Flat Roof Leaking

What's happening?

A flat roof is never truly flat — it is laid to a shallow fall so water runs to an outlet. When that fall is wrong, blocked or has settled, water ponds, and ponding is what kills flat roofs. The covering is a continuous waterproof membrane rather than overlapping units, so it has no second chance: one split, one failed lap or one lifted upstand and water is inside. Older UK flat roofs are three-layer built-up bitumen felt, typically reaching the end of their life somewhere in their second or third decade. Newer ones are single-ply, EPDM rubber, GRP fibreglass or torch-on high-performance felt, all of which last considerably longer when detailed properly.

What usually causes it?

• Ponding water from inadequate or reversed falls, a deck that has deflected between joists, or a blocked outlet, hopper or gutter, so the roof holds water rather than draining
• Split or open laps between sheets of felt, usually starting where the sheets were bonded in cold or damp conditions
• Blistering — moisture trapped under the membrane expanding in sun and lifting the bond
• Failed upstands and flashings at the abutment wall, or an upstand too shallow so water gets over it
• UV degradation and loss of mineral or chipping protection, leaving bitumen brittle and crazed
• Foot traffic and stored items puncturing the membrane, plus damage from ladders set on the roof
• Trapped condensation in a cold-deck construction with no ventilation path, rotting the deck from below

When should you call a roofer?

Call a roofer the same day if water is entering a habitable room, if a ceiling is bulging or sagging, or if the deck feels soft underfoot from below. Call within days for ponding, blistering or lifted laps with no active leak — those are the warning stage and repairs at that point are far cheaper than a deck replacement. Ask specifically whether the deck is sound, because that answer determines whether this is a patch, a re-cover or a strip and rebuild.

What will the roofer actually do?

• Assess the roof safely, including whether the deck can be walked on at all, and check for asbestos-containing materials on older garage and outbuilding roofs before disturbing anything
• Trace the actual entry point rather than the drip point, checking laps, upstands, outlets and penetrations such as vent pipes and cable entries
• Establish whether the deck is sound, saturated or rotten, since a new membrane over a wet deck fails again
• Clear and, where needed, re-form the outlet and correct the falls with tapered insulation or firrings so water actually leaves the roof
• Repair locally where the covering has life left, or strip and re-cover in built-up felt, EPDM, single-ply or GRP as appropriate
• Form upstands to an adequate height and dress them properly into the abutment with new flashing rather than sealant, and address ventilation or specify a warm-deck build-up where trapped condensation is rotting the deck from below