When you need to span 6 meters without intermediate support, the choice of section is not limited to applying an empirical rule like “5 cm per meter of span.” At this distance, it is the long-term deformation that dictates the section, not the bending strength. Many construction sites end up with a floor that sags after a few years because the sizing did not account for the creep of the wood.
Creep and deflection criteria: what really sizes a wooden beam at 6 meters
On a short span (3 or 4 meters), bending strength is often sufficient to determine the section. At 6 meters, the situation changes. The parameter that requires increasing the section is the allowable deflection at the serviceability limit state, not failure.
Specifically, Eurocode 5 and DTU 31.2 set deflection limits: L/300 for a habitable floor, L/200 for a roof. Over 6 meters, L/300 means that the beam must not deflect more than 20 mm under combined permanent and variable loads. And wood continues to deform over time under constant load (creep), especially in service class 2 (unheated indoor environment, garage, convertible attic).
This is why a calculation based solely on bending stress leads to an insufficient section. In glued laminated timber GL24h, field returns show that we reach sections of around 140 x 360 mm to 140 x 400 mm to meet these deferred deflection criteria. Therefore, choosing the right wood beam section for a 6m span requires checking deflection before anything else.
Solid wood, glued laminated timber, or I-beams: which material for a 6-meter span
The choice of material directly influences the necessary section. One does not size a Douglas C24 beam the same way as a GL24h glued laminated beam.
Solid wood: physical limits over 6 meters
In solid wood (fir, spruce, Douglas), a span of 6 meters remains feasible but pushes the sections towards cumbersome dimensions. A beam measuring 75 x 225 mm, common in joisting, is not sufficient as a single load-bearing beam over this distance. You quickly reach sections of 100 x 300 mm or more, with a weight per piece that complicates handling.
Solid wood is better suited for closely spaced joists than for isolated beams over 6 meters. With joists spaced 40 to 60 cm apart, sections of 75 x 225 mm can work for a lightweight floor, but each case requires a specific load calculation.
Glued laminated timber GL24h: the dominant choice in renovation and new construction
Glued laminated timber offers superior mechanical resistance compared to solid wood of the same section, thanks to the distribution of natural defects over several laminations. For a load-bearing floor beam over 6 meters, a section of 140 x 360 mm in GL24h covers the majority of common residential cases.
The main advantage on-site: you can order beams to exact dimensions, without having to search for large-section solid wood that is not always available at local sawmills.
I-beams: lightweight and useful height
I-beams (such as Steico Joist or equivalent) combine solid wood or LVL flanges with a fiberboard web. They easily reach heights of 240 mm and beyond, for a significantly lower weight than solid wood or glued laminated timber.
- Suitable for floor joists with regular spacing (often 40 to 60 cm)
- Easy passage of technical conduits through the web (a real time saver on-site)
- Less suitable as a single load-bearing beam, as their concentrated support strength is lower
Joist spacing and load distribution on a 6-meter floor
The question is often posed from the angle of “which beam for 6 meters,” while the answer depends as much on the spacing and type of floor as on the beam itself. Reducing the joist spacing from 60 to 40 cm decreases the load carried by each joist and can allow for a more modest section.
For a habitable floor with an OSB covering of 18 mm or more, common configurations are:
- Joists of 75 x 225 mm with a spacing of 40 cm in spruce or Douglas C24, for a lightweight storage floor
- Joists of 100 x 250 mm or I-beams of 240 mm with a spacing of 50 to 60 cm for a residential floor
- Central load-bearing beam in glued laminated timber GL24h (minimum 140 x 360 mm) if dividing the span into two 3-meter sections, which significantly reduces joist sections
The solution of a central post with a supporting beam remains the most economical when the layout allows. This then shifts from joists sized for 6 meters to joists sized for 3 meters, with sections two to three times lighter.
Checks not to overlook before finalizing the section
Sizing does not stop at the choice of a section in a table. Several site parameters change the final result.
The service class modifies the creep coefficients. An unheated garage (class 2) imposes larger margins than a permanently heated living room (class 1). Returns vary on this point according to engineering offices, but the trend is to size in class 2 as a precaution whenever there is doubt about the moisture content.
Support conditions also matter. A beam resting on a masonry wall with a 10 cm support does not behave like a beam bolted to a wooden post with a metal connection. Insufficient support causes local crushing of the fibers, even if the section is correct in span.
Finally, the type of load changes everything. A storage floor in an attic (low operating loads) and a residential floor with partitions (high permanent loads) do not require the same section. A structural engineering office or a calculation software compliant with Eurocode 5 remains the most reliable way to validate the choice, especially when exceeding 5 meters of clear span.