House Doctor

WHEN I first heard about the Trinidad asphalt lake I thought it was a joke - like the jam butty mines in Notty Ash. But while the Diddy Men beavering away to provide staple nourishment for the people of Merseyside is a product of the great Ken Dodd's fevered imagination, the extraction of asphalt to surface the world's highways from a bubbling black pond in the West Indies is very real.

Referred to locally as the eighth wonder of the world, the asphalt lake results from a geological fault which allows crude oil to seep up from below. Asphalt from the lake was allegedly used by Walter Raleigh to caulk his ships in 1595, and there are legends about its creation - something to do with God punishing the locals for killing hummingbirds. Some punishment - kill any more of those damned hummingbirds and I'll give you another oil well. That's legends for you.

Anyway, asphalt is a generic name given to any suitable bituminous material mixed with aggregates, such as sand, limestone or granite, which give it its wearing strength. Asphalt is therefore actually a type of concrete, with the sticky black stuff, rather than cement or lime, binding the aggregates together.

The bituminous component - around 50 per cent of the total - gives the asphalt its water-repellent qualities, plus the characteristic of melting when warm and hardening up again when cold, which makes it an ideal building material. Roads are surfaced with a coarse grade known as rolled asphalt which, as the name implies, is compacted by rolling. Tarmacadam, or "Tarmac", is a synthetic substitute made from tar - derived from coal or oil distillation - and coarse stone aggregates. MacAdam was the Scottish bloke who hit on the idea of making roads higher in the middle, so that rainwater drained away to the sides. Not a bad thing to leave your name to.

The finer stuff, used for roofs, steps and tanking basements, is called mastic asphalt, and is completely dense. That is, it contains no voids and therefore does not need to be rolled to compact it. Mastic asphalt is spread out and finished with a wooden float. Watching a skilled mastic asphalter at work can be a real pleasure - manual dexterity, timing and an understanding of the material combined together to produce an attractive, dense, waterproof surface that should last for 60 years.

So why do you see so many examples of mastic asphalt drooping forlornly down the front steps of houses? The answer, as so often, is that the work was done by cowboy builders. Mastic asphalt comes in different grades, and a roofing grade which will sit happily on a flat roof forever will not last two years under the forces of gravity when applied to a vertical surface, especially if heated by a southern sun.

In fact, if mastic asphalt has one inherent weakness, it is that, being black, it gets hot - and therefore soft - when the sun comes out, and cold and brittle when the sun goes in. Any sudden temperature change - such as a summer thunderstorm - can produce a condition known as thermal shock, where an asphalt roof goes brittle as it is shrinking, and suddenly cracks, "with a retort like a rifle shot".

Well, this is the story in all the textbooks, although nobody in the trade seems to have experienced it. It may be a myth - like God's revenge for the hummingbirds.

'Struck Off - The First Year of "Doctor on the House"' by Jeff Howell is available from Nosecone Publications, PO Box 24650, London E9 7XQ, price pounds 9 inc postage.

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