When a flat roof in Sawston starts to leak or show its age, the first real question is whether to repair it or replace it entirely - and the answer affects both the cost and how long you stay dry.
Flat roofs are used on extensions, garages, porches, and outbuildings. They are built in layers: a structural deck (timber joists and boarding, or concrete), insulation, and a waterproof membrane. In the UK, most flat roofs installed before 2000 used cold-applied felt - thin bitumen rolls that were not torched or bonded. These have a lifespan of around 10 to 15 years. Many are now well past that.
Newer roofs use torch-on built-up felt or EPDM rubber membrane. Torch-on systems use two or three layers of bitumen felt heat-welded together, and when installed correctly they last 20 years or more. EPDM rubber is a single-ply membrane bonded or mechanically fixed to the deck; it handles frost and UV well and can last 25 years or longer in good conditions. Knowing which system you have tells you a great deal about whether repair is worth attempting.
Repair is the right choice when the underlying structure is sound, the membrane is still largely intact, and the problem is localised. Common repairable faults include a split or blister in the membrane, a failed edge trim or drip edge, a cracked upstand where the membrane meets a wall, or a poorly sealed penetration around a pipe or outlet.
A roof that is five to ten years into a quality torch-on or EPDM installation and has a single leak caused by a specific event - a branch falling, frost lifting a flashing, or a poorly bedded outlet - is a good repair candidate. The repair cost is modest and you preserve the remaining life of the system.
Typical flat roof repair costs in the UK range from around £150 to £600 for localised work, depending on the size of the repair, the access required, and whether any deck boards need replacing beneath the failed area. A small EPDM patch repair with collar can sit at the lower end; cutting out and relaying a full corner section with new upstand and trim will cost more.
Replacement becomes the sensible option when the membrane has multiple failure points, when it is cracking or lifting in several places, or when it has reached the end of its expected lifespan. Old cold-applied felt with widespread surface cracking and multiple blisters is rarely worth repairing. Each repair patch covers a symptom, not the cause, and you often find yourself repairing the same roof every couple of years.
Replacement is also the better choice when the deck itself has deteriorated. Wet deck boards rot, and a rotten deck cannot hold fixings or support a new membrane properly. If a contractor probes the deck and finds soft or punky timber, those boards must come out. At that point the labour cost of a full replacement is not much more than a repair that leaves a compromised deck underneath.
A clear sign that replacement is overdue is visible ponding - water that sits on the roof for more than 48 hours after rain has stopped. This means the fall is either inadequate or the membrane has deformed enough to create low spots. Repeated ponding accelerates membrane degradation and can add load stress to the deck. Tapered insulation installed during a replacement can create a proper fall where none existed before.
In some cases, an overlay is possible: a new membrane is bonded over the existing one without stripping it back. This works when the existing membrane is stable, not blistering, and firmly adhered to the deck. It saves the cost and mess of stripping, and it can add an extra insulation layer at the same time.
Overlays are not always appropriate. If there is trapped moisture between the existing layers - a common problem on older felt roofs - laying a new membrane on top will seal that moisture in and cause the deck to continue rotting underneath. A roofer should test for moisture in the existing build-up before recommending an overlay. Thermal imaging or a simple moisture meter reading can identify a problem before it is covered up permanently.
When assessing a flat roof in Sawston, it helps to work through these points:
Flat roof replacement in the UK typically costs £70 to £120 per square metre for a quality torch-on or EPDM installation. A garage roof of around 15 square metres might come to £1,200 to £2,000 depending on the system chosen, the deck condition, and any edge or flashing work needed. A larger extension roof of 30 square metres might run from £2,500 to £4,000. These are realistic current ranges rather than guaranteed prices - a written quote after a proper inspection is the only reliable figure.
Always ask for the deck inspection to be part of the survey, and make sure any quote specifies what will happen if deck boards are found to be rotten. A clear allowance or day rate for additional deck work avoids unpleasant surprises on the day stripping starts.
Newmarket Roofers work across Sawston and neighbouring communities, covering repairs, full replacements, and new flat roof installations on extensions and outbuildings. If you are not sure whether your roof needs a repair or a full new system, call us and we will arrange a proper inspection and give you an honest assessment before any decisions are made.
No obligation · We usually respond within 2 hours
Storm damage, active leak, damaged ridge - we aim to be on-site within 24 hours.
