Structural timber - strength certification is mandatory in the production of structural timber.
Structural timber - how it is made
Structural timber is lumber of uniform strength, thus meeting the strength requirements specified by the building designer.
Strength grading involves assessing each piece of lumber. The wood’s characteristics that influence its strength are monitored (listed below). If any of these characteristics in a given beam exceed the limits specified in the technical standard, the beam is REJECTED. Raw material statistics prove merciless – the sieve in the form of the PN-D-94021:2013-10 standard screens out up to 60% of lumber (we use this wood in the production of garden architecture).
Structural timber (graded) is equivalent to KVH timber with the added advantage of being 100% solid timber.
The design will include a strength class. It’s designated with a symbol, e.g., C24 or C27. The designer assumes the truss will be calculated based on a specific class, such as C24. This means that the beams used in the calculations will be guaranteed to withstand a bending pressure of 24 MPa.
Only when such material is available does one decide what type of construction should be adopted for a given building, what cross-sections of beams, what spacing between rafters, etc.
What does this value mean: 24 MPa? It indicates destruction, or breakage, of a beam subjected to such stresses. So, will a truss made of unclassified wood collapse? Probably not. In Poland, almost no structural timber was used in house construction, only random beams. And the houses are still standing.
What is all this for?
If a beam is stressed by forces of 24 MPa, it has the potential to crack. But if it’s stressed by 10 MPa, what does the beam do? It BENDS! But when it’s stressed by 20 MPa? It bends MORE!
And that’s the key.
Everyone’s probably seen a crack in the wall near the ceiling. They’ve probably also heard about a leaking roof. Or perhaps crumbling tiles or rusting metal roofing? Or a torn membrane? Or cracking plaster?
Who, when recounting these events, linked them to a malfunctioning truss?
Exactly, no one.
Structural timber - the dangers of making the wrong choice
The truss is the framework. Not only for the roof, as it has a huge impact on the walls. If the designer assumed that their structure would perform properly provided that beams withstand 24 MPa were used, then if we use beams that withstand, say, 13 MPa, the truss won’t collapse. But when snow falls or a storm hits, the rafters won’t bend by 2 cm, but by 5. What happens next is beyond the imagination. Example consequences:
- WATER STRIPS AND MOLD: Excessive rafter deflection causes force to be transferred to the wall plate, causing wall cracks and plaster chipping. Warm, humid air from the house penetrates through the cracks. The further outside the wall, the colder it gets, so vapor condenses in the wall, creating a damp patch. Fungi develop in damp walls. Black spots appear, and the moisture increases. The fungus migrates upwards by capillary action, infecting the wood.
- ROOFING DAMAGE: Rafter deflection causes roof deformation. Roof tiles, or even more so, metal roof tiles, have a tolerance, but very little. Therefore, they begin to press against each other. The edge of one sheet strips the anti-corrosion coating from another. The pressure from the tiles causes one of them to crumble. LOSS OF THERMAL
- INSULATION: The membrane is stretched over the rafters. It’s winter, and the material, under tension, stretches due to the deformation of the roof. Cracks form through which water blown under the roof covering or snow melting beneath it will leak onto the mineral wool. Once wet, the mineral wool shrinks, an irreversible process. Thermal insulation is lost. As a side note, it’s better to replace mineral or glass wool with wood wool – it’s moisture-resistant and more thermally efficient.
Structural timber - important aspects
Therefore, the production of structural timber is not just ordinary sawmilling.
The sawmill produces general-purpose lumber of varying strengths.
Statistically, 50% of the lumber produced from Polish raw materials does not meet the requirements of any strength class, requiring appropriate sorting.
According to building regulations, every building material must have a Declaration of Conformity issued by the manufacturer. In the case of wood, this confirms compliance with a technical standard describing the characteristics affecting strength.
The construction manager MUST NOT approve material without a Declaration of Conformity.
How is it sorted? Technical standards describe the characteristics of wood that affect its strength. These include:
- general knotting
- knotting in the so-called beam margins (i.e., bending and tension zones)
- fiber twist
- graininess
- slope
- cracks
- curvature in various directions
- etc.
To be able to perform sorting, you must complete a relevant course and exam under the auspices of the Polish Committee for Standardization. In Poland, such a course is organized and conducted by the authors of the harmonization of Polish technical standards with EU versions (SGGW). As a side note, believe me, this exam is not easy.
The technical standard specifies, among other things, what defects a beam may have in order to meet the requirements, e.g. C24 or C18. Therefore, EACH BEAM is compared individually with the range specified in the standard. The knot area coefficients of the cross-sections and margins must also be calculated, previously reproducing the cross-section non-destructively (of course 😉 ).
Out-of-class elements are designated for other production. Therefore, such production organization is impossible in small sawmills – it would require producing at least twice the number of ordered beams to sort them into the appropriate quality material.
A significant fact for the Polish industry: out of 8,000 sawmills, we have approximately 330 qualified sorters. Most of them work for companies involved in the international lumber trade. Therefore, among building materials suppliers, we are one of only a few producers in the country providing genuine structural and construction lumber.
