Roofs: Flat Felt

Defect

Cause/Action

Ponding

Ponding is generally caused by deflection in the roof decking preventing surface water getting to the outlets, by blockage of the outlets by leaves or tennis balls or because outlets are proud of the decking.

Persistent ponding at the same place causes a gradual deterioration of the bitumen felt, especially if it is old and based on organic fibres. If the ponded areas do not leak they can be left until it is convenient to carry out remedial work, but should be inspected at regular intervals.

The material causing the blockage will have to be removed by rodding or jetting. If the roof outlet was blocked consideration should be given to a form of guard which will prevent debris from entering. If the outlet is raised above the roof level due to faulty setting it will be necessary to re-fix the outlet to its proper level.

Blistering, rippling and cockling

Blistering, rippled or cockled areas vary In size. They may not allow water to leak into the building but splits occurring at the top of a blister may spread so that leakage takes place. Blistering may be accompanied by sliding action of the top layer of felt revealing the edge of the layer underneath it. This may accelerate the rate of leakage. Blistering occurring between layers of felt will be due to the expansion of air or water vapour entrapped when the felt was laid. In hot weather the felt will be more flexible and the expansion of the air or water vapour will exert greater pressures so that blistering occurs more often in summer months.

Entrapped rainwater or water vapour from interstitial condensation may also be present between the felt and the substrate and may cause blistering.

Where leakage has occurred it will be necessary to open up the covering, ascertain the reason for blistering and carry out the appropriate repairs.

Split

This defect is usually due to neglect of the necessary precautions when laying. Flat roofs are subject to movements within the structure due to temperature changes or from movements in the base upon which the felt has been laid. The extent of any movement may be greater than the felt can withstand. If the base cracks it is likely that the overlying felt will crack at that point.

Remedial work depends upon the reasons for the splitting of the felt and the extent of the splitting. If due to shrinkage of the base the splits may be patched using traditional techniques, though the inclusion prior to patching of a loose strip of material not bonded to the roof covering either below the torn piece of felt or above it would be an advantage.

If due to more general movement of the roof, especially thermal movement, and particularly if the split is long, consider converting the split into a movement joint. Subsequent thermal movements of the roof can be reduced by applying layer of white chippings or brushing on a lime-tallow wash or a paint specifically formulated for the purpose. Normal paints should not be used since the shrinkage of the paint film can tear the felt making the situation worse.