A Guide To Roof Construction — Part 1 — Great Home

A Guide To Roof Construction - Part 1 - Great Home

Most homeowners only have to consider repairing or replacing a roof once in their lives so getting to grips with what it involves and how much it should cost can be a challenge. This article, spread into three parts, provides the basic information needed to understand what a roofing company is talking about, to help those unfamiliar with roof construction. To a non technical person, there appear to be a bewildering range of roof options but these can be simplified if you know the basics.

What is a roof?

A flat roof is generally defined as having a pitch not greater than 15° to the horizontal. A truly flat roof would not allow rainwater to drain away so most flat roofs have a fall on them so that the rainwater naturally flows to collection points. Whilst you can have tiled roofs of  10-15° these are really flat roof construction with a cosmetic covering of tiles.

Examples Of Flat Roofs

There are different types of roofing material used in the waterproofing layer of a flat roof construction:

  • EPDM is a type of rubber which comes in continuous sheets. (EPDM stands for ethylene propylene diene monomer if you are desperate to know)
  • Roofing felt is a bitumen coated sheet with a mineral coated surface to add colour and provide the bitumen with protection from the sun
  • Fibre glass is a rigid material 
A Guide To Roof Construction - Part 1 - Great Home

Flat Roof Construction

  1. A Waterproof layer which resists water.
  2. Thermal insulation which retains heat within the building rather than allowing it to escape through the roof.
  3. A vapour barrier to prevent moisture escaping
  4. A timber deck, often of plywood sheeting, which allows the roof to support a load such as someone walking on it
  5. Wooden roof joists which are supported by the walls
  6. Plasterboard ceiling

Flat Roof Construction Diagram Warm Roof

A cold roof consists of the same elements but in a different order.

Pitched roofs

A pitched or sloping roof has a pitch of at least 15° but often much more. A typical  roof pitch in the UK for a traditional house will be 40°-50° but at the extreme can go up to 70° (above 70° would actually be called a wall!)

Examples Of Pitched Roofs

Part of the choice of pitch is down to the architect and different period homes will have different pitches which are more aesthetically pleasing to the designer. Nathaniel Lloyd in his 1929 book, Building Craftsmanship In Brick And Tile And Stone Slates quoted Sir Edwin Lutyens describing 45° as the ugly angle for a roof pitch.

As well as the visual appearance, the choice of roof pitch is influenced by the materials to be used and the resulting overlap required if roof tiles are used. As an example thatched roofs are often 45° or more whilst a slate tile roof can be much flatter at say a 35° pitch if desired. In areas of high snowfall then the roof pitch can be much greater, to allow the snow to fall off the roof rather than settle and build up on the roof.

The majority of houses with pitched roofs will have what is now called a cold roof, where insulation is laid on the floor of the loft area, immediately above the ceiling of the upstairs rooms of the house. This means that the roof space is at close to outside air temperature. This is fine for most houses. However if you are looking to use the loft area as an extra room or maybe you live in a barn conversion where the roof rafters are exposed then it is not practical to insulate in this way. In these cases you need a warm roof where insulation is inserted between the roof rafters so that the roof space can be heated and will be at house air temperature.


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