Most flat roofs will need some form of surface protection for the following reasons.
- UV rays - The covering of a flat roof will spend many years exposed to ultra violet light from the sun, this can lead to oxidisation of the roof surface causing the plasticisers leak out and the surface to become brittle. This can significantly affect the performance and durability of the roof.
- Thermal movement - Heat from the sun can also cause solar radiation, where heat is transmitted to the roof system causing continual expansion and contraction which can be very damaging. This is a particular problem for built-up felt and asphalt roofs as dark membranes absorb more heat.
- Differential movement - The temperature difference between the roof covering and the structure below, in a warm roof, may be high, leading to thermal stresses and possible splitting of the membrane.
- Fire protection - The Building Regulations Approved Document B4 sets out certain specified performance levels of surface finishes on flat roof to control the spread of flame.
Surface protection should be light in colour because it will provide good solar reflectiveness and emit absorbed heat more efficiently.
Surface Protection Methods
The various methods of surface protection are detailed below.
Factory Applied Protection
- Self finished mineral- surfaced felts are available which limit UV radiation, these should be of Class 3E or 5E to BS747. Some of these felts also give an AA external fire rating although AB is more common.
- The felts are ready surfaced with a granular material of fine sand, green mineral aggregate, small crushed slate flakes or granite. Membranes faced with metal (aluminium, copper, stainless steel) are also available.
- However some self finished felts have little reflective quality and do not provide protection from the damaging effect of temperature rises in the membrane surface or solar gain.
These membranes are generally self finished with good reflectivity and resistance to UV ageing and so do not usually require additional solar protection. However the fire rating of a single ply membrane must be checked with the manufacture.
Solar Reflective Painted Finishes
Proprietary solar reflective paints can be painted on the surface of flat roof to provide solar reflection. This helps protect the roof from the potentially damaging effect of the ultraviolet solar radiation (compatibility with bitumen based materials should be checked).
However these paints are not very durable and reflectiveness deteriorates rapidly, therefore repainting is required every 3 to 5 years. They also do not attain the AA fire rating required for most flat roofs.
Solar Reflective Chippings
- Solar protection is often achieved by the application of white reflective chippings, usually limestone or light coloured spar, not less than 12.5mm. These are very effective at providing protection from ultra-violet light and reducing the surface temperature in hot weather. The density of the stones will also slow the rate of heat gain and heat loss from the building.
- The chippings can be laid loose or bonded in hot or cold bitumen compound to the roof covering.
- Chippings can be used provide a ballast layer to help hold down the roof surface in exposed locations.
- However, the chippings can move around or conceal defects in the roof and should not be walked on as they may puncture the membrane.
- Chippings should only be used on roofs at less than 10° as complete adhesion at steeper pitched is not possible.
Where flat roofs are designed for regular access such as a terrace or balcony the surface layer will need additional protection.
The form of protection will depend upon the anticipated usage and the appearance required, but will usually be in the form of slabs or paving tiles, placed on raised supports to allow rainwater to drain away.
The roof structure, the covering and insulation should be designed to support the dead weight of the paving and live loads from foot traffic.
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