Not every flat roof fault needs a new roof. This guide runs through the common failures on felt, rubber and fibreglass, the correct repair for each, and how to tell when patching has run out of road.
Start with where the water is standing
Before touching the covering, look at the roof the morning after rain. Water lying in a pool 24 hours later tells you the fall is wrong or the outlet is restricted, and no patch will survive long in a puddle.
Ponding accelerates every other failure. It softens bitumen, holds silt that abrades rubber and freezes into lifting wedges in winter. Correcting a fall with tapered firrings is more work than a repair, but it is often the repair that actually holds.
Felt: blisters, splits and lifted laps
A blister is trapped moisture or air expanding under the cap sheet. Small firm blisters can be left alone. Soft, spreading ones are cut in a cross, dried out, dressed back down and torched or bonded over with a patch that extends at least 150mm past the cut in every direction.
Split laps are the more serious fault because they let water travel between layers. Where more than a couple of laps have opened, the felt has usually lost its oils and the whole covering is at the end of its service life.
EPDM: punctures, edges and adhesive failure
Rubber is repaired properly with a rubber patch, primer and seam tape, never with mastic. The area is cleaned with the manufacturer's cleaner, primed, and the patch rolled down hard with a seam roller. Done that way a puncture repair lasts as long as the roof.
Where the sheet has released from the deck and ripples in the wind, the adhesive has failed rather than the rubber. A localised rebond is possible, but widespread lifting means the roof was bonded badly and re covering is the sensible spend.
GRP cracks are ground back to sound laminate, feathered, relaminated and topcoated, giving a chemically bonded repair. Crazing that has not gone through only needs abrading and a fresh topcoat.
Mastic asphalt, still found on older Oxford properties, is repaired hot by a specialist or cold patched as a holding measure. It is worth keeping in service where it is sound, because a well laid asphalt roof can outlast everything that replaced it.
Prices and the point of no return
Localised flat roof repairs generally sit between £180 and £650 depending on access and the covering. Reworking an upstand, outlet or lantern kerb properly is more often £450 to £1,200 because the detailing takes time.
If you are spending several hundred pounds a year, the deck feels soft underfoot, or the insulation below is saturated, a re cover puts that money into something with a 25 year outlook instead. Our flat roof replacement cost guide has the figures.
Can a flat roof be repaired in winter?
Rubber patches and cold applied repairs can be done in dry cold weather. Torch on felt and fibreglass need temperature and a dry deck, so a temporary repair sometimes bridges the gap until conditions allow a permanent one.
Why does my flat roof leak only in heavy rain?
That pattern usually points to an outlet or gutter that cannot cope, or an upstand that is only overtopped when water backs up. It is a detailing fault rather than a hole in the field of the roof.
Is liquid rubber coating a real repair?
Applied over a properly prepared, sound surface it can extend the life of a tired covering. Painted over a wet, split or delaminating roof it seals moisture in and makes the eventual replacement more expensive.
How do you find the leak on a flat roof?
We look at outlets, upstands and penetrations first, then trace back from the internal stain, lifting insulation or ceiling access where needed. Water on a flat roof rarely enters directly above the damp patch.
Will you repair a flat roof another firm installed recently?
Yes, though if it is still within the installer's guarantee we will tell you to go back to them first, because that costs you nothing.
Flat roof letting water in?
We will find the entry point, tell you honestly whether it is a repair or a re cover, and price both if it is a close call.
Arrange a Flat Roof Visit
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.