Asphalt flat roof overlay in London

What happens to an asphalt flat roof as it ages

Years of sun, frost and the building’s own movement wear an asphalt roof down, and the signs are usually the same: fine crazing and surface cracking spreading across the roof; blisters where moisture is trapped below the surface; the asphalt hardening until it can no longer flex; and splits opening at the upstands, skirtings and around outlets, which is where asphalt roofs almost always fail first. Water that sits on the roof after rain speeds all of it up. Once the crazing and cracking has spread across most of the roof, sealing one crack at a time no longer holds, because the surface as a whole has reached the end of its life.

The point many owners miss is that reaching this stage does not automatically mean a full re-roof. A worn surface over a sound structure is exactly the roof an overlay is built for.

Why overlay instead of stripping the asphalt off

Taking the old mastic asphalt off completely is heavy, slow work, and it leaves the building exposed while it is done. Where the asphalt has only worn on top but is still firmly bonded, and the structure beneath it is dry, you can avoid all of that. The asphalt is prepared and kept in place, and the liquid system is taken over the whole surface and into every upstand and edge detail, locking the existing roof in under one watertight layer with no laps or joints left to fail. With nothing torn off, there are no skips, no open deck, and a fraction of the upheaval of a full strip-and-replace.

The catch is that the asphalt and the deck have to be genuinely sound for this to work. If the asphalt has lifted and debonded, the structure has shifted, or damp has already tracked into the deck, an overlay would only lock the trouble in, and that has to be put right first. It is the reason the job begins with someone getting up on the roof to look: an Approved Contractor checks the asphalt, the falls and the drainage and tells you plainly whether it is fit to overlay or needs work before anything else.

What an asphalt roof overlay involves

Preparation is most of the job, and on asphalt it counts for more than on almost any other surface. The roof is cleaned back to a sound, stable base, then the asphalt itself is made good: crazed and cracked areas, blisters, and above all the upstands, skirtings and outlets, since those are where an asphalt roof lets go first. A primer chosen for asphalt goes down so the new layer grips the old surface, and only then is the liquid waterproofing applied, reinforced with mesh and carried across the whole roof so it sets into one seamless surface with no joints to fail.

The work is flame-free from start to finish. Nothing is heated and no torch goes near the roof, which is no small thing on a brittle, ageing asphalt surface. Most overlays are a few days’ work, not the weeks a full strip-and-replace can take, and because the asphalt is sealed in place rather than dug out, the building below carries on as normal. There is a bonus, too: a paler finish throws off heat that bare black asphalt soaks up in full sun, which you can read about on our cool roof page.

The system behind the work

HYDRONYLON holds European Technical Assessment ETA-23/0735, and it is the same system whichever surface it goes over. HYDRONYLON LTD supplies it to a network of Approved Contractors, who are trained and certified to fit it and work across London. When one of them overlays an asphalt roof, the materials carry a 10-year product guarantee, held by the contractor who did the work. If you want the detail, the applications page shows how the system is built up surface by surface, and the system page walks through the method.