Flat roof repairs divide neatly into two groups: the ones that fix a defect and the ones that cover it. Which group a repair belongs to depends almost entirely on whether the deck underneath is dry, and that is the question worth answering before spending anything, because a patch on a wet deck is a payment for a delay.
Flat roof repairs by system: felt, rubber and fibreglass roof repair
Reinforced bitumen felt is the most repairable of the three: a split or an open lap can be cut out and a new patch torched or bonded in, and a competent repair on a sound roof lasts years. Fibreglass cracks where the roof moves, and a repair grinds back the surface and lays new matting and resin, which works if the movement has stopped and fails again if it has not. Rubber sheet is repaired with a bonded patch and the difficulty is surface preparation, because the bond is everything.
Shed roof repair cost: the smallest flat roof
Few roofers publish a price for a shed roof, because most are refelted by owners or added to a bigger job. The nearest published figures are for small domestic flat roofs: Alliance Roofing gives £150 to £400 for a small repair to a GRP or EPDM roof, and Nationwide Roofing Specialists gives £800 to £1,500 for a 15m² garage roof in felt. On a shed, the call-out, access and the boards underneath set the price.
Roof felt repair cost
A patch on felt is priced as a small job rather than by area: Arkenstone Construction gives £150 to £500 for a leak trace and repair. Nationwide Roofing Specialists gives £40 to £80 per m² for felt, which is a replacement rate and the figure to compare against once a roof has been patched more than twice. A felt repair is only worth paying for while the deck underneath is dry.
Why repairing from inside rarely works
Owners look for a way to seal a flat roof leak from inside because access is easier and the stain is visible there. The problem is that water enters at one point and travels along the deck before it appears, so the visible spot is rarely above the defect, and any sealant applied from below traps water inside the build up where it rots the deck faster. Internal work has one legitimate use, which is finding the extent of the wet area before the repair is priced from above.
Leaking flat roof repair cost: when the repair stops being worth it
Three signals. The deck feels soft underfoot, which means the boards have absorbed water and will not recover. The roof has been patched more than twice in the same area, which means the defect is the falls or the deck rather than the covering. Or the insulation is saturated, which a moisture survey shows and which nothing laid on top will dry out. At any of those points the cost of another repair is better spent on the replacement it is postponing.
Questions people ask about flat roof repair cost
Can a flat roof be repaired in winter?
Torch applied and self adhesive systems need a dry surface and a workable temperature, so cold wet weather delays a permanent repair. A temporary cover is the usual answer until conditions allow.
Flat roof leaking? How to find where the water gets in
From above, working back from the wet area inside to the highest point on the roof that could feed it, checking the outlets, the upstands and the laps first. A hose test on a dry day narrows it further.
Is liquid rubber a real repair?
On a sound dry roof with a small defect, applied to a properly prepared surface, yes. Applied over a damp or dirty roof it delaminates, which is why the preparation is most of the labour.