The flat roof seems to be moving to centre stage, especially with the trend for cool modern structures and the growing interest in green roofs.

What Pitch is a Flat Roof?

If flat roofs are to work well, they have to have a slope, known as a fall, to shed rainwater. The fall should never be less than 1:80 and preferably about 1:40. There are several methods of building in a fall:

  • One is to set the cross walls at different heights. Most people shy away from this because they don’t like the idea of having an internal ceiling that slopes.
  • A far more common method is to use ‘firring strips’, which are battens cut at an angle and placed on top of the roof joists.
  • Another possibility is to cut a fall into the roof insulation, something several businesses now offer — it’s often referred to as tapered insulation.

How to Build a Flat Roof

Warm Roof, Cold Roof or Hybrid Roof?

How exactly a fall is built in largely depends on how the flat roof is built. There are three main choices:

  • The cheapest and simplest solution is to construct a warm roof. This is where the roof insulation sits directly on top of the timber structure (thus keeping it warm) and then the roofing membrane is laid over the insulation.
  • A cold roof places the insulation between the rafters, underneath the ply roof covering. It is the standard method employed on flat-roofed extensions but it is rarely used for new builds today. In general, a warm roof is preferable from a construction angle, but it does lead to a higher roof profile, which isn’t always acceptable from a planning perspective.
  • There a number of variations of the warm roof, referred to as hybrid roofs, which introduce different elements. In particular, some designs call for a ventilation gap above a warm roof in order to eliminate any possibility of moisture build-up inside the roof structure.

It is important that moisture doesn’t get trapped within the roof, which can happen from underneath if there is a pathway into the roof via air leaks around badly fitted boards or penetrations, such as downlights or pipes. Condensation risk analysis is habitually undertaken by designers to ensure that any potential dew point that might occur within the roof itself is eliminated.

In truth, it’s a confusing topic with claim and counter-claim often being made by manufacturers. Where problems do occur, 99% of the time it’s due to air leakage through readily identifiable routes, but however you do it, great care must be taken to avoid air leakage into the roof void. It’s no less important than providing a good waterproof covering above.

Flat Roof Coverings

Over the years many products have been used and none has emerged clearly as the best. In practice most people plump for the system their installer or designer likes the most. Generally speaking, the issues to watch are the cost and the guarantee offered. As a rule, don’t go for anything less than 20 years. Costs vary from around £35/m2 up to £80/m2.

Torch-on felt: applied hot in two or three layers, it remains potentially the cheapest covering but there are several different grades you can specify. The best felt systems are now comparable with any of the alternatives but cost just as much. Torch-on felt still has its admirers. Although it’s a cheaper product, it is built up in layers and this gives a more robust finish, more suitable for light foot traffic. And the fact that it is applied hot makes it easier to form around penetrations and corners.

Mastic asphalt: potentially the longest lasting, it is now usually used under roof terraces that will be walked over, so habitually it is laid as a base layer beneath paving slabs.

EPDM rubberised roofing: widely used and easy to install, it comes on cut-to-measure rolls with its own flashing systems. It is usually bonded to the substrate with adhesive and, done well, should last at least 30 years. Firestone, the tyre manufacturer, is the number one supplier.

Single ply membranes: these use a variety of materials, of which PVC is the most common, but others are available. Look out for PIB, TPE and TPO, the latter preferred by people wishing to avoid PVC on environmental grounds and consequently popular in Germany. They are usually fitted by approved installers and, more often than not, customers work with the system their installer recommends. Generally these are a little more expensive than EPDM, but with longer guarantees. Sarnafil remains the best known PVC brand; look at Bauder for alternatives.

Liquid roofs: occasionally, complex roof shapes require something a little different and paint-on solutions can be used effectively where other methods struggle. Products like Kemperol have a well-deserved niche, known for their reliability and capacity to overhaul and repair failing roofs. However, on new roofs they are expensive. Fibreglass roofs can also be laid wet and built up with matting.

Why Use a Flat Roof?

There are many advantages to using a flat roof:

  • One of the main advantages is that a flat roof frees up internal space.
  • You have a lot more freedom to plan interesting shapes beneath a flat roof without having to worry about the way the roof is held up. In a conventional house with a pitched roof everything relates to the way the roof works. In a flat roofed house you can place the internal walls pretty well where you want and incorporate all sorts of twists and turns.
  • Flat roofs offer immeasurable flexibility.
  • They help to keep the internal temperature stable.
  • With flat roofs you get none of the impression of bulk that you get with a pitched roof, but internal area is not compromised to achieve this.
  • Although specialist teams used to install a flat roof correctly can be expensive, flat roofs are still cheaper to construct than pitched roofs and need less material to cover the same span.

Green Roofs, Blue Roofs and Beyond

The fashion for extensive green roofs growing sedums, grasses and mosses shows no sign of abating and, done well, they make truly splendid features.

However, there is a lot more to them than a ‘normal’ flat roof. They require a root barrier to stop the plants interacting with the waterproofing beneath and a flat roof in particular requires a drainage layer to retain water.

Coupled with some edging features to stop the vegetation leaking away, this makes a green roof build-up quite expensive. They also add to the weight of a roof, and may well require the underlying structure to be beefed up. Another function of green roofs is to hang onto rainwater and reduce the amount of run-off.

A blue roof is one that aims to attenuate rainwater run-off to prevent localised flooding. This is like a green roof on steroids and involves a far more complicated build-up. It is unlikely that self builders will be specifying a blue roof unless planners insist on such a feature.

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