Can a new flat roof go over the old one?

Overlaying means leaving the existing covering in place and laying the new one on top. It saves the cost of stripping and disposal, which on a larger roof can be a noticeable share of the bill, and it keeps the roof protected throughout the job, which suits a flat roof over a living space that cannot be left open to weather.

It is only reasonable when the existing roof meets a short list of conditions:

• the deck is sound, dry and firmly fixed
• the old covering is still firmly stuck down, with no widespread blisters or lifting
• the insulation beneath is dry
• the falls are adequate and water drains to the outlets
• the extra weight and height are acceptable at upstands, doors and edges

Typically the old covering is a single layer of felt, because a roof already carrying several layers has little reason to take another. Extra layers add weight and make it harder to find where a fault lies later, and some manufacturers will not allow their system to go over more than one existing layer. Whichever way it goes, the new covering is chosen to be compatible with the old one, and the installer must follow the manufacturer's guidance on overlaying.

Moisture is the deciding factor. Once water has reached the insulation or the deck, a second covering locks the dampness in with no way to dry out. Warmth from inside the house and from the sun then drives that moisture around under the new membrane, causing blisters and deterioration of the timber.

A soft or spongy deck is the clearest warning. It means the boards or sheets have taken in water and begun to rot, and no covering will hold securely to them. Our guide to a blistered flat roof explains what bubbling means, and it is a frequent reason an overlay is turned down.

A survey checks for these problems before any decision is made, by checking the joists and boards from the loft or ceiling void, and by testing for moisture where the covering is open. Where there is any doubt, we would strip back to the deck rather than cover a problem. That also lets us see and put right any rot.

Signs that rule an overlay out

Spongy footfall, a roof that rings hollow when tapped, ridges where the old covering has lifted and water stains on the ceiling below all point towards stripping. So does a covering that has been patched repeatedly, because each patch is another place where moisture may have crept in unseen.

In England, Part L sets out what happens to a flat roof's insulation when its waterproof layer is renewed. Renewing that layer counts as renovation, and when more than 50% of the surface is touched, the wording calls for the entire roof to reach 0.16 W/m²K, so far as the building allows. A wholly new roof is judged against 0.15.

For an overlay, that matters because a new covering with no extra insulation beneath it may fall short of the standard, and the shortfall only shows up when building control asks. In some cases insulation can be added above the old covering as part of the new build-up, which raises the roof height. Raising the roof affects thresholds, window cills and upstands, so it should be planned from the start.

Whether your own job needs Building Regulations approval is a question for building control, and it is far easier to ask before the first roll of membrane arrives: the Gloucestershire Building Control Partnership for Stroud District, or a registered building control approver. Where the job is notifiable, we send in the notification and tell you what to expect.

An overlay is quicker and tidier, with less waste. A strip lets you see and fix the deck, adjust falls, renew insulation and start with a clean surface. Roofers differ on how readily they overlay, and you should be wary of anyone who proposes it without checking the deck. A quote that gives a price for both routes, with the reasons for recommending one, is a good sign.

BS 6229 guidance treats 1:80 as the finished minimum fall, designed from 1:40. A roof laid almost level to begin with does not gain any slope from an overlay, so ponding stays, and the new covering inherits the old problem. Our page on the minimum fall on a flat roof covers this. If you are still choosing a system, EPDM against GRP compares the two common options.

The right answer comes from a survey of your own roof, not a rule of thumb or a general table. We photograph the roof, test for moisture and report what we find. The written quote that follows gives every stage of the job a price of its own, so an overlay and a full replacement can be compared on equal terms.

For the broader decision, see flat roof repair or replacement: how to decide. More on systems and typical costs is on our flat roofing page, and either approach carries our written 10-year workmanship guarantee.