The Different Types of Flat Roof


What is a flat roof?

In contrast to the many varieties of sloped roofs, a flat roof is a roof that is almost level. A roof’s pitch is the correct term for its slope, although flat roofs often have a slope of up to 10 degrees. The roof area can be used as a living space or a living roof thanks to flat roofs, an old style that is typically used in arid climates, such as in the Mediterranean or Middle East.

Flat roofs, often known as “low-slope” roofs, are frequently seen on commercial and residential structures all over the world. A low-slope roof, according to the National Roofing Contractors Association, is one with a slope of 3 in 12 (1:4) or less.

How to cover a flat roof

A membrane is typically any sheet of material used to cover a flat or low-pitched roof, and its main function is to keep the roof area dry. Materials used to cover flat roofs often allow water to drain into a gutter system from a little incline or camber. Although gutter systems are helpful in keeping both walls and foundations dry, water from some flat roofs, such as those on garden sheds, occasionally flows freely off the edge of a roof. Smaller roofs frequently have gutters that discharge water either directly onto the ground or, preferably, into a soakaway. Larger roofs typically have gutters that drain water into the area’s rainwater drainage system.

Bitumen flat roofs

In order to keep a building waterproof, most flat roofs in the western world often have felt paper put over roof decking instead of tar or asphalt. In order to keep the sun’s heat, ultraviolet radiation, and weather off the felt paper and prevent it from cracking or blistering, it is next covered with a flood coat of bitumen (asphalt or tar), followed by gravel. This helps prevent degradation. Bitumen is mopped in two or more coats (often three or four), using a hot liquid that has been heated in a kettle. The felts are covered with a flooding covering of bitumen, and the hot bitumen is mixed with gravel.

The removal of the gravel from the roof membrane, also known as a ‘built-up roof’, by individuals or events exposes it to the elements and the sun, which is one of the main causes of these traditional roofs failing. As a result of cracking and blistering, water eventually seeps in.

Typically, roofing felts are made of fibres or “paper” that has been bituminously-impregnated. The felts are typically coated with bitumen and shielded by sheet metal flashings known as gravel stops since gravel cannot protect tarpaper surfaces where they rise vertically from the roof, such as on parapet walls or upstands. When heavy rains fall, the gravel stop that ends the roofing prevents water from washing off the gravel surface and running below the roofing.

Because of the unreliability of traditional bitumen roofing and petroleum’s environmental impact, Bristow & Reeve no longer use bitumen to construct flat roofs.

Rubber flat roofing

Anyone who wishes to install their own roofing system could choose EPDM rubber roofing because it is simple to install, fairly lightweight and cost-effective. Additionally, EPDM is strong and hardwearing, so you won’t need to replace your roof for a while. In contrast to conventional materials like bitumen (asphalt), you’ll also save time and money on maintenance and cleaning. Therefore, this can be the best option if you’re seeking a low-maintenance solution for your house.

How to install a rubber flat roof…

Firstly, we lay out the EPDM flat and let any creases, folds or bubbles straighten out. Then we use adhesive on the EPDM (and sometimes on the surface of the flat roof itself), and then we roll out the rubber on the flat roof structure.

Fibreglass flat roofs

Fibreglass is another extremely popular option for covering and strengthening this kind of roof, because it is very durable, sometimes lasting over 30 years.

Fibreglass roofs are sometimes known as GRP roofs (GRP glass-reinforced plastic). GRP describes a type of fibreglass where strips of fibreglass are mixed with a resin that quickly hardens. A top coat is needed to seal a fibreglass roof.

To lay a fibreglass roof, the day must not be too cold, or the mixture will turn too viscous and solid. (However, the hotter the day, the quicker the resin will set solid.) Air temperatures between 7 degrees to 26 degrees are best. It must also be a dry day so that moisture does not prevent adhesion.

We prepare the substrate (the flat decking onto which the GRP will adhere). There are small gaps between the decking boards to allow the resin to flow into them and affix the boards together.

We then lay edge trims and join them. Then, it’s time for lamination!

We mix buckets of resin, and lay out the strandmat on the decking. We then coat both the mat and the decking with resin, using a roller. We often do multiple layers of resin, sanding and acetoning between each layer to make an even and smooth surface.

Finally, we top coat in a colour chosen by the client to fit the look of the house.

Lead flat roofs

Listed buildings or characterful older houses look great with a lead flat roof. (Listed buildings are the most common lead flat roof projects we get called for, because of the cost of lead, it’s a fairly rare occurrence!) Lead flat roofing is a specialist skill; panels of rolled lead are laid over OSB boards, and upstands prevent the lead from cracking when it expands and contracts. Using the correct code is vital for this; generally, the more space, the thicker the lead. Using the wrong code can either result in cracking lead, or in a high project cost!


Contact us for your flat roofing repair or reroof.