Brettstapel

Brettstapel, also known as dowellam, is a massive timber construction system, fabricated exclusively from softwood timber posts connected with hardwood timber dowels. It is a relatively simple method of construction that exploits low grade timber, not normally suitable for use in construction, to form load bearing solid timber wall, floor and roof panels. Brettstapel, also known as dowellam, is a massive timber construction system, fabricated exclusively from softwood timber posts connected with hardwood timber dowels. It is a relatively simple method of construction that exploits low grade timber, not normally suitable for use in construction, to form load bearing solid timber wall, floor and roof panels. Brettstapel works by using hardwood dowels with a moisture content lower than that of softwood posts. Over time the dowels expand to achieve moisture equilibrium thus 'locking' the posts together and creating a structural load-bearing system. It is one of a few construction methods that can be entirely fabricated from timber. Although some variations do still use glue and nails, these are not necessary and avoiding these means a healthier indoor air quality can be achieved. The timber itself locks in vast amounts of carbon dioxide without emitting harmful toxins found in other materials, benefiting both the user and the environment. Brettstapel was invented in the 1970s by the German engineer Julius Natterer. It is now commonly used across Austria, Switzerland and Germany, whilst slowly emerging in the UK. Originally it consisted of low grade timber posts continuously nailed together to form panels strong enough to support mines and railways. In 1999 a German company developed a doweled system which relied on the varying moisture content to form solid structural panels however many companies introduced glue to the system to strengthen it. In 2001, a new variant of Brettstapel was developed where diagonal dowels replaced the perpendicular ones thus resolving separation issues and allowing for longer spans. Presently only one company uses the diagonal dowel system whilst all except for two (who still use nails) use perpendicular dowels, mostly without glue. There are now a range of companies producing CLT cross laminated timber (rotating layers of boards) using timber dowels based upon Brettstapel. Throughout the development of Brettstapel, low grade wood (predominantly spruce or fir) has remained the choice raw material for the posts where-as beech is more often used for the dowels. By ensuring natural defects, such as knots in the wooden posts are not adjacent to each other, poor quality timber can be utilised which makes for a highly economical way of using a fast-growing, underused resource, of which the UK and Scotland in particular, has an abundance. Brettstapel is manufactured to three quality grades depending on how visible the final product will be: Industrial, Standard and Exposed. With this in mind, the most rudimentary solution is to use unfinished timber where the panels are to be covered. Alternatively, the posts can be planed, chamfered and profiled to give a highly aesthetic finish and further still, the posts can be altered with hollow voids along the edge to enhance the acoustic performance. Brettstapel can also be combined with concrete and steel to form composite structures for more demanding projects such as large spans, bridges and trusses. Brettstapel is commonly produced as part of a prefabricated wall, ceiling or roof panel. The prefabrication takes place in dedicated factories using specialist machinery and an experienced team ensuring tight quality control and fast construction. The structural panels are generally manufactured in 600mm wide sections, secured together using timber joints and further combined with sheathing board, insulation, and a vapour barrier to form the completed panels. Structural openings are incorporated into the panels, and in some cases plumbing and electrical fittings can be too, whilst the external finish commonly varies between render and timber cladding which is applied on-site.