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Last edited 15 Sep 2020
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.
- 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.
 Concrete slab flat roofs
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.
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.
- 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.
 Find out more
 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.
- Mansard roof.
- Pitched roof.
- Roofing defects.
- Thatch roofing.
- Types of cool roofs.
- Types of roof.
- Wall plate.
- Warm roof.
 External references
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