Where deterioration occurs
There are a number of locations where deterioration can emerge in a Bituminous Sheet flat roof including:
- Gutters and outlets
- Eaves and verges
- Roof Lights and openings
- Vents and flues
- Expansion joints
- Roof surface
Problems associated with each of these locations occur for a number of reasons and have a variety of signs and symptoms which allow for their early detection. Likewise, there are a variety of methods of repair which can be used to ensure continued performance.
Methods of repair
How to repair a Bituminous Sheet flat roof where it has deteriorated:
Sometimes the parapet wall can become cracked or have stones dislodged. It can be caused by a number of factors but is most often the result of water ingress or wind. ‑e usual procedures for masonry repair should be followed with unsound masonry being rebuilt or re pointed as appropriate. A further problem that may be encountered along the edge of flat roofs is that of split, torn or damaged skirting resulting in water ingress. Skirting occurs where the edge of a flat roof meets the masonry parapet. Deterioration can be caused by thermal or moisture movement, a lack of an adequate key to the backing or simply a failure in the original design or workmanship. Where this occurs the skirting will have to be repaired or replaced, and is best undertaken by an experienced contractor.
There a number of potential failures which can occur in the area surrounding a roof light or other opening. Wind damage to flashings, defective sealing around the base of the roof light and rotting or degraded timber elements can occur. These should be repaired by making good flashings, ensuring seals are sound (replacing them where necessary) and renewing the frame if rotten.
Vents and flues
Often flat roofs will be penetrated by vents and flues to allow for the disposal of waste gas or smoke from a building. Problems can be experienced around these features with roof coverings becoming soft, discoloured or generally deteriorating. is can be caused by condensed solvents and other harmful agents leaching from the vent or flue. To combat this the affected area should be repaired and a new membrane introduced which can better resist such attacks. Raising the height of the vent may also solve such problems.
There are a wide range of problems which can affect the surface area. Each has its own cause and can be repaired in a variety of ways. Cracks and tears along the line of the support joists of the roofing deck can be caused by thermal or moisture movement, saturation of insulation or sagging of the roof deck. This can be repaired by cutting back the existing felt and allowing the area to dry before applying a new layer of high performance felt with a suitable overlap between the new and existing material. Cracks, splits and rucks can occur when there is thermal or moisture movement between the roof substrate and membrane. It is likely that localised re-roofing will be necessary to tackle this using a high performance felt. Surface crazing is caused where there is a lack of adequate protection from the harmful effects of exposure to the sun or, in rare occasions, chemical attack. If this is only in small localised patches no treatment is necessary but should be regularly rechecked. If more widespread the areas will have to be re roofed and specialist protection introduced.
Blisters can be caused by water vapour pressure occurring below the roof covering. e blister should be cut and then re-bonded to the underlay allowing any trapped moisture to escape first. e source of the moisture should also be traced and rectified. Punctures and rips are most often caused by impact damage by people or loose debris. ey should be repaired using localised patch repairs. Where surface problems are serious it may be necessary to re-surface the entire roof. is should be considered where patch repairs will no longer be sufficient to ensure the integrity of the weather proof covering and should be carried out by a contractor experienced in such work. Well maintained, a flat roof should perform adequately for many years.
Guidance on Roofs
- Technical Note 42: Chimneys, flues and hearths
- Technical Note 43: Roofs
- Technical Note 44: Roof drainage
- Listed Building Maintenance - Roof Coverings
For additional technical guidance please visit the Historic Environment Toolkit