Engineered timber design (LVL, I-joists)

Engineered timber is designed within BS EN 1995, Eurocode 5, using each manufacturer's characteristic values and technical guidance, since properties differ between products and suppliers. The familiar checks apply, bending, shear, bearing, buckling and deflection, with the kmod factor for load duration and service class, but the design values are higher than solid timber, so members are shallower or span further for the same load. As with all timber floors, deflection and vibration usually govern, and I-joist manufacturers publish span tables that we verify against the actual loads and support conditions rather than adopting blind.

I-joists need specific attention at the details, because their thin web is their weak point. Web stiffeners are required at supports and under point loads to stop the web crippling, service holes must be kept within the manufacturer's permitted zones and sizes, and blocking or rim boards are needed to hold the joists upright and transfer load at the ends. LVL beams, being solid, are more forgiving but need bearing, connection and lateral restraint checked like any deep beam. We set all of this out with the layout so the frame is buildable and passes Building Control.

Fire and moisture both need thought because the make-up of these products differs from solid timber. LVL chars in a broadly similar way to solid wood, but an I-joist's thin web has little material to lose, so an unprotected I-joist floor loses strength quickly in a fire and normally relies on a plasterboard ceiling for its fire resistance, which the design assumes and the build must provide. On moisture, most of these products are bonded for dry, service class one and two conditions and are not suited to permanent damp or exterior exposure without a specific product and protection.

Engineered timber suits domestic and light commercial floors, flat roofs, long-span beams and lintels and anywhere a straight, predictable, long member is wanted without the weight of steel. Its strengths are span, straightness, light weight and consistency, and it lets a floor cross a large room without a mid-span wall or beam. Its limits are cost against sawn timber, sensitivity to site damage and moisture, and the reliance on correct detailing at webs, holes and supports, so we specify the product and details clearly rather than leaving them to be worked out on site.