
- Brickwork should not be carried out when temperatures fall below 2°C.
- Good workmanship is essential in preventing any water seeping through the outer leaf in the gaps between the bricks.
- Use bucket handle, weathered or struck pointing. Recessed pointing should only be used in sheltered locations.
- Recessed joints should not be used with full fill cavity insulation.
- When constructing a cavity wall the height difference between the two leaves should never more than 6 standard block courses.
- Vertical chasing should be no deeper than 1/3 block thickness.
- Horizontal chasing should be no deeper than 1/6 block thickness.
- Avoid Back-to-Back chases.
- Hollow blocks should not be chased.
Movement joints in the outer leaf of external masonry walls prevent movement from expansion and contraction causing cracks in the brickwork.
- Movement joints are not usually required in the internal blockwork walls as they are regularly interrupted by party and partition walls.
- Movement joints are usually hidden in corners or behind rain water pipes.
- All movement joints provided in the substructure should run the full height of the masonry wall. However, movement joints are not normally required below DPC level because the moisture content and temperature should be relatively constant.
- Wall ties are required either side of the movement joint.
Movement Joint Spacing
Movement joints are usually created by providing straight, unbounded, vertical joints in the brickwork at spacings detailed below:
|Movement Joint Spacing|
|Material||Joint Width||Normal Spacing|
|Clay Brick||16 mm||12m (15m maximum)|
|Calcium Silicate Brick||10 mm||7.5 to 9m|
|Concrete Block & Brick||10 mm||6m|
|Any Masonry in a Parapet Wall||10 mm||1/2 of the above spacings & 1.5m from corners (double the frequency).|
|The spacing of the 1st movement joint from a return should not be more than 1/2 the above dimension.|
Install ties to each side of movement joints:
- Vertically - 300mm or each block course
- Horizontally - within 150mm of the joint
Movement Joint Filler
Movement joints should be filled with the correct compressible filler. For clay brickwork, flexible cellular polyethylene, cellular polyurethane or foam rubber faced with a flexible sealant at least 10mm deep to ensure a good bond.
Horizontal DPCs in external walls are essential to prevent damp rising from the ground into the superstructure.
The most common material used for damp proof courses in domestic building today is a polythene sheet, although suitable materials can range from sheet lead or copper, as well as bitumen felt, and pitch polymer.
It is also possible to use a semi-rigid materials, such as mastic asphalt or rigid materials, for example slates or a couple of courses of engineering bricks (DPC category).
- DPCs should be laid in two separate strips, one for each leaf of the cavity wall.
- DPCs should be installed at least 150mm above ground level.
- Polythene DPCs should be in one continuous length or with joints lapped by a minimum of 100mmm, bedded on a full bed of mortar with a further bed of mortar laid over the DPC.
- There should also be a 5mm projection beyond the external face. However, the DPC should not project into the cavity where mortar and debris could build up and bridge the cavity possibly causing damp penetration to the inner skin.
- The DPCs should be lapped by a minimum of 50mm with the DPM which protects the floor thus providing a continuous barrier against rising moisture.
DPCs around openings
- Vertical and horizontal DPCs around openings in cavity walls are often already combined within a proprietary cavity closer.
- Vertical DPCs should protrude at least 25mm into the cavity.
- The upper DPC should always lap over the lower.
- Extend vertical DPCs up to the lintel and turn back towards the inner leaf.
- All sills and copings should have a DPC underneath to prevent water penetration of the wall below.
- Cavity Trays should be provided above window and door openings and at all interruptions to the cavity such as lintels, roof abutments, air bricks and meter boxes.
- Ensure any water running down the cavity is directed out through weep holes.
- Provide a cavity tray over full fill insulation where the insulation is not taken up to the roof in order to prevent any water dripping from the wall ties higher up in the wall from falling and ponding onto the top of the insulation, leading to damp penetration to the inner leaf.
- Provide cavity trays for lintels which do not have a cavity tray incorporated in their design.
- Cavity trays over lintels should project at least 25mm beyond the cavity closer and cover the ends of the lintel.
- Cavity trays should be installed in one continuous length. If the tray is not continuous, provide a minimum 150mm stop ends to prevent any moisture running off the ends of the tray and back towards the inner leaf.
- The cavity tray should be extended 150mm beyond each side of the opening.
- Cavity trays should have a rise of at least 140mm from the outer leaf up to the inner leaf.
- The rise across the cavity should be at least 100mm.
- Return the upstand of the cavity tray into the inner leaf unless it is rigid enough to stand against the inner leaf without support.
Lintel cavity Tray:
- To drain water from cavity trays, weep holes should be provided by either installing proprietary plastic weep hole vents or by leaving gaps in the mortar perpends.
- Weepholes should be positioned in the first course of masonry above a cavity tray at 450mm (max) centres (at least 2 weepholes per opening).
Parapet walls are exposed to the elements on their both sides and top. This can lead to premature failure and possible water ingress.
When constructing a parapet wall, only bricks with a high level of frost resistance and low salt content should be used .
Parapet Wall DPCs
- Provide a throated coping or cap to prevent moisture penetration at the top of the wall, with a sealed DPC below.
- The DPC should be supported over the cavity to prevent sagging.
- There must also be a DPC, a minimum of 150mm above the roof surface, to lap with the upstand flashing, ensuring continuity with the roof covering.
Parapet Wall with DPC Support:
- Where the roof abuts a cavity wall, a cavity tray linked in with the flashing should be provided at 150mm above the roof surface and fitted into the cavity to ensure that any water which gets into the cavity is diverted out of weepholes provided and does not enter the enclosed area.
- For pitched roof use a series of small stepped cavity trays with a stopend and a weephole provided to the bottom of the cavity tray.
Where a masonry chimney penetrates a roof structure, a DPC may be needed to prevent water soaking the masonry inside the building.
DPCs at the base of partitions built off oversite where there is no integral DPM should be the full width of the partition.
- Cavities should be kept uniform and the residual clear cavity should be at least 50mm unless it is proven that the quality of workmanship, suitability of location and design can enable the cavity to be reduced down to a possible 25mm.
- Cavities must be kept clear of mortar droppings. This can be done using a protective batten positioned over the cavity as the wall is being constructed.
- Provide proprietary cavity closers, which can also act as DPCs to close the cavities around openings and at the tops of walls (do not close cavities with returns bricks or blocks which can cause cold bridging).
- When the windows and door frames have been fitted, the cavity closers should overlap them by at least 25mm.
- In modern highly insulated cavity walls there is an increased risk of gaps in the insulation leading to cold bridging and heat loss. Warm moist air coming into contact with these cold spots can cause condensation problems, damp patches and mould growth either on the surface or within the construction.
- A high standard of workmanship is critical to ensure the insulation is continuous at junctions. such as where the ground floor meets with the external walls, if cold bridging and air leakage in the construction is to be avoided.
Energy conservation requirements demand ever thicker levels of insulation. Around a third of all heat loss in an un-insulated home occurs through the walls. Insulation in external walls is normally positioned within the cavity.
Insulation can also be fitted on the outside of the cavity walls requiring an external finish such as render, tile-hung or timber-clad. Alternatively, the insulation may be installed internally as a drylining solution.
The insulation performance is measured as a u value expressed as W/m2K.
Insulation installed within the cavity can be either total fill or partial fill. This will depend upon the insulation material being used and site exposure.
- The partial fill solution will often use rigid foil-backed polyurethane sheets such as Celotex or Kingspan. It is reasonably expensive but does have around twice the thermal performance of the mineral or rockwools although the wools will provide good standard of protection from sound and noise transmission.
- Fully filled cavities in exposed locations are at risk from penetration of moisture through the external leaf, soaking the insulation and transferring damp through the inner walls. Therefore a fully filled cavity is not acceptable in severe weather locations such as Scotland.
- There are also more eco friendly insulation products available such as natural cellulose fibre made from recycled newspaper or sheep’s wool.
Partial Fill Insulation
- Partial fill insulation boards should be fixed tight against the inner leaf of the cavity and retained in place with the correct tie retaining clips before the outer brickwork is built up.
- Ensure the wall ties provide suitable structural integrity.
- Butterfly-type ties should not be used with partial fill insulation.
- Insulation boards should start 2 brick courses below the DPC with the first row of boards supported on the wall ties and each board on at least two wall ties per 1200mm board positioned at max 600mm centres horizontally.
- For partial cavity fill the spacing of the wall ties should coincide with horizontal joints (maximum 450mm centres vertically and 900mm centres horizontally). However, around reveals or movement joints etc, where wall ties need to be more closely spaced, ties can be installed by providing a clean neat cut in the insulation.
- Insulation boards should be tightly butted with staggered joints and no gaps to minimize heat loss and dampness.
- NHBC requires a clear 50mm residual cavity between the insulation boards and the external leaf. However, a cavity width of 25mm is possible in sheltered location, provided the workmanship is of a high standard, to minimise the risk of damp penetration.
- Damp problems can be caused by mortar droppings bridging the cavity. Therefore, during construction, it is essential to place a batten across insulation and cavity to stop mortar dropping into the cavity, and to remove any excess mortar from the wall and the top of the insulation materials.
Full Fill Insulation
- In fully filled cavities, the cavity should be 5mm wider than the full fill insulation batt specified.
- The insulation boards should be supported on the wall ties DPC at 450mm centres horizontally. Subsequent boards should be tightly butted together with staggered joints between the ties.
- The batts should be built into the wall as construction progresses.
- Ensure all mortar joints are completely filled with mortar. Do not use recessed joints in a full filled cavity wall.
- To prevent mortar snots bridging the cavity, leading to possible damp problems, a cavity batten should be placed across the insulation and cavity to prevent mortar dropping into the cavity. Any excess mortar must be removed from the wall and the top of the insulation materials.
During the mid 20th century, it was common to use concrete lintels. However in modern construction pre insulated steel lintels are more common as concrete lintels can lead to cold bridging.
- Steel and concrete lintels should comply with BS EN 845-2.
- Timber lintels should not be used externally unless they can be protected from the weather and they do not support brick or block work.
- Most modern lintels incorporate a cavity tray to direct any water out through weepholes away from the internal leaf. However, some lintels, for example IG lintels, require a separate cavity tray. This should be provided over the full length of the lintel with stop end s to stop water running into the cavity. Lintels can also come ready filled with insulation.
- The lintel company can specify the correct lintel type and its size by calculating the imposed loadings. However, the lintel specified should always be wide enough to provide adequate support to walling above.
- Lintels should be bedded on mortar on a full block or on a padstone under the lintel bearings, where required by the design.
- The inner and outer leaf of the cavity wall should be built up together to avoid twisting the flange. The height difference between the leaves should never exceed 225mm.
- Masonry should not overhang the lintel support by more than 25mm.
- Soft or non durable packing should not be used.
The table below provides the minimum bearing required for lintels:
|Minimum Bearing Length (mm)|
|Span (m)||Simple Lintel||Lintel combined with Cavity Tray|
|Up to 1.2||100||150|
Rendering the external surface of a wall will improve its air tightness and weather resistance, hopefully preventing any rain penetration.
- The rendered wall should comply with BS EN 13914 ‘Design, preparation and application of external rendering and internal plastering’.
- The specified mix should comply with BS EN 13914 ‘Design, preparation and application of external rendering and internal plastering’. Particular care should be taken when specifying a mix for aerated or lightweight concrete blocks.
- A render mix will comprise of cement, lime to increase workablility, water and sharp sand (grading type A). Admixtures may also be used (air entrainers should not be used with masonry cement.) (Refer to the NHBC Good Building Guide.)
- To prevent the render shrinking and cracking as it dries out, ensure the mix does not contain too much water or cement.
Insulation of rendered walls
- The lack of ventilation in the cavity of a full filled cavity wall may adversely affect the drying out process of the render and a specific render mix, as well as special bricks or blocks, may be required.
- In exposed locations subject to driving rain, full filled cavity insulation is not suitable for a rendered wall.
- A cavity wall which is to have partial fill insulation may be rendered, provided a 50mm residual clear cavity is maintained.
Preparing the surface
- The surface to be rendered should be free from dust, loose particles, efflorescence and organic growth. It must be moderately strong and porous thus providing an adequate key and ensuring a good bond.
- Dense blocks with a smooth surface are not suitable.
- Textured Blocks
- No preparation required.
- Clay brickwork and dense block
- Provide 15mm recessed joints for a sufficient key (by raking out joints).
- Hack the Surface.
- Smooth Blockwork or Bricks
- Provide a spatterdash coat (strong cement/sand slurry thrown onto the surface).
- Provide a stipple coat (strong cement/sand slurry possibly with a bonding agent brushed on to the surface).
- Provide a suitable adhesive.
- Hack the surface.
- Apply a bonding agent.
- Provide suitable metal lathing (see below).
Refer to NHBC 'Superstructure'.
- Painted Brickwork
- Provide suitable metal lathing (see below).
- Metal Lathing should be stainless steel compliance with BS EN 845
- For a good bond position the metal lathing slightly away from the wall surface so that render can be pushed through the mesh.
- When rendering on masonry cavity walls, two coats of render are usually sufficient, although in exposed areas , on solid wall construction or where a metal lathe is used, two undercoats and one finishing coat are usually required
- Ensure each coat of render is weaker and thinner than the previous coat or the material it is applied too.
The First Coat
- The first coat (undercoat) should be between 10mm to 15mm thick. It should be adequately levelled and combed to provide a good key for the second coat.
- Allow the first coat to shrink and dry for a minimum of 3 days so the render is hard but not completely dry before applying the next coat.
- Subsequent coats must be thinner and weaker than the first.
The Finishing Coat
- The finishing coat should be between 6mm and 10mm thick and may have a smooth, textured or course aggregate finish. (A severely exposed site would benefit from a rough textured finish)
- Do not use a strong mix for finishing coat.
- Keep the finishing coat damp for at least 3 days. (In very hot, dry weather it may be necessary to spray the finishing coat with water or cover with a polythene sheet)
- Do not apply render in hot temperature or in bright sunshine.
- Do not apply render in wet or frosty conditions of when temperatures reach 2°C and falling.
- Provide suitable details around openings service penetrations, movement joints etc.
- The render should be stopped just above the DPC.
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