Mastic asphalt

Mastic Asphalt Roof Repair & Refurbishment

London is full of asphalt roofs and most of them are older than the people who own them. Handled properly, a tired asphalt roof is one of the better candidates for renewal in place: heavy, hard-wearing, and usually still doing most of its job when the first leak appears. What it needs is an accurate reading of how much life is left, which is a question for an inspection rather than a glance from a ladder.

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What mastic asphalt is, and why London has so much of it

Mastic asphalt is a dense mixture of bitumen and fine aggregate, floated onto a roof hot and hand-worked to a smooth, jointless surface. It arrives as a solid material rather than a sheet, so a finished asphalt roof has no laps, no welds and no seams: the field, the upstands and the details are all the same continuous mass.

That suited the city rebuilt after the war and extended for decades afterwards, so asphalt ended up under mansion block parapets, on school and hospital extensions, across council estates, over shop units and on the flat sections of countless terraced additions. It was laid to last, and a great deal of it has. What ages is not the idea but the material's flexibility: decades of heat, frost and sunlight gradually stiffen it until it can no longer absorb the movement going on beneath.

How asphalt roofs actually fail

The failures are recognisable and they tell you different things about the roof, which is why a proper look matters more than a photograph.

Fine surface cracking. A web of hairline lines across an exposed area, most often where a protective finish has weathered off and ultraviolet light has been working on the surface for years. Shallow and local at first; a warning that the material has stiffened.
Blisters. Raised bubbles where vapour or trapped moisture beneath has expanded in the sun. Some are cosmetic and stable for years, some are pressurised and about to open, and only cutting one open tells you which sort you have.
Cracks at upstands and junctions. The important ones. Where the covering turns up a parapet or meets a different material, movement concentrates and the asphalt eventually splits at the change of plane. These are the cracks that let water into the building.
Slumping and thinning. On a steeper section or a sun-facing upstand, decades of warm days can move the material very slowly downhill, leaving it thin at the top of the run.
Ponding and staining. Not a failure of the asphalt itself, but standing water accelerates everything above and points at falls or outlets that need attention in their own right.

Repair, overlay or renew?

Three routes exist and asphalt genuinely supports all of them, which is unusual and worth using. The refurbishment options guide sets out how that choice is made in general; here is what it means for this material specifically.

Local repair is the traditional answer and still the right one when defects are isolated. Asphalt is repaired in kind: the area is cut back, heated and reworked until the mend is fused into the same continuous mass rather than stuck on as a patch. A sound roof with one split upstand does not need anything more ambitious.

Overlay is usually the economic answer for a roof that has aged all over rather than failed in one place. The system we specify is liquid-applied waterproofing, HYDRONYLON, prepared and worked over the asphalt already there so that the old covering stays put and keeps earning its place in the build-up. Nothing is broken out, nothing heavy goes into a skip, the deck is never left open to the weather, and the building below carries on being used. On a roof where the asphalt is firm, dry underneath and structurally intact, spending money on removing it buys you very little.

Renewal earns its place when the material has gone beyond that: widespread splitting, water already through into the structure, or an underlying build-up that has failed. Then the honest route is taking the roof back and rebuilding it, and we would rather price that than sell an overlay that is being asked to do the impossible.

What the survey checks before any of that

The evidence comes first. An inspection looks at how deep the cracking goes and whether it stops at the surface; whether blisters are dry or wet inside; what state the upstands, outlets and edge details are in; whether the asphalt is still firmly held to what it was laid on; and crucially whether anything beneath it is wet. Where the build-up includes insulation, moisture in it changes the answer completely, because a wet layer under a new surface stays wet.

Only then does a specification exist. The written condition report sets out what was found, which route the findings support and why, so the recommendation can be checked against the evidence rather than taken on trust.

When an overlay is the wrong call

Adhesion is the honest limit. A liquid system bonds to the surface it is given, so a surface that is loose, dusty, greasy, damp or breaking up will not hold one however carefully it goes on. Old asphalt finished with a reflective or mineral treatment may need that removing first, and there are surfaces no amount of preparation will bring up to a state worth building on. Roofs still moving enough to have cracked repeatedly need that movement designed for, not covered.

And where the survey finds saturated insulation below, the covering is no longer the problem. Renewing the surface at that point traps the water in the build-up and leaves the roof performing worse than it did while it was leaking. If that is what your roof turns out to be, we will tell you so before you spend anything, and whatever guarantee applies to the route you do choose is confirmed in writing alongside the price.

Frequently asked questions

How do I know whether my roof is mastic asphalt?

It looks and feels like a solid, seamless dark surface rather than sheets with visible joints, and it is often finished with fine stone chippings or a reflective finish. Underfoot it is hard and slightly springy in warm weather. If you can see overlapping edges every metre or so, it is a sheet system, not asphalt.

Can asphalt be patched, or does the whole roof have to go?

Patching is genuine and often correct. Asphalt is repaired by heating and reworking the material itself, so a sound roof with a local defect can be made good without a new covering. The question a survey answers is whether the defects are local events or the first visible signs of a covering that has hardened throughout.

Why does asphalt crack in the same places every time?

Because those places move and the field of the roof does not. Upstands at parapets, junctions with brickwork and edges around outlets are where the structure flexes and where the material is thinnest, so thermal movement concentrates there. Repeated cracking at one junction is a movement problem to be detailed for, not a patching problem to keep repeating.

Is overlaying cheaper than replacing an asphalt roof?

Where the survey supports it, considerably, and the saving is mostly in what does not happen: no breaking out, no skips full of heavy material, no exposed deck, far less disruption to whoever is underneath. That is only an advantage if the existing asphalt is genuinely sound enough to build on, which is the whole point of inspecting first.

Does old asphalt contain anything hazardous?

Mastic asphalt itself is a bitumen and aggregate material, but roofs of a certain age can include other products in the build-up, and surveys occasionally turn up materials that need specialist handling. Where anything looks doubtful we stop and have it identified properly rather than guessing on site.

Cracked asphalt, or just not sure what you have got?

A photograph of the surface and one of the upstand usually tells us most of it. Send those and we will arrange a proper look.