When building a flat roof, it is vital that a degree of falls is built into the construction to ensure rainwater run-off.
The definition of a flat roof, according to BS 6229, is “having a pitch less than 10o to the horizontal.” However, some in the industry would consider up to 15o as their definition of a flat roof. Others would treat even steeper pitches as flat roofs. For example, if a green roof was to be installed on a pitch steeper than 15o it would likely be a flat roofing contractor who would undertake the work, therefore it could be classified as a flat roof.
A correctly built flat roof will drain rainwater quickly and effectively into gutters without allowing water to pond – a small amount of ponding is still possible but this very much depends on the level of drainage. Inadequate drainage can result in the following.
- Silt deposits and freezing
- Slip hazards caused by algae, mould growth and ice
- Progressive deflection
- Increased water ingress following mechanical damage
- Poor aesthetic appearance
- Increase maintenance costs
- Odour / hygiene issues
There are several different ways of creating a flat roof and each has its own pros and cons. There are also a variety of types of insulation available for flat roofs including PIR, tapered insulation, vacuum insulated panels and XPS insulation. The below diagrams show different flat roof build-ups.
Warm roof (built-up roof)
A warm flat roof or a built-up roof (BUR) is one where all the insulation is above the joists/deck, making them part of the warm fabric of the building. The insulation is immediately below the waterproofing layer. The vapour control layer is above the decking but below the insulation. The insulation can be adhered, mechanically fixed or loose laid.
A cold flat roof is one where the insulation is between or between and below the roof joists. This is used for refurbishment e.g. refurbishing an existing timber joisted flat roof which has no insulation. The insulation is below the deck, but above the plasterboard. 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 a 50mm airgap for ventilation above the insulation and the roof deck to prevent condensation occurring.
Warm green roof
A green roof, or a living roof, is a type of warm roof that 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’). A green roof can provide a habitat for wildlife and creates usable areas for recreational activities. They provide a visually more attractive finish than protected membrane roofs with gravel or paving slab ballast. A green roof can also retain rainfall thus preventing water surges into the drainage system. Other benefits include an improvement of sound insulation and the fact that green roofs can help to gain BREEAM Credits.
Warm inverted roof
Inverted roofs are a type of warm flat roof where the insulation is 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.
The most common deck types used in flat roof constructions are concrete, timber and metal. Each has both time and cost advantages and disadvantages. Each deck type is commonly associated with a different method of creating a fall – concrete often uses screed, timber often uses firrings and metal often uses a tilt. All of these roof types are compatible with tapered insulation.
A common design problem
There is a growing trend in which a layer of insulation is specified above the deck, whilst the rest is specified below the deck. While this may seem like a good idea, as it requires no ventilation plus keeps the build-up as thin as possible, this practice should not be encouraged.
In a warm roof, condensation is prevented by keeping the deck at a warm temperature, therefore there is no cold surface for the condensation to form on. In a cold roof, condensation is prevented as the ventilation taking away the moisture vapour before it can form as condensation.
In this type of roof, you remove both of these levels of protection – thinner insulation means the deck isn’t kept warm, and no ventilation space means the moisture is not removed before it forms as condensation.
If this practice is going to be attempted, a full condensation risk analysis must be conducted before the design is finalised. Even then, it is not advised to use this method as the workmanship standard will be assumed to be almost perfect – and if this is not the case, the condensation risk is greatly increased.
Tapered insulation as a flat roof solution
Tapered insulation is a great flat roof solution which allows falls to be created with specially shaped insulation, rather than requiring screed or timber firrings. Tapered insulation is compatible with many different roof types and in both new build and refurbishments. It is a flexible solution which allows for waste to be minimised through predesigned and factory-made processes. There are several advantages of using tapered insulation over alternative solution. It is up to 1.5% of the weight of screed, reducing the stress on the rest of the structure. It also eliminates the need for drying time which saves money by speeding up the overall construction time.
There are lots of important factors to consider when installing a flat roof. Whether it’s a new build or refurbishment, what waterproofing system you will use, what your budget is and much more. Whichever build up you choose, it’s vital to ensure that you adhere to building regulations and that the fall in your flat roof provides efficient drainage.