Jargon buster: Types of roof construction
There are a number of different roof constructions, each of which has its own insulation requirements as well as ventilation and waterproofing methods.
These can often get a bit confusing and we hope that our quick jargon busting guide helps by giving a brief introduction to them, their construction and how they should be insulated. More detail about the correct methods of insulation for a particular roof build up can be found on the roofing section of our website.
This is the usual construction that is seen on domestic houses, with the roof meeting at an angle often in the centre of the building. A pitched roof can be warm or cold, depending on the location of the insulation. See our structural quick guide for full details of the different build ups for pitched roofs.
Warm Pitched Roof
A warm roof is where the insulation is on the outside of the structure e.g. above the rafters, it keeps the timbers completely warm, near or at room temperature hence the term warm roof. Insulation may also be placed inbetween the rafters as well as on top of the rafters. When using the latter method it would be advisable, especially in a domestic property, to keep the insulation either at equal performance e.g. 50 / 50 split between and above the rafters or have a higher proportion of insulation on the outside of the structure.
The most common of the two warm pitched roof methods is where the insulation is above and between the timber rafters, with a breathable membrane above the insulation. This would be finished with vertical counter battens, tiling/slating battens and then tiles or slates.
Cold Pitched Roof
A cold pitched roof is where the rafters are not insulated. All the insulation is either between or below the rafters and there will be no insulation above them. Cold roofs are not recommended in high humidity areas (such as swimming pools). Cold Roofs are an easier method if you are carrying out a refurbishment as you do not have to raise the roof line, which would involve removing all the roofing material such as the tiles or slates.
One that is flat, not pitched! This is a roof where the pitch is less than 10 degrees. Flat roofs can have zero degrees of fall i.e. one that is perfectly flat, although this is not a common method of construction due to the problems that it can cause with water drainage. Again the roofs can be of a warm or cold construction.
Flat roofs often use tapered insulation (on a completely flat deck) or have a fall built into the deck, either in the concrete via a screed or timber filletts to provide the fall on timber joists. It is recommended that the minimum fall for a flat roof is 1 in 80 which will allow water to drain easily. For a NHBC project the fall required is 1:60 when using tapered insulation or 1:40 if using regular/flat board insulation. More details about flat roof build ups can be found in our Flat Roofing Quick Guide.
Hybrid Warm Flat Roof
A hybrid warm flat roof is one where at least 2/3rds of the insulation is above the roof deck or joists, however this is a general guide and could change depending upon the building use, location and humidity class. We would recommend a condensation risk analysis be carried out on every hybrid flat roof construction to determine the insulation specification.
Warm Flat Roof
A warm flat roof is one where all the insulation is above the joists/deck, making them part of the warm fabric of the building. Inverted roofs are a type of warm flat roof where the insulation is above the waterproofing layer.
Cold Flat Roof
A cold flat roof is one where the insulation is between or between and below the joists of the roof. This is a common technique when refurbishing an existing timber joisted flat roof which has no insulation. It allows insulation to be added without having to replace the waterproofing on the existing roof or raising the roof line. It is important to make sure that there is at least 50mm airgap for ventilation above the insulation and the roof deck to prevent condensation occuring.
This is a flat roof where the slope of the roof is created by the insulation, allowing water to drain off. Quicker and much more lightweight than laying a tapered screed on top of the deck, each roof is designed to each customer’s requirements by our tapered roofing team. This will take into account the U-value for the roof designs. For more detailed advice our tapered roofing service can provide all the guidance you need, from initial thoughts to installation.
A warm deck flat roof is where the waterproofing would sit above the insulation layer, keeping the insulation and all beneath it dry. However on an inverted roof this is altered and the insulation layer/s go above the waterproofing layer. This means that the insulation should be highly resistant to water absorption – for example, an extruded polystyrene based product which will not be affected by water or repeated freeze/thaw cycles. Any rain water will drain from the roof surface as well as passing through the gaps in the insulation layer. We would recommended the use of double entry type drains so that the water can drain at surface and deck level. This is a similar construction to the warm roof as insulating above the deck ensures that the structure of the building is warm.
A green or living roof is one where it is covered with plants or grass on top of a growing medium, all planted over the waterproofing layer of the roof. It could be a shallow growing medium, planted with sedum for example (sometimes referred to as an ‘extensive’ green roof’), or a full blown roof garden with a variety of different sizes of shrubs and plants (often called an ‘intensive’ green roof’). Visit our green roof application page for more information.
Curved roofs are roofs which incorporate an arc in order to dispel water. They are often used as part of an architectural design feature. This sort of roof requires insulation which can flex over curves, both concave and convex while still providing the required level of insulation. Rigid insulation boards can be grooved in order to provide high levels of insulating performance without the problem of extra thickness associated with common fibre-based insulation materials.
Share this blog post with your friends and colleagues by clicking on the social media icons below.