Felt roof repair

How felt roofs fail

Felt is laid in overlapping layers bonded together, usually with a mineral or chipping finish on top to shield it from sunlight. Ultraviolet light, summer heat and winter cold slowly dry out the bitumen. As it ages it loses flexibility, and the movement of the timber deck below starts to open up weak points instead of being absorbed.

Most felt failures therefore start small and local: a bubble here, a crack along a board joint there, a corner lifting at the drip edge. Spotting them early is what keeps a repair cheap.

Blisters and bubbles

A blister is a pocket where the layers have parted and trapped air or moisture. The sun warms it, the pocket expands, and the raised dome takes extra wear from weather and foot traffic. A blister that is intact and not leaking is not an emergency, but one that has cracked across its top will let water track between layers.

Repair involves cutting the blister open, drying and re-bonding the layers, then sealing a new piece of felt over the area with a generous overlap. Dozens of blisters across the whole roof point to wider trouble, often damp trapped under the covering.

Splits and cracks

Straight splits often follow the joints between deck boards underneath, because each board moves slightly with temperature and humidity. Crazing, a network of fine surface cracks, is a sign of age rather than a single defect.

A clean split on otherwise supple felt can be patched. Widespread crazing means the top layer has lost its life, and covering it with patches tends to create more edges for water to get under.

Lifted laps and edges

Every place where one sheet overlaps another is a potential entry point. Wind gets under a loose lap at the perimeter and peels it back further each storm. At the eaves, felt usually dresses into a gutter or over a drip trim; if that trim has worked loose or the felt has shrunk back from it, rain runs straight onto the fascia board and into the deck edge.

Lifted laps can be cleaned, warmed and re-bonded, and loose trims refixed, as long as the timber they are fixed into is sound.

Upstands and flashings

Where a flat roof meets a house wall, the felt turns up the wall as an upstand and should be covered by a flashing tucked into the brickwork. Many leaks blamed on the roof surface actually start here: the upstand cracks at the fold, the mortar holding the flashing falls out, or the flashing was never fitted. A similar weakness appears around rooflights and soil pipes.

These details can usually be repaired without disturbing the main surface. Our guide to flat roof leaks covers other places water commonly gets in.

Patch repair: when it works

A patch is a sensible choice when:

• The fault is in one place and you can see what caused it.
• The felt around it still bends without cracking.
• The deck beneath is dry and does not give when walked on.
• The roof drains, with no large pools sitting for days after rain.

When a felt roof is past repairing

Repeated leaks in different spots, soft areas in the deck, widespread crazing and ponding across a large part of the roof all suggest the covering has reached the end of its useful life. At that point the money spent on patches would be better put towards a new roof, laid on sound boards with proper falls.

What goes into that price is explained in our guide to flat roof replacement cost. Laying a new covering over an old, failing one is rarely a good idea, because any trapped damp and rot stays hidden underneath.

Garages and extensions

Felt is still very common on detached garages and older single-storey extensions. Garage roofs are often thinner, with chipboard decks that swell badly once wet, so a small leak can spread further than you would expect. If a garage roof has been patched several times, read about garage roof replacement before paying for another repair.

On an extension, the inside ceiling usually shows a stain long after water first got in, so it is worth checking the roof surface from a window or ladder once a year.

If you would like the roof looked at, our flat roofing page explains how we approach repairs and replacements. We look at the roof, explain the options and give a clear written quotation before any work starts.

Frequently asked questions