The availability and knowledge about the properties of different tree species have always affected the choice of timber for building. In Sweden, we have historically used many different tree species, including deciduous.
“Very few are used in modern Swedish construction and, in my opinion, this has considerably more to do with the availability of timber products than demand,” says Eric Borgström, timber design engineer at engineering consultancy Bjerking.
Increased availability of different types of construction timber
In Sweden, various types of construction timber are increasingly available in pine, a species that in recent decades has primarily been used for interiors and pressure-impregnated products.
“In terms of structural strength, stiffness and density, pine is actually slightly better than spruce under laboratory conditions. In practice, however, for construction purposes there is no difference between different softwoods in these parameters, as they are all classified in standardised structural strength classes, such as C24. So, in structural strength tables, all softwood is treated as structurally equivalent,” says Borgström.
Areas where the choice of tree species matters
However, there are areas of use in which the choice of tree species is important and where, historically, spruce has been the deliberate choice, primarily tongue and groove underlayment and external panel boards. This is because spruce absorbs less moisture than pine sapwood. One could say that spruce timber is more forgiving from a damp point of view.
Pine sapwood, the outer part of the trunk, absorbs significantly more moisture than spruce sapwood. According to Borgström, using the right material for the job also applies to species:
“In this regard, it’s important that the timber industry provides the right information at all stages. Another example is timber columns in Climate Class 3, or other applications exposed to high levels of moisture, where pressureimpregnated pine is best suited to achieving a long working life, but where spruce is often incorrectly used, which creates problems.
For any wood that is built-in, kept dry or only used temporarily, such as structural timbers, glulam beams, cross-laminated timber, formwork and interior wood, it is therefore possible to replace spruce with pine without any problem,” he says.
“Of course, visually there are differences, but the commercial types are the same and in large structures where the timber is seen from a distance, it’s unlikely that many people will notice the difference.”
Text: Carl Johan Liljegren Photo: Bjerking
