Last edited 15 Sep 2020

Flat roof


[edit] Introduction

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.

This is in contrast with a pitched roof which slopes.

The Scottish Technical Handbook - Domestic, describes flat roofs as roofs, '...the slope of which does not exceed 10º from the horizontal'.

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.

[edit] Coverings

Common flat roof coverings include:

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.

[edit] Deck

[edit] 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.

[edit] Timber flat roofs

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):

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.

Garages may be un-insulated, but most roofs above the habitable part of the house will need to be insulated to comply with Building Regulations.

Conservation of energy can be achieved in two ways:

For more information, see Flat roof defects.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External references

  • Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007)


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