17 AUGUST 05

It's the height of summer and the canopy of Tree House is finally filling out. Great branches of Douglas Fir unfurl in the big open-plan study at the top of the house, ready to lift our long-awaited roof into place. It feels as if the flourishing tree at the front of our site and the house that ever so slightly wraps around it have begun a long and intimate embrace (though none of our current team of joiners - Steve, Mark, George and Pete - show any inclination towards tree-hugging).

Reflecting on our guiding metaphor, I am struck by how freely we draw on the natural world to illuminate our own, yet rarely return the favour with any sympathy, perhaps because projections of our manufactured world on to the natural landscape tend to expose uncomfortable truths about the relationship between the two. At best the biosphere is a garden, more often a playground, warehouse or rubbish dump.

The world would be a happier place if our first intuition was to treat the natural world as a university, a place to seek wisdom, skills and technology. This approach is most fully articulated by the advocates of 'biomimicry', who point out that many of our energy-intensive, pollution-saturated technical achievements have long been realised in the natural world at ambient temperatures with minimal resources and entirely beneficial side effects. The lotus stays remarkably clean in muddy Asian swamps because dirt particles cannot hold to the microscopic mountain ranges on the surface of its leaves. A spider excretes a material for web-building that, weight for weight, is five times stronger than steel. The abalone shell is one of the hardest materials known to man yet it is made in the sea rather than in a furnace (see www.biomimicry.org).

Trees are fine examples of biological engineering, but our efforts in Clapham to build a house that 'works like a tree' are focussed on achieving the ecological performance of a tree without necessarily using arboreal technology. For example, a tree harvests solar energy using tiny organic photosynthetic solar cells, largely made from air and water, whereas our photovoltaic solar roof is made from highly refined materials, leaving a significant trail of energy emissions and waste behind.

The nearest we will get to working literally like a tree is in our extensive use of wood. When you chop a tree down, the transport of water and sugar within wood ends immediately, but the extraordinary strength, resilience and durability that sustained the tree remain (has mankind ever built such an elaborate, towering structure on a single slender post, fit to last hundreds of years?) In fact, the best timber comes from the inside of the tree where the wood is effectively already dead. As a tree grows and its trunk thickens, the tree packs oils and resins into its core. This heartwood protects and strengthens the tree and helps to support the sapwood around it where the living processes continue.

Although most timber sold commercially is heartwood, it sometimes comes with edges of sapwood. These are vulnerable to insect attack but the heartwood will only get munched if it has already decayed, usually because of prolonged exposure to damp. So don't rush to spray your home with nasty pesticides if you see tiny holes in exposed timbers as this is likely to be expendable sapwood. Such 'flight holes' are usually historical anyway: your guests may have left decades ago, especially if you have central heating that dries out the wood and makes it unpalatable. Where wood treatment is unavoidable, use boron-based products which have low toxicity to both humans and the environment.

Foreman Steve is not entirely persuaded by our branching roof, as not all of the branches are strictly necessary. Happily, though, we are not inspired by hard-line modernism but by a tree, and in the natural world lots of things are not strictly necessary: abundance is the reward of truly sustainable design.