Signs Your Flat Roof Needs Replacing (Not Just Patching)
By Leonard Walton, owner & lead roofer4 min read
Flat roofs on garages, extensions and dormers don't last forever, and there comes a point where patching only buys a few months. A flat roof usually needs replacing rather than repairing when the covering has reached the end of its life, when water is ponding and getting in across several points, or when the deck underneath has started to rot. An isolated split on an otherwise sound roof, on the other hand, can often be repaired. Here's how to tell which you've got.
Quick answer: repair or replace?
- Repair if the roof is a modern system in good condition with a single, localised fault — one split, one lifted edge, one blister.
- Replace if the covering is old (especially felt past 15 years), water is ponding or getting in, the surface is widely cracked or blistered, or the deck feels soft underfoot.
How long a flat roof should last
Lifespan depends almost entirely on the system that was installed — and old pour-and-roll felt is the weak link.
- Torch-on felt: around 10–20 years
- EPDM rubber: 20–30+ years
- GRP fibreglass: 25–30+ years
The relevant standard is BS 6229, the code of practice for flat roofs. If your roof is felt and over 15 years old and starting to fail, it's usually at the replace end of the scale.
The warning signs to look for
Most failing flat roofs show several of these at once. One on its own may be repairable; a handful together points to replacement.
- Ponding water that sits for more than a day or two after rain (BS 6229 expects roofs to be laid to drain).
- Blisters or bubbling in the surface, where the layers have lost their bond.
- Cracks, splits or tears, especially at the upstands and edges where movement concentrates.
- Leaks or damp patches in the room or garage below — often appearing away from the actual fault.
- A spongy or soft feel underfoot, which usually means the timber deck below is wet and rotting.
- Moss, organic growth or bare patches on old felt where the protective granules have worn away.
Why patching an old flat roof rarely works
Once a felt covering has become brittle with age, fixing one split just moves the problem. The surrounding material is at the same stage and tends to fail next to the repair within a season or two.
Ponding is the other reason patches fail. If the falls (the slight slope that drains the roof) are inadequate or the deck has sagged, water keeps sitting in the same spot — so any repair is working against standing water. Correcting the falls means re-covering, not patching.
What replacing a flat roof involves
A proper replacement is a chance to fix the causes, not just the symptoms. A full re-cover typically includes:
- Stripping the old covering and checking or replacing the timber deck
- Upgrading insulation to meet current Building Regulations (Part L) where it's a warm-roof build-up
- Correcting the falls so water drains instead of ponding
- Fitting a new membrane with properly detailed edges and upstands — see how GRP, EPDM and felt compare
Good falls, a sound deck and neat detailing matter more than the material name — a mid-range system installed well outlasts a premium one installed badly.
What does a new flat roof cost?
As a rough 2026 guide for a single garage or small extension: modern torch-on felt commonly runs around £800–£1,500, while longer-life EPDM or GRP systems typically range from £1,200 to £3,500+, depending on size, access and whether the deck or insulation needs work. A free survey gives you a fixed figure.
Frequently asked questions
Can a flat roof be repaired instead of replaced?
Yes, if the covering is modern and sound and the fault is isolated — a single split or lifted edge can be repaired. If the roof is old, ponding, or leaking in several places, replacement is the more cost-effective fix.
How do I know if the deck under my flat roof has rotted?
A soft, spongy or bouncy feel underfoot is the classic sign, often alongside interior damp. It's confirmed once the covering is lifted; a survey and, where useful, a drone inspection help assess it first.
Why does my flat roof keep pooling water?
Flat roofs are actually laid with a slight fall to drain. Persistent ponding usually means the falls are too shallow or the deck has sagged — which is corrected during a re-cover, not a patch.
Which flat roof system lasts longest?
GRP fibreglass and quality EPDM both offer 25–30+ years; torch-on felt is shorter at 10–20 years.
Not sure whether your flat roof needs repairing or replacing? We survey and install GRP, EPDM and felt flat roofs across Hull, Beverley, Hessle and the wider East Riding — free quotes, fully insured, up to a 15-year workmanship guarantee.