Spotted gum + concealed steel: composite design

For pergolas and structural carpentry with long spans, the timber-alone solution often gets bulky fast. Spotted gum is strong (F22 stress grade routine) but if you want a 4-metre cantilever or a 6-metre clear span, the timber section size starts to dominate the design.

Concealed steel solves this. A flat steel plate (typically 8-12mm thick) is housed into a routed slot in the head beam, fixed with hidden bolts. The steel carries the bending moment; the timber wraps it and reads as a solid timber beam from outside. To anyone looking at the finished work, it's a timber pergola with surprisingly slim proportions.

The engineering: you size the steel to carry 80-100% of the bending load. The timber carries shear and provides the fixing surface for everything else (rafters, slats, fittings). The connection between steel and timber needs to allow for differential thermal expansion, we use slotted holes at one end of the steel so the timber can breathe seasonally without splitting where the bolts go through.

The visible result: a 4.8 metre pergola with a 1.6m cantilever, head beam looking like a 200×100 spotted gum section. Without the steel that beam would need to be 280×140 to carry the load. The cantilever would have looked clumsy. With the steel, you don't notice the engineering, you just notice that the proportions are right.