Flat Roof Replacement: When a Patch Will No Longer Do
A flat roof can be patched for years, but there comes a point where each repair buys less time than the one before it. At that point a flat roof replacement is usually the better use of the money: the old covering comes off, the deck underneath is checked and put right, and a new system goes on. This guide covers the signs that a flat roof has reached that point, what a replacement involves, and what decides the cost.
Signs Your Flat Roof Needs Replacing, Not Repairing
One leak after a storm is usually a repair. The signs below, especially two or more of them together, usually mean the covering as a whole has failed:
- Leaks that keep coming back: the roof has already been patched more than once, or water is getting in at more than one place.
- Damp inside in more than one spot: stains, damp patches or mould on the ceilings and walls below the roof, which can mean water has already reached the deck or the insulation.
- Blistering, cracking or splitting across the surface: damage spread across the whole roof, rather than in one area, points to a covering at the end of its life.
- Seams and laps opening: joints lifting or splitting along their length. Seams are where flat roofs usually fail first.
- Standing water that does not drain away: ponding long after the rain has stopped often means the falls have gone or the deck has sagged, and a new covering on its own will not fix that.
- A deck that gives underfoot: if a roofer finds the boards soft or spongy, water has usually got into them, and they need replacing along with the covering.
- Edges and upstands pulling away: the covering shrinking back from edge trims, walls or rooflights, which opens gaps for water to get in.
If you are not sure which side of the line your roof is on, our flat roof repair guide sets out the faults that can still be repaired sensibly, and when a patch is still the right answer.
Flat Roof Material Lifespans
The lifespan of your flat roof depends on the system installed. GRP fibreglass typically lasts 25 to 30 years and EPDM rubber 25 to 40, while built-up felt roofs may need attention after 15 to 25 years. For a full comparison of how different materials perform, see our guide to flat roof materials.
Age on its own is not a reason to replace. A roof past its expected life that is still sound may only need watching, while a younger roof that was badly laid can need replacing early.
What a Flat Roof Replacement Involves
Replacing a flat roof means more than laying a new covering over the old one. The work usually runs in this order:
- Survey: the roof is inspected, the deck and the falls are checked where they can be, and the system and the scope of work are set out in writing before anything is booked in.
- Strip: the old covering is taken off back to the deck, along with any insulation that is wet or has failed.
- Deck check and repair: with the roof open, soft, rotten or saturated boards are replaced. This is the part nobody can price exactly until the old covering is off.
- Insulation: in England, replacing more than half of a flat roof usually counts as renovating a thermal element under Part L of the Building Regulations, so the insulation is normally brought up to the current standard where that is technically, functionally and economically feasible. Our guide to Part L compliance explains the rule.
- Falls and outlets: the slope is corrected where water has been ponding, and outlets and gutters are set so the roof drains properly.
- The new system: the chosen covering goes on, whether that is built-up felt, EPDM rubber, GRP fibreglass, single ply, a liquid membrane or a green roof.
- Edges and detailing: edge trims, upstands, flashings and the details around rooflights and pipes are finished. This is where a flat roof is most likely to fail if the work is rushed.
Our flat roofing installations carry a 10-year insurance-backed warranty on workmanship, alongside the relevant manufacturer guarantee on the membrane fitted.
Flat Roof Replacement Cost: What Drives the Price
There is no fair price for a flat roof replacement without seeing the roof, because the biggest variables only show up on site. These are the things that move the cost:
- Size and shape: the area of the roof, and how many corners, junctions, rooflights and pipes the new covering has to be detailed around.
- The system chosen: the six systems differ in their materials, in how they are laid and in how much labour the detailing takes.
- Insulation: bringing the roof up to the current standard adds material, and the extra depth can mean raising upstands, edge trims or fascias to suit.
- Deck repairs: how much of the deck is sound once the old covering is off. A dry, solid deck can stay; boards that have taken on water have to be replaced.
- Access: a garage roof reached from the drive is a different job from an extension behind a terraced house, or a roof that needs scaffolding.
- Removing the old roof: stripping and disposing of the old covering, which takes longer where earlier re-covers have left several layers on the deck.
When you compare quotes, check that each one covers the same scope: the same system, the same insulation specification, and the same level of detailing at edges and upstands. The cheapest quote is not always the best value if it leaves out the deck check or the insulation.
For a domestic flat roof we price from a survey rather than from a plan. A free survey lets us see the deck, the falls and the existing covering, and the quote that follows sets out the system and the scope in writing.
Choosing the New System
A replacement is the chance to choose a different covering from the one coming off. Felt is the covering we are asked to replace most often, but the right system for the new roof depends on its size and shape, whether anyone needs to walk on it, how much detail it has, and how it looks from the windows above.
- Built-up felt: the traditional layered system, and still one of the most cost-effective ways to cover a flat roof properly.
- EPDM rubber: laid as a single sheet, so most domestic roofs have no joints across the field of the roof at all.
- GRP fibreglass: seamless, hard-wearing and walkable, which suits balconies, terraces and roofs crossed for access.
- Single ply: the standard covering for commercial flat roofs, and it works equally well on a large domestic one.
- Liquid membrane: suits roofs with a lot of pipes, plinths and awkward details packed into a small area.
- Green roof: a sedum or deeper growing layer over the waterproofing, where the structure can carry the extra weight.
Our flat roof installation guide explains what to expect from the installation itself, from the initial survey through to final sign-off and warranty. Our flat roofing service sets out all six systems we fit, including green roofs.
Replacing Flat Roofs in Mid Sussex
Around Haywards Heath, Burgess Hill, Lindfield and the surrounding villages, the flat roofs we replace sit on extensions, loft conversions, garages, porches and summer houses, as well as on blocks of flats, offices and commercial buildings.
On a block of flats or an office, the roof usually has to be replaced while people carry on using the building underneath, so access, hot works and weather protection are planned before the old covering comes off. On a home, the first question is usually access: how scaffolding and materials reach a rear extension, and how the rooms below are kept dry while the roof is open.
We work from our base in Scaynes Hill and cover Sussex, Surrey and Kent. If your property is not in one of the towns named here, it is still worth asking.
Book a Free Flat Roof Survey
If your flat roof is showing more than one of the signs above, or has been patched more often than you would like, book a free visit. We will tell you in writing whether it needs a repair or a replacement, with the reasoning set out, and if it is a replacement we will recommend the system that suits the roof and quote it.