Serious damage to a flat roof will result in water seeping in and damp patches will soon appear on either the ceiling or adjoining walls. The position of the damp patch will not necessarily indicate the source of the leak, as water can enter through the covering at one point and travel along either the ceiling or joists to emerge elsewhere. For this reason, make a careful inspec-tion of the roof covering before undertaking any repair. And remember, poor ventilation and condensation can cause the same symptoms as leaks.



Blisters and cracks are the most common types of damage found on felt roofs. These can be caused by ageing, the effects of sunlight, and extremes of temperature or poor original fitting. Felt roofs can be kept well main?tained by painting them with a renewing compound such as Aqua seal waterproofing paint every three or four years. How?ever, don't expect this treatment to prolong the life of the roof covering indefinitely - the practical maximum is often as little as 10 to 15 years.

Blisters:

1. Providing the blister is restricted to the top layer of felt, it can be repaired in much the same way as you would repair a blister in wallpaper.

2. Use a sharp trimming knife to cut an X across the surface of the blister; Make sure you do not cut into the second layer of felt. Gently lift the four corners of the cross-cut with a putty knife, paint scraper or other relatively blunt tool.

3. Use your putty knife to paste in ample quantities of mastic under the repair. Nail back the cut pieces using flat-headed galvanized nails.

4. Next cut out a patch of roofing felt large enough to cover the repaired area with an additional overlap of about 50mm on all sides. Bed the patch down on a layer of mastic and secure with galvanized nails.

Patching:

If the roof felt is damaged over a large area - up to 300mm square - repair it with a double patch.

Cut out a piece of new roofing felt, large enough to cover the damaged area. Using the patch as a template lay it over the damage and mark out the area of felt to be removed. 5. Most roofs are covered with overlapping layers of felt - on outbuildings or garages there may only be one thickness. Use a sharp trimming knife or chisel to cut away only the top, damaged layer. Prise away the damaged piece with a paint scraper. 6. Coat the area to be patched with a thick layer of roof mastic, and then bed the patch in place.

Use galvanized flat-headed nails to secure the patch: drive them in around the edges. 7. Finally, cut out a larger patch so that it overlaps the repair by at least 50mm. Apply this in exactly the same way as the first.