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.

Jeff@doctoronthehouse.demon.co.uk.

'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.

PROMOTED VIDEO
Have you tried new the Independent Digital Edition apps?
Life and Style
ebooksA superb mix of recipes serving up the freshest of local produce in a delicious range of styles
Life and Style
ebooksFrom the lifespan of a slug to the distance to the Sun: answers to 500 questions from readers
News
Renee Zellweger as Bridget Jones
i100
Arts and Entertainment
music

News
Russell Brand at an anti-austerity march in June
peopleActor and comedian says 'there's no point doing it if you're not'
Arts and Entertainment
Banksy's 'The Girl with the Pierced Eardrum' in Bristol
art'Girl with the Pierced Eardrum' followed hoax reports artist had been arrested and unveiled
News
i100
Voices
Oscar Pistorius is led out of court in Pretoria. Pistorius received a five-year prison sentence for culpable homicide by judge Thokozile Masipais for the killing of his girlfriend Reeva Steenkamp
voicesThokozile Masipa simply had no choice but to jail the athlete
Arts and Entertainment
Sister Cristina Scuccia sings 'Like a Virgin' in Venice
music

Like Madonna, Sister Cristina Scuccia's video is also set in Venice

Arts and Entertainment
James Blunt's debut album Back to Bedlam shot him to fame in 2004
music

Singer says the track was 'force-fed down people's throats'

Life and Style
The Tinder app has around 10 million users worldwide

techThe original free dating app will remain the same, developers say

News
news

Endangered species spotted in a creek in the Qinling mountains

News
peopleJust weeks after he created dress for Alamuddin-Clooney wedding
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

ES Rentals

    iJobs Job Widget
    iJobs General

    IT Security Advisor – Permanent – Surrey - £60k-£70k

    £60000 - £70000 Per Annum plus excellent benefits: Clearwater People Solutions...

    MI Analyst – Permanent – West Sussex – £25k-£35k

    £25000 - £35000 Per Annum plus excellent benefits: Clearwater People Solutions...

    English Teacher

    £110 - £130 per day: Randstad Education Preston: The Job ? This is a new post...

    Primary General Cover Teacher

    Negotiable: Randstad Education Southampton: We are looking for Primary School ...

    Day In a Page

    Two super-sized ships have cruised into British waters, but how big can these behemoths get?

    Super-sized ships: How big can they get?

    Two of the largest vessels in the world cruised into UK waters last week
    British doctors on brink of 'cure' for paralysis with spinal cord treatment

    British doctors on brink of cure for paralysis

    Sufferers can now be offered the possibility of cure thanks to a revolutionary implant of regenerative cells
    Let's talk about loss

    We need to talk about loss

    Secrecy and silence surround stillbirth
    Will there be an all-female mission to Mars?

    Will there be an all-female mission to Mars?

    Women may be better suited to space travel than men are
    Oscar Pistorius sentencing: The athlete's wealth and notoriety have provoked a long overdue debate on South African prisons

    'They poured water on, then electrified me...'

    If Oscar Pistorius is sent to jail, his experience will not be that of other inmates
    James Wharton: The former Guard now fighting discrimination against gay soldiers

    The former Guard now fighting discrimination against gay soldiers

    Life after the Army has brought new battles for the LGBT activist James Wharton
    Ebola in the US: Panic over the virus threatens to infect President Obama's midterms

    Panic over Ebola threatens to infect the midterms

    Just one person has died, yet November's elections may be affected by what Republicans call 'Obama's Katrina', says Rupert Cornwell
    Premier League coaches join the RSC to swap the tricks of their trades

    Darling, you were fabulous! But offside...

    Premier League coaches are joining the RSC to learn acting skills, and in turn they will teach its actors to play football. Nick Clark finds out why
    How to dress with authority: Kirsty Wark and Camila Batmanghelidjh discuss the changing role of fashion in women's workwear

    How to dress with authority

    Kirsty Wark and Camila Batmanghelidjh discuss the changing role of fashion in women's workwear
    New book on Joy Division's Ian Curtis sheds new light on the life of the late singer

    New book on Ian Curtis sheds fresh light on the life of the late singer

    'Joy Division were making art... Ian was for real' says author Jon Savage
    Sean Harris: A rare interview with British acting's secret weapon

    Sean Harris: A rare interview with British acting's secret weapon

    The Bafta-winner talks Hollywood, being branded a psycho, and how Barbra Streisand is his true inspiration
    Tim Minchin, interview: The musician, comedian and world's favourite ginger is on scorching form

    Tim Minchin interview

    For a no-holds-barred comedian who is scathing about woolly thinking and oppressive religiosity, he is surprisingly gentle in person
    Boris Johnson's boozing won't win the puritan vote

    Boris's boozing won't win the puritan vote

    Many of us Brits still disapprove of conspicuous consumption – it's the way we were raised, says DJ Taylor
    Ash frontman Tim Wheeler reveals how he came to terms with his father's dementia

    Tim Wheeler: Alzheimer's, memories and my dad

    Wheeler's dad suffered from Alzheimer's for three years. When he died, there was only one way the Ash frontman knew how to respond: with a heartfelt solo album