If your flat roof in Cambridge is showing early signs of trouble, catching them now is the difference between a repair and a full replacement.
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Flat roofs fail gradually, not overnight. The problem is that the early signs are easy to miss or dismiss until water has already found its way into the structure beneath. Knowing what to look for - and what is causing it - puts you in a much stronger position before the damage gets costly.
The most obvious sign is a water stain on the ceiling below the roof. But by the time you see that, water has already been sitting on or in the roof covering for some time. A damp patch on a bedroom or garage ceiling below a flat roof should be treated as urgent, not something to keep an eye on.
Blistering is one of the most common signs of felt roof failure. It happens when moisture trapped between layers of felt expands in heat, pushing the surface up into bubbles. A blister does not always mean an immediate leak, but it does mean the felt is delaminating and the surface has been compromised.
Once blistered felt cracks under foot traffic or further sun exposure, water can get in through those cracks directly. On garage roofs and extension roofs, people sometimes walk on the surface to clean gutters or retrieve something - that is enough to split a blister that has already formed.
Flat roofs are not truly flat - they should have a slight fall (a slope, even a very slight one) to allow rainwater to drain away. When a roof has no effective fall, or when the decking beneath has sagged over time, water pools in low spots. Standing water accelerates the breakdown of felt and causes premature failure of virtually every flat roof material.
If you look at your garage or extension roof after rain and see a puddle still sitting there hours later, that is a drainage problem. It may be fixable without replacing the whole roof, but it should not be left. Over a single winter, persistent ponding water adds significant weight and stress to the deck below.
Felt contracts and expands with temperature change. Over years, this movement causes cracking, particularly at edges, seams and where the felt meets a wall or parapet. These splits are often only a few millimetres wide, but they are enough to let water in every time it rains. The joins at upstands (the vertical sections where the roof meets a wall) are particularly vulnerable.
Lead flashing at these junctions can also crack or pull away from the wall, leaving a gap. This is a separate issue from the flat roof covering itself, but it causes identical symptoms - water tracking down inside a wall or appearing at ceiling level. A roofer should check both the roof surface and all the flashings together, not just one in isolation.
Standard mineral felt has a practical lifespan of around 10 to 15 years on a garage or extension, sometimes less if it was laid in a single layer or if the roof gets a lot of direct sun. The UV exposure degrades the bitumen binder in the felt, making it brittle. Once brittle, it cracks. Once cracked, it leaks.
Felt roofs also fail early when the original installation was poor. Common problems include insufficient overlap at seams, felt taken straight to the edge without proper drip trim, and no vapour control layer beneath - leading to condensation within the build-up rather than rain penetration from above. The result looks the same from inside (a damp ceiling) but the cause is different, and the fix needs to address the right problem.
Decking failure is another cause. The boards beneath the felt can rot, particularly on older garages where there has been repeated minor leaking over time. A roofer stripping a felt roof sometimes finds the deck is in poor condition and needs partial or full replacement before a new covering goes on. This is not something that can be assessed properly from the ground.
Not every flat roof problem means a full replacement. A localised split at a flashing or a small area of cracked felt can often be repaired if the rest of the covering is sound and the deck underneath is dry and solid. A repair is the right answer if the damage is genuinely isolated.
Replacement becomes the better choice when the felt has reached the end of its natural life across the whole surface, when there are multiple failure points, or when the deck has suffered rot or movement. Patching a roof that is failing throughout costs money without solving anything - within a year or two, another patch is needed, and then another.
The honest way to assess it is to have the roof stripped in the affected area so the deck can be seen. Any roofer who tells you what is underneath without taking the old felt off is guessing. A proper assessment requires a hands-on inspection.
When a flat roof on a garage, extension or dormer is replaced properly, the old felt comes off completely - not laid over. The deck is inspected board by board, and any rotten or soft sections are replaced. A vapour control layer goes down first, then the new covering - either torch-on felt or EPDM rubber, depending on what suits the roof. Drip trim is fitted at the edges, and all upstands and flashings are dressed properly before the job is signed off.
EPDM rubber has become a popular alternative to torch-on felt. It is a single-ply sheet membrane that is bonded to the deck, with far fewer seams than a built-up felt system. It handles temperature movement well and does not become brittle in the way mineral felt does. Many roofers now use it as their standard flat roof system for domestic garages and extensions.
We carry out flat roofing work in Cambridge and across the surrounding area from our base in Haverhill. We also work in Linton, Saffron Walden, Haverhill itself, and neighbouring villages including Steeple Bumpstead, Sturmer, Little Walden and Thaxted. If your flat roof is showing any of the warning signs described above, call us for a roof survey so we can tell you what is actually going on rather than what it might be from the ground.
These are not awkward questions - any experienced roofer will answer them without hesitation. If you get vague answers, that tells you something useful before any money changes hands.
A flat roof that is blistering, pooling water or cracking at the edges will not heal itself over winter. Frost gets into existing cracks and forces them wider. A small repair job in autumn can become a full deck replacement by spring if it is left. Call us to book a survey of your flat roof in Cambridge and we will give you a straight assessment of what needs doing and why.
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Call any time. Calls are answered day and night and passed straight to our roofers.
