Flat Roof Systems:
- Single-Ply Roofing (TPO, PVC, EPDM)
- Hot/Cold Asphalt Built-up Roofing (BUR)
- Reflective Coatings
- Restoration Systems
- Foam Roofing
Traditionally most flat roofs in the western world make use of tar or asphalt more usually felt paper applied over roof decking to keep a building watertight. The felt paper is in turn covered with a flood coat of bitumen (asphalt or tar) and then gravel to keep the sun's heat, UV rays and weather off it and helps protect it from cracking or blistering and degradation. Roof decking is usually of plywood, chipboard or OSB boards (OSB = Oriented Strand Board, also known as Sterling board) of around 18mm thickness, steel or concrete. The mopping of bitumen is applied in two or more coats ( usually 3 or 4) as a hot liquid, heated in a kettle. A flooded coat of bitumen is applied over the felts and gravel is embedded in the hot bitumen.
A main reason for failure of these traditional roofs is ignorance or lack of maintenance where people or events cause the gravel to be moved or removed from the roof membrane, commonly called a built-up roof, thus exposing it to weather and sun. Cracking and blistering occurs and eventually water gets in.
Roofing felts are usually a 'paper' or fiber material impregnated in bitumen. As gravel cannot protect tarpaper surfaces where they rise vertically from the roof such as on parapet walls or upstands, the felts are usually coated with bitumen and protected by sheet metal flashings.
In some microclimates or shaded areas these rather 'basic' felt roofs can last well in relation to the cost of materials purchase and cost of laying them, however the cost of modern membranes such as EPDM has come down over recent years to make them more and more affordable. There are now firms supplying modern alternatives.
If a leak does occur on a flat roof, damage often goes unnoticed for considerable time as water penetrates and soaks the decking and any insulation and/or structure beneath. This can lead to expensive damage from the rot which often develops and if left can weaken the roof structure. There are health risks to people and animals breathing the mould spores: the severity of this health risk remains a debated point. While the insulation is wet, the “R” value is essentially destroyed. If dealing with an organic insulation, the most common solution is removing and replacing the damaged area. If the problem is detected early enough, the insulation may be saved by repairing the leak, but if it has progressed to creating a sunken area, it may be too late.
One problem with maintaining flat roofs is that if water does penetrate the barrier covering (be it traditional or a modern membrane), it can travel a long way before causing visible damage or leaking into a building where it can be seen. Thus, it is not easy to find the source of the leak in order to repair it. Once underlying roof decking is soaked, it often sags, creating more room for water to accumulate and further worsening the problem.
Another common reason for failure of flat roofs is lack of drain maintenance where gravel, leaves and debris block water outlets (be they spigots, drains, downpipes or gutters). This causes a pressure head of water (the deeper the water, the greater the pressure) which can force more water into the smallest hole or crack. In colder climates, puddling water can freeze, breaking up the roof surface as the ice expands. It is therefore important to maintain your flat roof to avoid excessive repair.
An important consideration in tarred flat roof quality is knowing that the common term 'tar' applies to rather different products: tar or pitch (which is derived from wood resins), coal tar, asphalt and bitumen. Some of these products appear to have been interchanged in their use and are sometimes used inappropriately, as each has different characteristics, for example whether or not the product can soak into wood, its anti-fungal properties and its reaction to exposure to sun, weather, and varying temperatures.