Felt garage roofs are everywhere in the UK. When they’re installed well and maintained, they can last for years. But when they start to fail, the symptoms usually look the same: blisters/bubbles, cracks, and splits — often followed by leaks that “only happen in heavy rain”… until they happen all the time.
This guide explains why felt roofs fail, what each symptom normally means, and when a repair is worth doing versus replacing the whole roof.
Quick overview: the 6 most common reasons felt fails
- Age and UV/weather exposure
- Ponding water (standing water stressing seams)
- Failed joints/seams and poor detailing
- Moisture trapped beneath the felt (causing blisters)
- Movement in the roof deck or structure
- Cheap materials or rushed installation (thin felt, poor adhesion)
What a felt garage roof is supposed to do
A felt garage roof is basically a waterproof covering made from bitumen-based layers. It’s designed to shed water off the roof, and it relies heavily on:
- tight, durable seams
- strong edge details (drip edges, corners)
- proper outlet and drainage design
- a sound, dry roof deck beneath
When any of those are compromised, felt starts to show the classic failure signs.
1) Blisters and bubbles (why they happen)
What you see
Raised bubbles or blisters in the felt, sometimes soft to the touch, sometimes hard.
What’s actually happening
A blister usually means air or moisture is trapped under the felt. Heat from the sun warms the roof and expands trapped air/moisture, creating a bubble. Over time, blisters can split, creating a leak point.
Common causes
- Moisture in the deck when felt was installed
- Condensation trapped under the covering
- Poor adhesion (felt not fully bonded)
- Installing over an old wet layer without correct preparation
- Inadequate ventilation in some garage roof build-ups
Why blisters matter
Blisters can be harmless early on, but once they crack or split they become a direct path for water.
2) Cracks (surface cracking and “crazing”)
What you see
Fine cracks across the surface, often spreading over time, especially on older roofs.
What’s actually happening
Felt becomes less flexible as it ages. UV and temperature swings slowly dry and harden the bitumen. Once the material loses flexibility, it starts cracking as the roof expands and contracts.
Common causes
- old felt reaching end-of-life
- high UV exposure, especially on south-facing roofs
- cold winters + warm summers causing repeated expansion/contraction
- thin felt or low-quality system that degrades faster
When cracks become urgent
- cracks appearing at seams, corners, or edges
- cracking combined with damp patches inside
- multiple cracks across the whole roof area
3) Splits (the “this is now leaking” symptom)
What you see
A visible split or tear, often at:
- the front edge/drip edge
- corners
- seams/joints
- around outlets
What’s actually happening
A split is a failure point. Water will track into the roof layers and eventually into the deck. Splits often start small but grow quickly once water gets into the system.
Common causes
- felt lifting at edges, then wind-driven rain gets underneath
- poor corner detailing (corners are high-stress points)
- movement in the roof deck (boards flexing or failing)
- ponding water putting constant stress on seams
- repeated “sealant fixes” that fail again
4) Ponding water (standing water) — the silent felt killer
Flat roofs should still shed water. If your garage roof holds water, felt seams and joints are under constant stress.
What you’ll notice
- puddles on the roof after rain
- leaks that continue even after rain stops
- blisters and seam failures repeating in the same areas
Why it causes failure
Standing water finds micro-gaps, weak seams and tiny cracks. Over time it forces its way in, especially if the felt has aged.
Typical solution
If ponding is consistent, the long-term fix often involves improving drainage or correcting falls during replacement, rather than endless patching.
5) Failed seams and poor detailing (edges, corners, outlets)
In practice, many felt roof leaks happen at the details, not the middle.
High-risk felt roof areas
| Area | Why it fails |
|---|---|
| Front edge/drip edge | wind-driven rain lifts felt, water tracks underneath |
| Corners | stress points; poor detailing splits |
| Seams/joints | weak bonding; age and movement open gaps |
| Outlets | water concentrates here; small failures become leaks fast |
| Wall junctions (attached garages) | water runs down wall and finds tiny gaps |
If you keep repairing the same edge or corner, it’s often a sign the overall system is at the end of its life.
6) Decking problems underneath (soft boards = repeat leaks)
If water has been getting in, the deck boards can soften. A soft deck flexes when weight or wind loads it, which then cracks the felt and opens seams.
Signs of deck issues
- sagging visible from inside the garage
- stains on roof timbers
- recurring leaks in the same “low spot” area
- musty smell or visible mould
Deck repairs can be local, but widespread deck problems often push the job towards replacement.
Is it repairable, or does it need replacing?
Here’s a simple way to decide.
Repairs often make sense when:
- there’s one split or one weak seam
- the rest of the felt is still flexible (not cracking everywhere)
- there’s no ponding water and no soft deck
Replacement is often better when:
- there are multiple blisters across the roof
- cracks are widespread (the felt is ageing)
- you’ve had repeated patch repairs
- ponding water is present
- the deck feels soft or sagging
What you can do next (practical steps)
If it’s leaking now
- manage drips inside (bucket/tray)
- avoid climbing on the roof
- take a few photos from ground level for a quote request
- note where the leak is and when it happens (heavy rain / windy rain / after rain)
What to request in quotes
Ask for a roofer to specify:
- whether the deck needs repair
- what edge trims/drip detail will be used
- how ponding/drainage will be addressed
- whether a patch repair is realistic or not
FAQs
Are blisters on a felt roof always a problem?
Not always immediately, but they’re a warning sign. Blisters can split over time and become leak points, especially if the felt is ageing.
Why does my felt roof keep cracking?
Felt becomes less flexible as it ages. UV, weathering and temperature swings can dry and harden the bitumen, leading to cracking.
Can you patch a split felt roof?
Often yes for a local split, but if the felt is cracking widely or the deck is soft, replacement is usually better value.
Do “roof leak sealants” fix felt roof splits long-term?
Sometimes briefly, but they rarely fix the underlying issue. If felt is lifting, cracked widely, or ponding water is present, sealants tend to fail again.
