Overlay or strip

Can you lay a new flat roof over the old one?

A second layer saves the strip-out but hides what lies underneath. Below: what decides it, the weight and Part L questions, and when the old roof must come off.

The short answer

Yes, but only over a dry, firm deck and a sound covering, and only once the extra weight has been checked. A wet layer, a soft deck or a pond that will not clear rules it out, and only a survey shows which you have. The Planning Portal asks for approval where a new covering would make the existing roof covering 15 per cent or more heavier.

5 min read · Part of Flat roofing

Rooflights in a flat roof over an extension

What an overlay is and when it can work

An overlay means a new waterproof membrane goes on top of the existing covering instead of the old roof being stripped off first. It saves taking the old layers off and carting them away, and the roof stays closed to the weather while the work goes on. Whether that is sound depends on what lies underneath, because the new layer inherits every problem the old one has.

On the survey, an overlay is only worth pricing where all of these hold:

the deck is firm underfoot, with no springy patches or dips
the old covering is dry all the way through, checked by lifting small samples in several places
the ceiling below shows no stains that point to a wet roof build-up
the maker of the new system accepts it over the surface that is there now
the existing fall carries water to the outlet, so nothing ponds

If one of these fails, the verdict moves towards stripping back. A roof that has already had one overlay is a harder call, because two old layers hide twice as much, so we look for wear in the first before agreeing to a second.

Why wet felt and soft boards rule it out

Trapped moisture is the problem an overlay cannot answer. A new membrane sealed over wet felt or wet insulation holds that moisture in, and the deck beneath stays damp. Timber that has sat wet for months weakens, so a deck can look tidy from above and still give way underfoot. Nothing laid on top will stiffen it. From inside, a ceiling that is stained or bowing says the build-up is wet, even where the surface of the roof looks neat.

Small samples settle the question. We lift a few at the places most likely to be wet: beside the outlets, along the upstands, under any pond, and over the stained part of the ceiling. A wet layer, or dark and crumbling board under it, means the answer is to strip back, check the joists and re-board.

Stripping to the deck has one more benefit, because it puts the joists in plain view. An overlay never shows them. Rot at the end of a joist, where it sits in the wall, is the kind of fault that stays hidden under two layers until a ceiling comes down.

Weight, Building Regulations and Part L

A second layer is extra covering. The Planning Portal asks for approval where the new material makes the covering 15 per cent or more heavier, and Approved Document A calls a load increase of more than 15% a significant change. So we compare the weights from the makers' data sheets, and the Portal's guide advises having an engineer or surveyor assess the roof before the work begins.

Part L, which covers saving energy, is the second question. Under para 11.2(d) of AD L 2021, stripping a flat roof and laying a fresh waterproof layer is renovation. Once renovation passes half of the roof's surface (more than 50%), the entire roof is held to the improved figure in Table 4.3, 0.16 W/m²K against a 0.35 threshold, wherever that is feasible. Whether an overlay counts as replacing the membrane is for building control to say, so we ask first. Extra insulation laid over an overlay lifts the roof at the wall and at the door thresholds, so we measure those as well.

At a Dover district address, Dover District Council puts re-covering pitched and flat roofs "even if this is just like for like" among the jobs needing approval, so an overlay goes to the council first.

How we decide between overlay and strip

The deciding facts come from the roof, not from a liking for one method. A survey sets out what the roof is doing now, and the table shows how the usual findings point.

Survey findings on an old flat roof
Finding	Likely route
Dry, firm deck, sound covering, good fall	An overlay is worth pricing
Wet layers in places, deck still firm	Strip the covering, keep the deck
Soft or rotten boards	New deck on checked joists
A pond that will not clear	Strip back and correct the fall

The fall matters because a membrane laid over the old surface copies its dips. Our answers on flat-roof falls and ponding water explain what that means at the outlet.

Whichever route fits, the written quote names it with the reasons, as one total price. Where the answer is close, a route that looks cheaper on paper can cost more in the end, so the quote says why one was chosen and what would change it. If an overlay is chosen, the notes on the samples we lifted are kept with the quote. The wider picture of repair against replacement sits on our flat roofing page.