Jambe De Force

What is a jambe de force truss?

A Jambe De Force truss is a type of heavy timber roof truss designed for extra long spans. The main center piece is a vertical king post, angled rafters leaning on the king post, and diagonal timber beams that project out from the rafters.

Joinery

The trickiest joint is where the angled rafter meets the timber beam. This connection has to handle significant bending forces. It is made using a specialized bridled tenon joint.

At the end of each rafter, instead of a simple tenon, a wider bridled tenon shape is carved out. This bridled end fits securely into the mortise hole in the timber beam.

How does it work?

When loads push down on the roof, the forces get transferred through the rafters and into the beams via these bridled tenon joints.

The timber beams allow this type of truss to achieve longer span widths than simpler king post trusses. By extending the beams out from the rafters in an arched curve, the roof loads can be distributed over a wider area.

Diagonal and vertical wood bracing members are incorporated to create triangular sections that transfer lateral wind forces into the primary timber members following the load paths.

When constructed properly using quality timbers and reinforced joints, this truss can provide very long clear spans for wide open interior spaces using traditional heavy timber framing methods.