Flat Roofs Explained – The components that come together to make a flat roof


Base components

A flat roof is constructed from a series of layers each one of which serves a specific purpose.

  • Ceiling – We all know what one of these is. They are usually made our of plasterboard.
  • Load bearing supportive structure – The ceiling cannot bear the weight of the flat roof, this is born by the load bearing walls of the property. The load bearing supportive structure transmits the weight of the roof down into the walls. In most domestic buildings this will usually be formed of timber joists but concrete and steel structures are also sometimes adopted.
  • Vapour Control Layer – This layer helps to control condensation and reduce the risk of damp.
  • Roof Deck – This forms a base layer for the waterproof membrane and/or insulation.
  • Thermal insulation – This helps to prevent heat from escaping the property and to maintain a stable temperature. It may also offer additional support to the waterproof membrane, particularly in a warm roof system.
  • Waterproof membrane and protective covering – this should prevent any damp from the outside world reaching the roof structure and causing water damage as well as preventing any moisture from entering the room below.
  • Outlets and Gutters.

The materials used to build these layers will be chosen by their compatibility and also by the kind of roof that is being built.  Some building components can react badly when in contact with other materials and such issues must be taken into account in the design of the roof.  Types of roof are cold deck, warm deck and inverted.

A closer look at built up Bituminous Membranes or Felt

These are by far the most economical form of roof covering and as a result they are also likely to be the most familiar to great many of you. They can be used on pretty much any kind of supportive structure and are common for domestic applications. As commonplace as they are, they have still come a long way since the old days.

Modern felt membranes are reinforced with polyester, glass or a combination of the two and are impregnated and further coated with bitumen. Typically two layers of felt will be used to create the roof surface. These will be bonded together using hot bitumen. The thinking behind this approach is that each layer will act to reinforce the other, if there is damage to one, the other will act as backup reducing the risk of wear and tear becoming a much bigger problem.

Felt is purchased on a roll and is generally referred to by the unique ingredients used in its manufacture, which will in turn impact upon its application. AAP is made from atactic and
polypropoylene and is usually used for torch on techniques for instance, while SBS is made from butadiene sandwiched between two laters of styrene and is generally used for pour and roll techniques.

There are various different types of felt which will be used at different levels in the development of a layered structure, all of which should confirm to BS EN 13707:

  • Type 5
  • Class 5B or 5U felt is generally used for base and intermediate layers. 5B can additionally be used for top layers but only when it has been provided with adequate surface protection.
  • Class 5E comes with mineral granules which acts and surface protection and this makes it ideal for top layers.
  • Type 3- All type 3 felt is less durable than type 5 felt. As a result it is generally used for intermediate layers only. It can be used as a base layer if it has been partially bonded.
  • Type 3G is perforated which, if partially bonded, can be used as a venting base layer if required.
  • Type 1 – These are the traditional rag based felts that were used back in the day. They have been removed from British standards and should no longer be in regular use.

Roof Covering Waterproofing System

The waterproof membrane is obviously a vital component of flat roof structure and must, by necessity, be hard wearing and durable. It should be able to maintain integrity and prevent leakage in the face of any and all potentials onslaughts.

  • Rain
  • Snow and Ice
  • Wind
  • Debris like dead leaves and fallen branches.
  • The effects of UV radiation.


There are various types of waterproof membrane available:

  • Built up reinforced bitumen, otherwise known as felt.
  • Mastic Asphalt roofing.
  • Single ply roofing systems.
  • Green roofs.
  • Metal sheet roofing which may be constructed from zinc, copper or lead.


In order to stabilise and seal decks and substrates before a waterproof membrane is applied, a primer may also be used. A primer can help to create a long lasting watertight finish as well as a good key.