Mastic Asphalt Roof Repair & Refurbishment

The short answer: A cracked or blistered asphalt roof is usually repairable. Slumping and crazing are surface conditions on a covering thick and heavy enough to be sound underneath, so the common answer is a patch repair or a new surface over the existing asphalt.

Background

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. The longer story of the material, and why London has so much of it, is worth ten minutes if you have inherited one.

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 it fails

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.

The decision

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, and our own crews carry it out. We repair asphalt in kind: the area is cut back, heated until it will work again 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. Heat is hot work wherever it comes from, so the same discipline applies as with a torch: what could catch is identified before anything is lit, the heat is kept away from it, and the area is checked again before anybody leaves. Laying a brand new mastic asphalt roof is the one thing we leave alone: that belongs to a specialist spread, and if it is genuinely the right answer for your building the report will say so.

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. Asphalt is not among the substrates named in that system's assessment, so nothing is promised for it here: it is judged on the roof, by preparation and an adhesion test, and whatever any guarantee covers goes into the quotation in writing before work starts. 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.

Liquid is the first proposal rather than the only one. Where the detail count is low, the structure will carry the extra weight and the calendar allows a torch on site, a built-up bitumen overlay is bonded over prepared asphalt instead, and the reasoning for choosing it is written down rather than left to be inferred.

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.

Evidence

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.