Last edited 15 Aug 2016

Flat roof

Contents

[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.

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:

  • 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.

[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:

  • 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.

[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)