2 FEBRUARY 05

The clouds hung low over our Clapham building site this week. Someone has been fly-tipping next to our skip, spilling rubble across the street. Site foreman Steve has been doing his best to clear it up, but it's a miserable business. Let's hope the perpetrators choke on their own cement.

I got out of the way, taking the Caledonian Sleeper to the north of Scotland to visit a factory that produces a truly remarkable construction material. It's similar to fibreglass in its design, a composite of tightly woven fibres and resin that gives great strength while remaining light and flexible. It's also tough - absorbing serious knocks without shattering - but is stiff enough to hold up considerable loads without buckling. To cap it all, it's made from renewable resources using 100% renewable energy.

Bizarrely, many people remain doubtful about the material's ecological credentials. I am not among them - after all, if Tree House is to express the beauty and integrity of the tree that towers over it, we can only build with the wonder material that is wood. Anxieties about wood run deep. Logger's trucks and wrecked rainforests have become iconic images of ecological devastation and the most high-profile eco-warriors in Britain have been dendrites, tree dwellers, facing down the forces of the state from the canopy of ancient woodlands. Trees are venerated across the world; we rarely take pleasure in their loss.

Such anxieties are well-placed. It is scandalous, for example, that we continue to import tonnes of illegally felled timber from the rainforests of Indonesia and Brazil. Nearer to home, logging in the old growth forests of Finland has been criticized for disrupting the reindeer grazing grounds of the Saami people.

Happily, thanks to the work of the Forest Stewardship Council (FSC) and others, the ecological contradictions of buying wood can be overcome. If you buy wood that is FSC certified, it will have a documented and independently audited 'chain of custody' tracing its journey from a forest that is managed in a manner that is 'environmentally appropriate, socially beneficial and economically viable'.

My visit to the Moray Firth retraced part of the chain of custody of the wood that will provide the structural skeleton for Tree House. The manufacturer, James Jones and Sons (www.jji-joists.com), has just gained FSC certification for its specially engineered timber products. For all its qualities, wood does present the odd problem, above all its tendency to shrink, twist and move - witness all those Elizabethan houses, delightfully free from right angles. Our 'I-beams', made from compressed timber particles, will never shift; the only creaky charm in Tree House will come from ageing fat cat Trevor. I-beams also provide excellent thermal insulation in walls, use relatively little timber, and are easy to handle on site.

FSC-certified products, from sheds to salad bowls, are now widely available. The big DIY stores stock FSC wood, so it's not hard to find - check out the product selector on the FSC website (www.fsc-uk.org). If you're buying from a timber yard, ask for FSC and your invoice should have a number on it with 'COC' (chain of custody) in the middle. If you want to know exactly where the wood came from, ask to see the documentation. If you're in B&Q or Homebase, you won't get this level of detail but their FSC timber should be clearly packaged.

I got the sleeper back from Inverness on the evening of the 25th, Burns night. As we sped through the Grampians, I imagined a hundred thousand haggis being slain around me, bursting from their shiny skins. As my brother Hugh will tell you, I'm about as Scottish as he is green. One day, perhaps, I will return to my roots - and he will change his lightbulbs.