SCISSOR TRUSS CONSTRUCTION

SCISSOR TRUSS CONSTRUCTION

The Scissor Truss is one of a number of trussed rafters which incorporate a vaulted ceiling into their design. Others include the Gambrel Truss and the Raised Collar (or Raised Tie) Truss.

A vaulted ceiling is a nice way to add character or interest to a room. It can make cramped, low buildings feel airy and light. The ceiling pitch can be used to accommodate roof windows. We find scissor trusses are used frequently in extensions and conservatories.

Scissor Trusses

Creating the vaulted ceiling means elevating or angling of the bottom chord or ceiling tie. This permits within the truss some degree of horizontal movement and hence a force will be generated at wall plate levels.

Research into normal domestic masonry wall construction has shown that up to 12 mm of total deflection can be tolerated by the walls. Designs are therefore constrained by this limitation.

The use of slide-shoes particularly with girders (or multiples) allows reasonable horizontal movement across the walls. As the truss is loaded the slide-shoe accommodates the horizontal deflection without imposing horizontal thrust.

SCISSOR TRUSS DESIGN NOTES
  • The vaulted ceiling can be specified by providing a ceiling pitch or ceiling apex height
  • The vaulted ceiling can be duo pitch or designed with an offset centre apex.
  • Remember you cannot have the ceiling pitch too close to that of the rafter as the design needs to incorporate structural webbing.
  • Truss clips can be used to fix single trusses but slide shoes will be specified for girders.