What type of flat roof is the best?

Over the last few decades in the pursuit of fast track cheap construction many buildings or parts of buildings have been covered over in a flat roof. These roof coverings are not entirely flat, they have slight falls towards rainwater outlets.

Going back centuries rather than decades traditional flat roofing was done in stepped sections to falls and clad in lead, copper or zinc sheet metal – with the sheets joined by lapping over rounded lengths of timber known as mop rolls, usually on lead roofs and folds known as welts on copper and zinc roofs. The problem with these types of roof construction is that although they are very durable and long lasting, a roof holding water caused by blocked outlets from leaves and debris or snow and ice will leak through its joints. Another disadvantage is that the value of copper and lead has become so high on the world market that metal theft from opportunistic predatory lowlife has become almost inevitable. Roof cladding is often removed and weighed in at scrap yards for a few pounds leaving a roof covering which is no longer watertight causing tens of thousands of pounds worth of damage when moisture ingress occurs during rainfall before the roof can be repaired and made watertight.

Types of Flat Roof

Mastic Asphalt
This type of flat roof was often used on buildings in the earlier parts of the 20th century, these roofs worked well on buildings constructed in dense, bonded materials not subject to a great deal of movement. Although asphalt is still used today failures can occur in modern buildings while lighter materials and expansion joints allow the building to move freely. While the building has intermittent movement like this, the brittle asphalt can crack and allow moisture ingress unless correct expansion provision has been made in the surface of the asphalt – which is very difficult to achieve.

Mineral Felt
In the 1960′s and 1970′s the use of pour and roll hot bitumen and mineral felt began to replace sheet metal and asphalt on modern buildings as a cheap, fast track alternative to the more expensive traditional flat roof systems used on high class work.

Throughout the 60′s and 70′s mineral felt flat roofing was used on housing estates, garages, porches, bay windows, many public buildings such as schools and medical centres and house extensions.

These felt flat roofs have many disadvantage, as well as being unsightly in many instances, the workmanship was often poor as was the quality of material used and the lifespan of the roof was often as little as ten years. Often the life of the roof was reduced further when the falls to the roof were incorrect or where stones placed on the surface of the roof to prevent sun damage were walked on in hot weather by maintenance workers such as window cleaners. As the bitumen-based felt became soft stones would be pushed through the felt underfoot, puncturing the roof membrane and allowing moisture ingress. These damaged areas of the roof allowed water to seep in, saturating the timber decking and insulation below. Problems also occurred when the roofing systems were being laid. The hot bitumen used would often find its way through gaps in the decking and would drop on people below causing injury. The areas below should have had restricted access, but when work is in progress on a busy site this was often overlooked and accidents occurred.

Torch-On Felt
This was developed to remove the need for hot bitumen but the process carried a risk of fire from the naked flame of the torches used to heat the felt and, if operatives laying the felt were on price (piece work), an adequate bond to the substrate was not achieved and seems were inadequately bonded as the works were rushed. As a consequence of this the roof system would often fail.

Rubber and EPDM
The use of rubber-based materials and EPDM have also been used to cover the decking of flat roofs the products are glued to the decking boards or substrate. EPDM is subject to extremes of movement leaving folds in the surface material when temperature changes occur.

Rubber type roof coverings can work well but are easily vandalised and can be cut with a sharp knife, raising questions about security if the rubber is pulled back and the decking below lifted up to allow access.

GRP and Fiberglass
A robust and long lasting type of modern flat roofing is the GRP system. This type of roof covering will last over 30 years if installed correctly, it’s life can be extended further still by lightly sanding the fiberglass topcoat after a period of years and applying a new topcoat layer. This process can be carried out in just a few hours and can give the roof many more years of useful life.

Fiberglass is lightweight, very strong, will take foot traffic and accommodate the movement that occurs in modern buildings.

Consider the hull of a fiberglass boat at sea. These vessels are pounded in heavy weather but continue to be used year after year. Imagine a mineral felt boat or rubber-clad boat frame giving the same number of years service at sea.

GRP roofs are available in a variety of colours but grey always give a pleasing aesthetic appearance as it has the look of lead which always looks well on high class work. GRP roof systems also have a variety of accessories such as trims, tilt fillets, outlets and cappings.

In brief, the system is laid on durable engineered rebated timber boards, glued at their joints and laid to falls running to rainwater outlets. The joints in the boards are then taped with glass fiber matting and a basecoat of fiberglass resin. A basecoat of fiberglass matting and resin is then applied to the whole of the decked roof area. Finally, a top coat of resin with the desired pigment in is applied to finish the roof surface and then any flashing or cappings required are installed and the system is complete.