Solid timber
Solid structural timber: strength classes (C24), density, common sizes, standards and what to watch out for in design.
sawn timber, structural timber
The simplest timber product: sawn, dried timber graded by strength. Grading follows standard rules, so the load-bearing capacity of the timber can be calculated in advance. The strength class designation also gives the bending strength: C24, for example, means 24 N/mm².
How is it made?
Timber sawn from the log is dried and then graded by strength (EN 14081-1). EN 1912 assigns a wood species and grading rule to a strength class; the classes themselves are set out in EN 338.
Almost every property of timber – its weight, resistance to fire and pests, load-bearing capacity and dimensional stability – depends on its moisture content. That is why it is installed dry: it shrinks less, and the risk of fungal decay is lower.
Technical data
- Build-up
- timber sawn in one piece, dried and graded by strength
- Raw material
- spruce, fir, pine, larch and Douglas fir; among hardwoods, beech and oak
- Strength
- C14–C50 (softwood), D18–D80 (hardwood) – the number is the bending strength in N/mm²
- Density
- spruce 440–470, pine 500–540, beech 690–750 kg/m³
- Adhesive
- none
- Common sizes
- boards 16–38 mm, planks 44–75 mm thick; squared timber 6/6–20/26 cm
- Standard
- EN 14081-1 (grading), EN 338 (strength classes)
- What is it for?
- load-bearing structures, cladding, formwork
Applications
- load-bearing members of wall, floor and roof structures and frames
- cladding and formwork
- hardwood for internal bracing elements and high-quality visible surfaces – hardwood is one and a half to three times stronger than spruce
What to watch out for
- Timber is a good insulator: the thermal conductivity of softwoods is 0.13 W/mK, of brick 0.44 and of concrete 1.8 – but moisture impairs all its properties, so it must be installed and kept dry.
- The coloured heartwood of larch, pine and oak is more weather-resistant; in spruce and fir, heartwood and sapwood are not distinct.
- Thick timber sawn in one piece is difficult to dry evenly, and its length is limited. For larger cross-sections and lengths, glued products – finger-jointed structural timber, duo/trio beams, glulam – are the better choice.