Brettstapel: another way of building with wood
Kennedy & Violich described the building material in the Soft House as " a traditional solid wood panel and deck construction with wood dowel joints." That didn't mean much to me, but Mike Eliason of the Brute Force Collaborative commented: "the wood is most likely brettstapel - basically 2x laid on end and fixed w/ dowels. no glue."
Regular TreeHugger readers will know that I am a big fan of wood construction; wood is renewable, sequesters carbon for the life of the building, and if the forests are replanted it is just about the most sustainable form of construction. Cross-Laminated Timber, (CLT) is another way of building with wood, where the timber is glued together to make strong, fire resistant panels that can replace concrete in medium height buildings. But a few readers have wondered about the safety and the longevity of the glues holding CLT panels together.
Brettstapel solves that problem. Invented by Professor Julius Natterer in the 1970s, it was originally made by simply nailing low-grade lumber together with long nails. This created some problems, particularly if you wanted to cut a panel without destroying your sawblade.
James Henderson of the UK site Brettstapel describes developments in the late 90s:
Dübelholz, German for “dowelled wood” refers to the inclusion of wooden dowels which replaced the nails and glue of earlier systems. This innovation involved inserting hardwood dowels into pre-drilled holes perpendicular to the posts.... This system is designed to utilise a moisture content variation between the posts and dowels. Softwood posts (usually fir or spruce) are dried to a moisture content of 12-15%. Hardwood dowels (mostly beech) are dried to a moisture content of 8%. When the two elements are combined, the differing moisture content results in the dowels expanding to achieve moisture equilibrium which locks the posts together.
So the expanding dowel locks the whole panel together, although over time, the panel might loosen up through the processes of expansion and contraction. Some manufacturers added glue to prevent the panels from opening up.
Here is where it gets really clever. James continues:
An Austrian company trying to address this issue developed a system of inserting timber dowels at an angle through the posts in ‘V’ and ‘W’ formations. This provides a very rigid jointing system which virtually eliminates the potential for movement gaps opening up between the posts ensuring, once again, a 100% timber product.
© Gaia Group
So, unlike CLT, the result is a solid, rigid panel of wood, and nothing but wood. Samuel Foster of the Gaia Group, Scottish architects of very green buildings like the award-winning Plummerswood house, tells Building.co.uk:
Gaia didn’t want to use anything that might contain toxic materials. “The reason we stay away from glues is that the Eurocodes governing structural adhesives mean these products might be a risk to health,” says Samuel Foster, associate at Gaia Architects. “We work on the precautionary principle and we haven’t seen any evidence that these products aren’t harmful to health.”
And indeed, this is one healthy wall section, right down to the wood fibre insulation. It looks so natural you could eat it.
Brettstapel is not as flexible or as strong as CLT, but for many low-rise buildings, it would do the job. The wonder of the stuff is that it creates a really healthy interior environment, has good thermal mass, creates a moisture permeable, breathing wall, has good acoustic properties and looks terrific. It also gets rid of the damn drywall.
We have some serious crises in North America; we need a lot of greener buildings, but we also have millions of acres of trees dying from the spruce pine beetle that should be harvested before they rot. Cross-laminated timber plants need a lot of expensive equipment and cost a couple of million bucks; Brettstapel looks a lot simpler. Someone should be importing this technology to North America, like now.
Read lots more at the UK site of Brettstapel.org, and watch their really good, explanatory video.