What is Brettstapel?

Brettstapel is the term commonly used for solid timber construction that does not generally use glues or nails. Fabricated from softwood timber posts connected with hardwood timber dowels, this relatively simple method of construction has the potential to utilise low grade timber that would otherwise be unsuitable for use in construction, to form load-bearing solid timber wall, floor and roof panels.

The system works by using dowels with a moisture content lower than that of the posts; over time the dowels expand to achieve moisture equilibrium thus ‘locking’ the posts together and creating a structural load-bearing system.

With Brettstapel normally manufactured entirely out of untreated timber, it is important to stress that glue is not necessary. The exclusion of glue and nails (which are seen in other solid timber systems) means a healthier indoor air quality can be achieved, while the timber itself locks in vast amounts of carbon dioxide.

The System

Conception Brettstapel was originally conceived by the German engineer Julius Natterer in the 1970s. It is a variation of massive timber construction and is widely used throughout central Europe today.

Natterer (born 1938), graduated with a civil engineering degree from Munich in the timber engineering field allowing him to acquire a comprehensive knowledge of timber construction methods. He founded his first company, Natterer GmbH in 1970, now running Bois Consult and since then has held several senior positions in his field, including the Chair of Timber Construction and Director of the Institute for Wood Structures at Lausanne University. Natterer is passionate about innovation in timber construction and it is his belief that “only the use of wood in the construction field can save and renew the forests of the world…”. More information can be found in Issue no.7 of Fourth Door Review.

1970s The earliest form of Brettstapel consisted of posts of sawn timber laid side by side, continuously nailed together to create solid structural elements around 600mm wide. The thickness of the posts varied from 80-200mm allowing the long nails to penetrate 3-4 planks each. The nature of this product allowed for low grade timber to be used meaning that it was an affordable way of constructing solid, environmentally friendly structures.

Low grade timber could be used because human selection ensured knots and defects found in posts were never placed next to one another. One manufacturer claims that this system was once used to support mines and small railway passes in Germany. This original form of Brettstapel evolved to include the use of glues as a way of strengthening it thus enabling it to span longer distances. There were a number of disadvantages to this system however, particularly as problems arose when trying to modify elements - the randomly placed nails meant it was almost impossible to cut into the timber.

1999 Brettstapel was used in this original format until 1999 when a German company developed the system known as Dübelholz. 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. Brettstapel could now be made entirely from solid timber with the potential to span up to 8 metres using panels 600mm wide and 80-300mm thick.

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.

Completely dry wood expands and contracts with temperature and moisture levels. With this arrangement of posts and perpendicular dowels it is important to note that exposure to excessive temperature or moisture variations (e.g. on site) can, over time, result in contraction and expansion and, potentially, separation along the axis of the dowels. This compromises the strength of Brettstapel, and the issue is sometimes addressed by reintroducing glue or nails between the posts. Although this resolves the issue of separation, it means that the product is no longer 100% timber.

2001 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. Other manufacturers have different means of dealing with the issue, including driving dowels through rotating layers of boards, though this is perhaps the most innovative.

2010+ Today, almost 20 companies are known to be manufacturing Brettstapel in Austria, Germany, Switzerland and, more recently, Norway. While most companies only build within fairly close proximity to their factories, some export as far as America. The most common form of Brettstapel today is the perpendicular dowel method with the majority of systems not utilising any glue. Two companies are known to still use nails. Brettstapel is now finished to a wide variety of specifications that take into account individual aesthetic, structural, financial and acoustic considerations and companies are continually striving to innovate and make their product a market leader.

History and Development

Above Brettstapel made with rotating layers of boards fixed with perpendicular dowels