If your flat roof is showing early signs of trouble, understanding what to look for and why it happens can save you a costly full replacement further down the line.
Call any time. Calls are answered day and night and passed straight to our roofers.
Flat roofs are a practical choice for extensions, garages and outbuildings. Unlike a pitched roof, a flat roof relies on a slight fall - typically 1:40 or steeper - to shed water to gutters or outlets. Get the fall, the deck and the membrane right and a flat roof performs reliably for many years. Get any one of those elements wrong and you will know about it quickly.
We install and repair flat roofs using felt and EPDM rubber membranes. Both are well-proven systems when properly detailed, and both suit the range of properties we work on in and around Cambridge.
Most flat roof failures do not appear overnight. They develop gradually, and the visible signs at ceiling level are usually the last stage of a problem that started at the membrane or the deck. Spotting the early signs means you can often repair rather than replace.
Blisters form when moisture or air becomes trapped between layers of the roofing system. With felt roofs, this is a common result of the original installation being carried out in damp conditions, or of a layer being applied over a surface that was not fully dry. The blisters themselves do not immediately let water in, but once they split - which they will under foot traffic or after a sharp frost - the membrane is breached. A blistered felt roof should be inspected to assess whether spot repairs are viable or whether the whole surface needs stripping back.
A small amount of standing water after heavy rain is not unusual. Water that still sits on the roof 48 hours later points to an inadequate fall, a blocked outlet or a dip caused by deck movement or decay. Persistent ponding accelerates the degradation of any membrane system, softening the surface and eventually forcing its way through joints and laps. On older felt roofs, ponding often sits over a section of deck that has already begun to rot.
The edges of a flat roof - the upstands at walls and parapets, the trim at fascias, the drip edge over gutters - are the points most likely to fail first. Felt becomes brittle as it ages, and seasonal movement in the structure below causes the membrane to pull away from upstands or crack along laps. EPDM is more tolerant of movement, but even a rubber membrane can lift at poorly bonded edges. When you see cracking along a seam or a gap appearing between the roof surface and the wall above, water has either already entered or is about to.
A damp patch or brown stain on the ceiling of an extension or garage is the sign most homeowners notice first. By this point, water has been working through the roof structure - through the membrane, into the deck and through insulation - for some time. The entry point on the roof surface is rarely directly above the stain inside. A proper inspection of the whole roof area is needed to find where the water is actually getting in.
A flat roof deck should feel solid underfoot. Any softness, flex or depression indicates that the deck boards or joists below have absorbed moisture and begun to break down. OSB and plywood decks deteriorate quickly once water reaches them. If the deck has failed in patches, those sections will need cutting out and replacing before any new membrane is laid. Attempting to roof over a rotten deck simply traps the moisture and causes the new membrane to fail prematurely.
Age is a factor, but age alone rarely explains a failed flat roof. The more common causes are below.
Torch-on felt - a reinforced bitumen membrane applied in layers - has been the standard for flat roofing for decades. A correctly installed three-layer torch-on felt system is durable and well-suited to standard extensions and garages. The layers bond together to form a waterproof finish, with mineral-surfaced capsheet on top to reflect heat and resist UV degradation.
EPDM rubber is a single-ply membrane bonded to the deck with adhesive. It is highly flexible, which makes it particularly good on roofs with complicated shapes, multiple penetrations or a history of deck movement. EPDM does not become brittle in cold weather in the way that felt can. For a straightforward flat garage roof, either system installed properly will perform well. For a larger extension roof with several outlets and a parapet detail, EPDM's flexibility often makes it the more practical choice.
Before any material goes on, the existing roof should be fully stripped and the deck inspected. This is the step that separates a properly done job from a quick overlay. Any soft, spongy or delaminated deck sections are cut out and replaced with matching-thickness board. The fall is checked; if outlets or drip edges are in the wrong position, this is the point to correct them.
Insulation, where included, goes onto the prepared deck before the membrane. The membrane is then installed according to the system's requirements - heat-welded laps for felt, bonded seams and edges for EPDM. Upstands at walls are taken to the correct height (at least 150mm above the finished roof surface is the standard figure you should check with any contractor). Edge trims and drip edges are fitted and sealed.
Access for a typical extension or garage roof does not usually require scaffolding, though low-level staging or a tower may be used. For roofs above a first floor or those adjacent to a public pavement, scaffolding is worth considering for safety and to protect the property below.
Flat roof replacement in and around Melbourn typically costs in the region of £70-£120 per square metre, depending on the system chosen, the condition of the existing deck and the complexity of the detailing. A small garage roof of around 15 square metres might cost £1,000-£1,800 fully replaced. A larger extension roof of 30 square metres will typically fall in the £2,000-£3,500 range. Repairs rather than full replacement are considerably less, though the right decision depends on the condition of the deck and membrane, which only a proper inspection can confirm.
Any quote you receive should specify the membrane system, the number of layers, what happens to the existing deck, and how upstands and edge details are to be finished. If a quote does not break these elements down, ask for that detail before agreeing to the work.
A repair is the right answer when the deck is sound, the membrane failure is isolated and the overall system still has serviceable life in it. Full replacement makes more sense when the deck has rotted in multiple areas, when the existing membrane is cracked throughout rather than at one point, or when the roof is more than 20 years old and has had repeated repairs. A reliable roofer should be willing to explain which option applies to your roof and why, and to show you the deck condition when the old material comes off.
If you have noticed any of the warning signs above - staining inside, blistering on the surface, soft spots underfoot or water sitting on the roof after rain - call us to arrange a flat roof survey. We cover Melbourn and the surrounding area including Royston and Cambridge. A survey gives you a clear picture of what is actually happening before you decide whether repair or replacement is the right course.
No obligation · We'll call you to arrange a free visit
Call any time. Calls are answered day and night and passed straight to our roofers.
