Repairing Flat Roofs

Repairing Flat Roofs

Last week I discussed durable alternatives to bitumen flat roofs, this week I’ll explain how to repair what you can’t afford to replace. But be prompt! Ignoring a dripping ceiling will cause the decking and the ceiling joists to rot rapidly and remedial work will then be much more expensive.

The position of the drip gives little indication of the position of the leak. Water can run under the felt to a different position, drip to the ceiling, then run to a different position again before finding its way into the room below. A small neat hole should be made in the ceiling to look for the drip, and the actual leak in the felt will be somewhat uphill of that. Flat roofs have a slight pitch, commonly 1:25 or 1:50, so rainwater drains off without puddling.

First check the flashing above the drip. This is the lead or zinc strip that covers the joint between wall and roof. It should have been ‘chased’ into the wall. A groove should have been cut into the wall about an inch deep, the lead fed in, then held with little rolls of lead, flattened into wedges and hammered into place. The groove is then filled with cement. Sometimes the groove will need chiselling out and the lead re-fitting. If this is impossible a makeshift repair can be made with 2 – 3” wide self-adhesive metallised flashing tape stuck over the joint.

Next examine the roof above the leak for cracks, raised flaps or brittle areas. If there is widespread damage it may be best to pay a specialist contractor to replace all the felt and seal it back with hot bitumen. If you are prompt with your repair, the decking and joists may still be OK.

If repairing it yourself, wait until after prolonged period of dry weather to dry out trapped water. Remove any chippings, clean around the split, and carefully lift any loose flaps. Dry out the space underneath with a hot air gun if necessary, being careful not to singe the felt and cause further damage. Pour or brush bitumen adhesive under the lifted flaps and press down firmly. Fill any other spaces with adhesive.

Cut a patch from a piece of new felt, to give a 50mm overlap beyond the repair. Apply bitumen adhesive over the repair and press the patch firmly into place, sealing all edges thoroughly. Brush more adhesive over if necessary.

Small blisters in the felt may be best left if not leaking. Repair larger or leaking blisters by cutting a cross in the top layer with a Stanley knife. Fold back the flaps and dry as necessary. Apply bitumen adhesive then firmly press the segments back into place. A patch of new felt reaching at least 50mm beyond the repair should then be stuck in place on top, as described above for splits.

For multiple areas of minor damage, brush of and save any chippings then apply a whole-roof treatment. There are bitumen-based products, and rubberised emulsions which claim they can be applied in wet conditions (which I don’t recommend). Even if your flat roof is not actually leaking, applying one of these may extend its life by a few years.

Finally, some coatings allow a metallised paint to be applied, reflecting harmful rays from the sun, greatly increasing the life of the roof. If this isn’t possible replace the light-coloured chippings.

Next week I’ll discuss why you can’t always rely on a structural survey from RICS, or on your Building Inspector to assure that your builder is following best practice. It may be controversial.



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