A flat roof is a roof that is completely, or almost level. However, whilst they are described as ‘flat’ almost all flat roofs are actually laid to a fall to ensure that rainwater can run off to the lower side. Typically they are designed to have a minimum fall of 1:40, which given on-site inaccuracies should result in a minimum fall of at least 1:80 in the finished construction.
While very common in countries with warm climates, flat roofs were only widely adopted in the UK after the Second World War. They were seen as a cheaper alternative to traditional pitched roofs. However, the longevity of some flat roofs has been poor, ranging from 6 years to 35 years depending on the quality of the covering and the structure.
Common flat roof coverings include:
- Sheet materials such as bituminous felt.
- In situ material materials such as asphalt.
- Metal such as lead or copper.
These roof coverings are generally designed to be laid as flat, continuous sheets, and they are prone to weakness where they are bent; for example at parapets and verges, where they are penetrated by ducts, flues and so on, and at joints or seams.
A concrete slab flat roof is normally made up of a structural layer of concrete finished with a smooth screed onto which a water proof layer such as a membrane is laid. The roof should incorporate insulation and usually a vapour control layer to protect from interstitial condensation.
Timber flat roof construction usually consists of structural joists topped with a decking of plywood or a similar sheet material. Wherever possible, joists should span the shortest distance of the roof plan . The pitch is governed by the roof covering and the required rate of rainwater discharge.
There are a number of different possible methods of creating a fall (slope):
- Joists cut to falls with flat soffit:These are simple to fix but may not be very economical in terms of timber usage, unless two joists are cut from one piece.
- Joists laid to falls with sloping soffit:Economic and simple but the sloping soffit may need to be hidden by a flat suspended ceiling.
- Firrings (tapered strips fixed above the joists) with joist run:Simple and effective but it does not provide a means of natural cross ventilation.
- Firrings against joist run:Simple, effective, and provides a means of natural cross ventilation, but uses more timber.
The loadings and span of the flat roof will determine the spacing and sizes of the joists required.
As with concrete flat roofs, a timber construction will be finished with a waterproof covering such as a membrane, or a sheet material such as lead. Insulation will be incorporated within the roof build up, together with a vapour control layer to protect from interstitial condensation.
Conservation of energy can be achieved in two ways:
- Cold roof: Insulation is placed on the ceiling lining, between the joists.
- Warm roof: Insulation is placed below the waterproof covering, above the roof decking.
- Inverted roof: Insulation is placed above the waterproof covering.
For more information, see Flat roof defects.
 Related articles on Designing Buildings Wiki
- A-frame house.
- Blue roof.
- Cold roof.
- Conical roof slating.
- Cool roofs.
- Domestic roofs.
- Dormer window.
- Flat roof defects.
- Green roofs.
- Inverted roof.
- Long span roof.
- Roofing defects.
- Thatch roofing.
- Types of roof.
- Wall plate.
- Warm roof.
 External references
- ‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007)
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