A damp patch on the ceiling under a flat roof rarely appears where the water is actually getting in. Flat roofs on garages, extensions and dormers across North Yorkshire fail for a handful of predictable reasons, and most of them are cheap to fix if caught early. Here is how to work out what is going wrong and what to do about it.
A flat roof should never be truly flat. It needs a fall of at least 1 in 80, and ideally 1 in 40, so rain runs to the gutter or outlet within a day or so. If water is still sitting on the roof 48 hours after rain, that is ponding, and it is the most common cause of leaks we see around Bedale, Leyburn and Northallerton.
Ponding happens when the deck sags between joists, when the original roofer laid the covering with too little fall, or when a later extension or fascia repair changed how the roof drains. Standing water puts constant pressure on any weak seam, freezes and expands in winter, and grows moss that holds moisture against the covering for weeks.
The flat area of the roof is rarely where the water gets in. Leaks almost always start at a joint: the lap between two rolls of felt, the corner of a parapet upstand, the flashing where the roof meets a house wall, or the seal around a soil pipe or rooflight.
On older three layer felt roofs, laps lift as the bitumen dries out and cracks, usually after 10 to 15 years. Lead flashings pull away from the mortar joint when the pointing fails, and a bead of mastic that someone put on five years ago will have long since split. On EPDM rubber and GRP fibreglass roofs the membrane itself can last 25 years or more, but the edge trims and upstands still depend on the quality of the original detailing.
If your ceiling only gets wet during a downpour, or when the wind is from a particular direction, drainage is the likely culprit rather than a hole in the covering. Leaves, moss from a nearby pitched roof, and tennis balls are all regular finds in outlets around here.
When the outlet blocks, water backs up over the upstand or under the edge trim and finds its way behind the fascia into the wall. Clearing the outlet, fitting a leaf guard and checking the gutter falls will fix a surprising number of leaks with no roofing work at all. Do this twice a year, ideally in late autumn once the trees are bare and again in spring.
Bitumen felt blisters when moisture trapped in the deck or between layers heats up in summer sun and expands. A blister is not a leak on its own, but once it cracks or gets trodden on it becomes one. Alligator cracking across the surface, where the felt looks like dried mud, means the covering has reached the end of its life and patching will only buy months.
Age matters. A felt roof from the 1990s or early 2000s is probably due for replacement rather than repair. A well laid GRP or EPDM roof from the last 15 years should have plenty of life left, so a leak there is more likely a detail failure that can be sorted for a fraction of the cost.
Condensation is the other thing worth ruling out. A cold flat roof over a kitchen or bathroom with no vapour barrier can drip from the underside of the deck in winter, and it looks exactly like a leak. If the damp appears on frosty mornings rather than wet days, that is the more likely explanation.
A localised repair to a lap, flashing or outlet detail on a small domestic flat roof generally costs in the low hundreds of pounds, depending on access and what is found once the surface is lifted. A full replacement of a typical single garage roof of around 18 square metres in EPDM or GRP usually runs to a four figure sum, with the final price depending on whether the deck boards need replacing, the height of the roof, and whether insulation is being upgraded at the same time.
Be wary of anyone offering to paint a liquid coating over an old felt roof without inspecting the deck underneath. If the timber is soft, no coating will save it. A proper inspection should include walking the roof, checking every upstand and outlet, and pressing the deck for softness, followed by a written quote that separates repair from replacement so you can make a sensible choice.
Water runs along the top of the deck boards or vapour barrier before dropping through, so the stain can be a metre or more from the actual entry point. The leak is usually uphill of the patch, towards a seam, upstand or penetration.
A temporary patch with flashing tape over a clean, dry seam can hold for a few months and is a reasonable stopgap in winter. It is not a permanent fix, and applying mastic or bitumen paint over a wet or dirty surface often makes the eventual proper repair harder.
Three layer felt typically lasts 10 to 20 years, while EPDM rubber and GRP fibreglass usually last 25 to 30 years when properly detailed. Exposure to wind and frost on the edge of the Dales, and how well the outlets are kept clear, make a real difference either way.