Pitched Roofs - Overview
The function of a roof is to protect the building below from the weather. In order to satisfactorily fulfil this function over a period of years it must be strong, stable and durable.
In addition, roofs must provide good thermal insulation and prevent the spread of fire from adjacent or adjoining properties.
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- The majority of houses in this country are constructed with pitched roofs.
- The angle of the pitch may be dictated by aesthetic or structural factors.
- It may also be influenced by the nature of the roof covering.
- Modern tiles permit shallow pitch but some of the older traditional coverings, such as hand made clay tiles, require quite steep slopes to ensure rain does not penetrate the roof covering.
- Shallow pitches are generally cheaper to construct with savings in both timber and tiling.
- In traditional pitched roofs, the rafters are supported by a wall plate at the bottom and a ridge board at the top.
- Intermediate support is supplied by a horizontal purlin.
- Most modern roofs for new houses are built from prefabricated components erected on site.
The simplest of pitched roofs is the Lean-to roof, commonly found forming the rear extensions to terraced housing.
- The sloping timbers are known as rafters and are supported at the top by building them into the solid wall (not recommended nowadays) and at the bottom by securing to a wall plate.
- The wall plate is a strip of timber which is bedded in mortar on top of the wall, and which evenly distributes the load from the roof and provides a good fixing for the rafters.
- Note: Mortar cannot actually bond the timber to the wall.
- The wall plate should be 100mm x 75mm 0r 100mm x 50mm.
- Ceiling joists are often built into the wall or supported on a wall plate bedded within the wall (not recommended).
- The rafters are skew nailed to the plate
- The depth of the rafter depends on its span and loading, and the width is primarily to prevent twisting and to provide a sufficiently wide surface on which to nail the battens supporting the tiles
- It is good practice to notch the bottom of the rafter where it sits on the plate as this gives a good bearing and aids alignment of the rafters.
- The rafters are usually spaced at 400mm centres.
- In most houses, the guttering is supported by the facia board. This is fixed to the feet of the rafters and it can be flush against the wall or it can form an overhang.
- Lean-to roofs can achieve only modest spans.
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