Cambridge Roofers installs and repairs flat roofs on homes, extensions, garages, and outbuildings across Cambridge, using EPDM rubber and torch-on felt systems suited to each job.
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Flat roofs rarely fail without warning. The signs are there if you know what to look for, and catching them early is the difference between a minor repair and a full replacement. This page walks through the most common warning signs on Cambridge homes, what causes them, and when you need to act.
A flat roof is never truly flat. It should have a fall of at least 1 in 80 - roughly 12mm for every metre of run - to shed rainwater to the outlet or gutter. When water sits in pools for more than 48 hours after rain, the fall has failed or the deck has deflected. Over time, standing water accelerates the breakdown of any membrane surface, softens the underlying deck, and adds significant structural load.
The cause is usually one of three things: the deck has sagged between joists, the outlet or gutter is blocked, or the roof was never laid with enough fall in the first place. A roof that ponds almost certainly had inadequate falls from day one. That is a design fault, not wear and tear, and patching the membrane will not fix it.
Blisters - raised bubbles in a felt or mineral-surface membrane - form when moisture is trapped between layers during installation, or when the membrane separates from the deck below. In summer heat the trapped air or moisture expands and lifts the surface. In frost it contracts and cracks the skin of the blister.
Small blisters that are still intact are not an immediate emergency, but they are a clear signal the membrane is failing. Once a blister splits, water gets in. At that point the damage accelerates quickly, because wet insulation loses almost all its thermal value and wet timber decking rots from below without any visible sign at ceiling level.
Older felt roofs, particularly those laid more than 15 to 20 years ago, develop a fine network of surface cracks as the bitumen dries out and loses flexibility. You can see this as a map-cracking pattern, sometimes called crocodiling, on the mineral surface. At this stage the roof is at the end of its serviceable life and is likely letting water in at the splits even if you cannot see an obvious hole.
Splits most commonly appear at upstands - where the membrane turns up the wall or parapet - and around pipe penetrations, rooflights, and drainage outlets. These are the points of greatest movement and the points that are most often rushed during installation. A thorough survey will check every upstand and penetration in detail.
Internal damp directly below a flat roof can mean the membrane has failed, but it can also mean a rooflight seal has gone, a parapet flashing has lifted, or condensation is forming on cold pipework inside the roof build-up. These causes look identical from below. Probing the deck from above - checking for soft or spongy areas - helps identify where water has been sitting, but some ingress tracks sideways before it shows at ceiling level, so the wet patch inside is not always directly below the entry point.
If your ceiling is stained or sagging, or you can feel movement in the deck when you walk on it, get a proper inspection rather than simply patching the nearest visible crack. Water will have been present for longer than you think.
Lead flashings where a flat roof meets a wall are a separate but related problem. Lead expands and contracts with temperature change. Over years, the mortar holding it into the raglet joint works loose and the flashing lifts, allowing water behind it. This is often the cause of damp on internal walls rather than the roof membrane itself. Point this out to any roofer you ask to survey the job, because flashing repairs are straightforward and inexpensive if dealt with before water has sat behind the lead for a full winter.
Most premature flat roof failures come down to a handful of avoidable causes.
For new flat roofs and full replacements in Cambridge, EPDM rubber is currently the most practical choice for most domestic applications. EPDM (ethylene propylene diene monomer) is a single-ply rubber membrane bonded to the deck in large sheets with very few joints. Fewer joints means fewer potential failure points. The surface does not degrade in UV light the way older felt does. EPDM flat roofs typically last 25 years or more when correctly installed.
Torch-on felt (also called built-up roofing or modified bitumen) uses two or more layers of polyester-reinforced bitumen felt, each torched down with a gas flame to bond them together. It is a well-understood system that has been used for decades and remains appropriate for certain applications, particularly where the roof has a more complex shape or where cost is a primary consideration. A two-layer torch-on system, correctly installed with a cap sheet, can last 15 to 20 years before major remedial work is needed.
The right system depends on the size of the roof, the access available, the substrate condition, and how the building is used. We will explain the options and the trade-offs before any work starts.
Flat roofing in Cambridge typically costs between £70 and £120 per square metre for a full installation, depending on the system and specification. A single-garage roof of around 15 square metres might run to £1,200 to £1,800 installed. A typical rear-extension flat roof of 25 to 35 square metres would normally fall between £2,000 and £4,000 including stripping the old surface, making good the deck, and fitting new outlets and flashings. These are working estimates; the actual price depends on deck condition, access difficulty, and the specification chosen.
Repair costs vary widely. A localised patch around a failed outlet might cost £150 to £350. Flashing repairs are often £200 to £500 depending on length. Full re-covering is always cheaper in the long run than repeated patching once a membrane is past its serviceable life.
Before agreeing to any flat roofing work, ask the roofer to explain what fall they are building in, how the insulation is handled, and what the upstand detail at the walls will look like. Ask whether the existing deck will be inspected before the new membrane goes down. If a contractor wants to overlay a new membrane onto an existing failing surface without stripping it first, that is a short-term saving that usually costs more within five years.
If a flat roof is showing any of the signs above, arrange a roof survey so you know exactly what you are dealing with before committing to any spend.
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