Despite this complexity, it strikes me that very few of the details I currently obsess over are absolutely essential to the concept of "house". Even the super-reductionist epithet "four walls and a roof" falters before the likes of Farnsworth House, Mies van der Rohe's glass box poised in a verdant glade. Perhaps it is only a "roof over our heads" that truly connects us to all other human habitation.
Roofs, however, present just as many questions as any other aspect of house design, questions that are not necessarily clarified by a specification of top-notch environmental performance. As the rafters of Tree House are cut and installed by Steve and George, it's time to make sure that this corner of the jigsaw fits together.
After providing shelter and shade, the most important function of a roof in Britain is to keep the heat in. Consequently, the underside or ours will be packed with the most thermally effective insulation on the market - Kingspan phenolic boards (www.kingspan.co.uk). Although this is a synthetic product, the energy costs of manufacture are quickly off-set by the energy saved in the building itself. Furthermore, by making the insulation an integral part of the roof, we won't be wasting space on a cold, dark loft.
But what of the roof covering? Here the eco-choices get more complicated. The manufacture of roofing materials can consume valuable natural resources, burn a lot of energy and emit toxic waste. But a roof must be robust and durable to get the most value from the materials and to reduce waste (and hassle) in the long run. A light roof is better than a heavy roof because it requires less structure to support it, and a recyclable roof is always better than one that will eventually end up being thrown away.
In fact, a large part of our roof will be covered by a material that does not perform brilliantly on any of these criteria: photovoltaic cells. In this strategic trade-off, we are accepting a little profligacy in provenance in order to achieve our principal goal of energy self-sufficiency. For the remainder of the roof, we are spurning slate, clay and cement for an economical eco-alternative: Ardesia slates, made from recycled plastics and crushed limestone. Although the most ecological choice would be to use reclaimed tiles, we are happy to use a product that has all the robust, developer-friendly attributes of new slates but is made from recycled materials. Ardesia slates are lightweight, durable and shaped to fit tightly together on the roof. They resemble Welsh slate but cost only half as much. For more information, contact EBC UK Ltd (www.e-b-c-uk.com, 01909 479276).
Sadly, we don't have the space or suitable access for a "living roof", for this is literally the greenest option, improving biodiversity and slowing the impact of storm-water. See the current edition of Building for a Future and the online archive (www.newbuilder.co.uk) for advice on this and a multitude of eco-issues. Of course, the more options you explore, the more you risk getting bogged down in the details. But this is not such a bad thing. As Van der Rohe famously pointed out, it ain't the devil that resides in the details, it's God.