Flat, Felt Covered Roofs – What To Spot

Posted 10th May 2018

Bitumen felt is the most common traditional material used for covering flat roofs in domestic construction. It normally appears a black to dark green with pigments of grey. It is widely known that flat roofs are more likely to fail than traditional pitched roofs and for this reason they are more often used when the span of the roof is relatively small. It is important to remember that flat felt roofs have a limited life span typically between 10 and 20 years and are prone to leakage. More often than not flat roofs fail because of poor workmanship at initial installation, however mechanical damage and lack of solar protection also contribute to the high number of flat roof failure.

Building materials expand and contract with changes in temperature at different rates. Depending on the type of flat roof, a felt covering will expand and contract at a very different rate to the timber deck which supports it and the two materials should be isolated from one another. Splitting to the felt can occur when the absence of an isolating board allows the supporting timber deck to expand beyond the maximum capacity of the felt covering. Roofs that are exposed more frequently to direct sunlight are more likely to suffer from this type of defect.

Poor detailing where the roof abuts an external wall, parapet, roof light or flue can often shorten the lifespan of a felt roof. Flashings of unsuitable materials such as flashband are occasionally provided and even traditional materials such as cement are prone to deterioration. Flashings should be dressed into the mortar joint and under the cavity tray (if applicable) at the upstands to prevent water penetrating the abutment. Ideally a flat roof should have flashings of lead or zinc and incorporate angle fillets to reduce the risk of splitting at the junction.

Without the provision of solar protection, prolonged exposure to direct sunlight is likely to cause some degree of brittleness. If the covering does not incorporate mineral granules then stone chippings should be used to absorb the UV radiation which can damage the membrane. The benefit of using stone chippings is they can absorb some of the radiant heat from the sun, slowing down temperature changes in the felt. The drawback of stone chippings is it can be difficult to apply them over the entire covering and stones are likely to fall away from edges with gradients.

We refer to a flat roof as being flat, but in fact it should not be flat. It should be laid with an appropriate fall to accommodate the expected amount of rainfall and the span of the roof. Inadequate fall will invariably result in ponding. When ponding occurs alongside minor damage to the covering, penetrating dampness becomes a very real prospect. It must be remembered that the buildup of moss can also affect rainwater runoff, particularly on flat roofs.

Despite the inherent defects, if professionally constructed with proper detailing; flat, felt covered roofs can be a cost effective way of keeping out the elements.