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Timber Frames

Half-Lap Scissor Truss

June 1, 2024
Pavilion built with half-lap scissor truss

What is a half-lap scissor truss?

A scissor truss is a timber framing system used to construct roofs where tie beams are not desired below. The main elements are paired rafters that cross and overlap at the ridge, creating an open concept below with no tie beam. Vertical posts extend up from the walls to support the scissors where they cross.

Joints

The connection is the half-lap joint where the paired scissor rafters overlap at the ridge. Instead of the rafters simply butting together, a lap joint is used where half the thickness of each rafter is cut away to create an interlocking lap.

Precisely cut lapping surfaces are created on the undersides of the rafters so that when overlapped, the top surfaces remain smooth and level across the ridge. Wooden pegs or treenails are then driven through the overlapping laps to lock the joint together.

The lapped and pegged joint prevents the scissors from being pulled apart vertically at the ridge. It also interlocks the rafters to transfer lateral forces between them when wind loads push inwards on one side.

Scissor Truss Design

if you want to see more trusses, check out 5 common timber frame trusses

When roof loads push down on the scissors, the compressive forces get channeled across the lapped joint at the ridge and down into the vertical posts supporting the scissors below. These downward loads flow directly through the interlocked scissor members without requiring a tie beam underneath.

When properly constructed with tight lapped joints and solid pegged connections, the half-lap scissor truss provides an open roof structure with clear span below. The forces follow a triangulated load path - down the rafters, through the lapped ridge joint, into the crossing scissors, and into the vertical posts. This efficient structural design allows large interior spaces to be framed with timber.

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