Can you put a new roof over an old flat roof?

Overlaying means laying a new waterproof layer on top of the old one instead of removing it. It can be reasonable when the old roof is basically sound and is merely tired on the surface. The old layer acts as a base: it must be firmly stuck down, free of blisters and splits, and reasonably smooth.

It is less disruptive too. There is less noise, less waste and less time with the roof exposed. Some systems are designed for it. A liquid coating or a glued single-ply sheet can be laid over a prepared, sound surface, and our page on liquid flat roofs describes one of them.

Start with the deck. A soft or spongy board will not hold a new covering, and covering it hides the problem for a season at most. Next is moisture. Water trapped between layers or in the insulation will not dry out beneath a new waterproof skin. A moisture check, with a meter or a few small cut-outs to inspect the layers, shows whether the roof is dry.

Then the surface. Blisters, splits and wrinkles must be cut out and made good, and loose material removed. Finally the details: upstands, trims and outlets must be sound and tall enough to receive a thicker build-up. Our page on flat roof edge trim shows why the edges decide so much.

Height is the quiet snag. Adding a layer raises the surface by a few millimetres or more, and a threshold, a window sill or an outlet that was already low can end up with the new surface above it. Door thresholds on a balcony roof are the classic case, because water then runs towards the door. Measure these before agreeing to an overlay, and ask the roofer to confirm the finished heights in writing, so that nothing ends up lower than the water it is meant to keep out.

Each additional layer adds weight to a deck that was not designed for it. The Planning Portal says Building Regulations approval applies if the new covering makes the roof covering 15 per cent or more heavier. Approved Document A, which deals with structure, puts the threshold at more than 15 per cent. They are worded differently, so check the figure with your building control body when the margin is close.

The Part L energy rules matter as well. Approved Document L lists a change of flat roof waterproofing among the works that renovate a thermal element, and the roof may then need to reach the improved U-value, a measure of heat loss. Raise it with building control before any covering work, because the answer varies with how much of the roof is touched.

Stripping takes the old layers away and shows what is underneath. It is the only way to see the deck, correct the falls, replace wet insulation and cure rot. A roof that has already been patched or overlaid once is a likely case for it, because each hidden layer adds weight and conceals defects. Our page on felt roof layers discusses how many layers a roof can reasonably carry.

There is also the fall. An overlay follows the shape of the old roof, dips included. If the old roof ponds, so will the new one. A strip allows tapered insulation to correct the slope, as described in our page on the minimum fall for a flat roof.

Old asbestos-containing material is a further reason to look before covering. Buildings built or refurbished before 2000 may contain asbestos, as HSE advises, and a flat roof on an old garage or outbuilding is no exception. A suspect sheet should be identified before anyone drills, cuts or buries it under a new layer.

Finally, think about the next owner. A roof with three buried layers is difficult to assess and costly to strip, whereas a clean deck and a single covering are easy to inspect and easy to document for a buyer's surveyor.

An overlay usually costs less on the day, since there is less labour and less to dispose of. MyBuilder's flat roof guide, updated in August 2026, gives replacement prices per square metre of £70 to £100 for bitumen felt, £90 to £130 for EPDM and £95 to £150 for GRP. Treat these as national averages for full replacement, not prices for your roof, and an overlay would sit lower.

The saving can be false if the overlay hides a fault. The new covering is limited by the roof beneath it. See how long a flat roof lasts for typical ranges. For a view across materials, our guide to flat roof materials compares the options.

We look at the roof, take the moisture readings, and photograph the layers we can see. Following the survey, the photographs and a written price reach you together, which shows why we lean towards an overlay or a strip. An overlay is our answer only when the roof beneath is fit to carry it.

Where notification is required, we explain the consent and file the Building Regulations notice for you. Workmanship carries a 10-year guarantee, in writing, and we take our waste with us. To see where leaks usually start, read do modern flat roofs leak, or visit the main flat roofing page.