Cambridge Roofers works with homeowners in Cambourne on flat roofs for extensions, garages and outbuildings, helping you understand whether a repair will hold or whether a full replacement is the better investment.
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When a flat roof starts to cause problems, the instinct is to patch it and hope. Sometimes that is exactly the right answer. Sometimes it costs more in the long run than replacing the roof outright. The key is understanding what condition the existing roof is actually in - not just the surface you can see, but the deck underneath and the way water is moving across it.
A flat roof that is leaking does not automatically need replacing. But a flat roof that has been leaking repeatedly, in different places, over several years, is telling you something different. Here is how to work out which situation you are in.
A repair is the right answer when the damage is isolated and the rest of the roof is structurally sound. Specific situations where repair works well include:
In these cases, a targeted repair by a competent roofer using compatible materials can add several years of service life. The condition of the deck matters here: if the boarding beneath the damaged area is dry and firm, a repair is low-risk.
There are signs that a flat roof has reached the end of its useful life and patching will not get you much further. A felt roof that is heavily blistered, cracked across the surface, or where the top layer has lost most of its stone chippings has typically aged out. EPDM that has pulled away from multiple edges or developed widespread crazing has the same problem.
The other issue is the deck. If water has been getting through for long enough, the OSB or plywood decking absorbs moisture and begins to break down. Soft or spongy boards cannot hold fixings, and a membrane laid over them will not perform - it may even trap moisture underneath and accelerate the rot. A deck that is significantly damaged needs replacing, and at that point the labour involved means a full re-roof is usually more cost-effective than a repair.
A common pattern is a roof that has had two or three patches applied over the years. Each patch holds for a while, but the underlying membrane is aging and new weak points keep appearing. At some stage - and only inspection can tell you when you have reached it - further patching becomes false economy.
You cannot always tell from the ground, or even from standing on the roof. The most reliable approach is to have the covering stripped in one area so the deck can be inspected directly. A thorough survey will look at:
Torch-on felt systems installed to a good standard can last 15 to 20 years. EPDM can last 25 years or more. If a roof is 10 years old and has had one isolated failure, repair is the obvious answer. If it is 18 years old and showing widespread deterioration, the arithmetic on replacement starts to look different.
If the roof needs replacing, the process follows a set sequence. The existing covering is stripped back to the deck. Every board is checked and any that are damaged or wet are replaced. If the roof lacks adequate fall, this is the point to correct it - either by adding tapered insulation or re-firring the joists. A new membrane is then laid, the edges are dressed with drip trim, all upstands are dressed and correctly terminated, and drainage outlets are fitted with compatible collars.
For a typical extension or garage roof in Cambourne, EPDM is usually the most practical choice. It is laid as a single sheet on smaller roofs, which means very few seams and fewer places for water to get in. Torch-on felt in three layers is also a proven option and is sometimes preferred where the roof has complicated details or where cost is a factor.
A targeted repair on a flat roof - patching an isolated split or redressing a failed upstand - might cost £150 to £400 depending on the extent of the work and how accessible the roof is. A full re-roof, including stripping, deck inspection, and a new EPDM membrane, typically runs between £70 and £120 per square metre for the finished job. A 20 square metre garage roof might cost £1,400 to £2,400; a larger extension roof, or one needing significant deck work, will cost more.
Torch-on felt replacement tends to come in at £50 to £90 per square metre for a complete job. The lower material cost is offset by the labour involved in laying multiple layers correctly.
These are honest typical ranges for the UK market. The only way to get a firm figure for your roof is a proper survey that takes in the deck condition, the access situation and the exact area involved.
Before committing to either a repair or a replacement, it is worth asking the contractor the following:
An experienced roofer should be able to give you straight answers to all of these. If the advice you get is vague, get a second opinion before spending money on a repair that may not last.
We work across Cambourne and into Cambridge and the surrounding villages. If you are unsure whether your flat roof needs repair or replacement, call us to arrange a survey and we will give you an honest assessment before any work is agreed.
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Call any time. Calls are answered day and night and passed straight to our roofers.
