TRUSS INSTALLATION

LIFTING

Trusses must always be lifted in a manner that minimises lateral bending stresses. It is preferable that they be kept strapped in bundles until they are erected, as this reduces the chances of damage.

 

Trusses may be lifted either by crane or manually. The choice will depend on the truss weight and wall height. They should be maintained in a vertical plane as much as possible when being handled individually.

 

Trusses lifted by crane require slings or spreader bars as shown in the diagrams. Where trusses are place in bundles directly onto the top plate, it is imperative that they are supported by internal walls and that the supporting structure is stable in its own right.

 

  • Crane 

 

The following recommendations are guidelines only, as the details are the responsibility of the roof truss installers. Refer to AS4440-2004 for more detail.

 

  • SET OUT

Prior to lifting any truss into place, it is often convenient to mark out the truss locations on the top plate using the supplied truss layout for reference.

 

Girder and truncated girder trusses should be set out first as they have specific fixed locations. Dual purpose trusses such as truncated girders / girders need special attention as they may appear similar to other trusses which must not be used accidentally instead.

 

Standard trusses must then be set out, taking care not to exceed the design spacing. Generally they would be evenly set out over runs of similar trusses. But it is also acceptable to space them at the design spacing and have a closing gap smaller than this – adjacent to a more heavily loaded truss, if possible.

 

VERTICAL LIFTING OF TRUSSES – SPAN LESS THAN 9m

 

Crane

Vertical chain or sling

Chain for brace on lateral movement of truss

VERTICAL LIFTING OF TRUSSES – SPAN FROM 9m TO 16m

Crane

 

HORIZONTAL LIFTING OF TRUSSES

 

When erected manually, they may be slid flat over the side walls on skids spread at 3m intervals, then rotated vertically into position – supported at the apex and panel points to ensure that they do not distort or sag between supports during this process.

 

When positioning multiple span or cantilever trusses, take care that they are the correct way around. Such trusses will have markings on the bottom chord showing the point of internal support.

 

Also refer to Section 8.5 of the Safe Work Australia publication, “Preventing Falls in Housing Construction – Code of Practice – July 2012for safety aspects when lifting trusses.

 

  • FIRST TRUSS
  • Gable Roofs

On gable roofs, start with the gable truss which is located over or just inside the end wall. Then brace it back to the ground, or to some other stable part of the structure.

TEMPORARY BRACING FOR GABLE END ROOF METHOD 1 – POST WALL FRAME

TEMPORARY BRACING FOR GABLE END ROOF METHOD 2 – PROP TO GROUND

Some gable end trusses are designed to sit on the end wall. In these cases, it must be supported at every bottom chord panel point along its length, as it cannot act as a clear span truss.

  • Hip and Dutch Gable Roofs

On hip and Dutch gable roofs, start with the truncated girder, apex girder or Dutch hip girder truss and brace it back to the corner of the building as shown. It is important that this truss be correctly plumbed and aligned, as other trusses must fit exactly up against it.

 

Where it is not feasible to install bracing of the first truss in the manner described, the first two or three trusses can be erected and cross-braced between them to form a stable unit.

 

TEMPORARY BRACING FOR HIP OR DUTCH-HIP END ROOF

  • SUBSEQUENT TRUSSES

As each truss is installed, fix it to the top plate at the required location, usually indicated by set-out marks or by using a set-out rule. Use a gauging rod and ties for spacing the trusses, and a string line along the apex to ensure correct alignment.

It is important that trusses are lined up along the apex, not the heels.

Any multi-ply truss must be fixed together prior to being installed. If the truss manufacturer has not done this at the factory, it is his responsibility to supply the fixing information, and the erecting crew’s responsibility to ensure that this fixing is properly carried out.

  • ERECTION BRACING AND TOLERANCES

As the trusses are erected, they must be braced longitudinally. This is to provide stability to the trusses during the erection process, and the bottom chord ties should be maintained in place after full installation is completed.

However, the temporary top chord ties may be removed once the roof battens are adequately fixed in place.

Refer to AS 4440-2004 for the full details for temporary bracing, however the following is a brief summary.

Temp braces on TC at each panel point (max. 3000 mm apart)

Plumb – trusses shall be installed so that no part of the truss is out of plumb by more than the smaller or height/50 or 50 mm.

 

TYPICAL TEMPORARY BRACING

  • Top Chords

Temp braces on BC at every mid-panel (max. 3000 mm spacing)

These tolerances will produce a good roof line, and the performance of the trusses will deteriorate rapidly if these are exceeded, producing excess deflections and overstress in the truss.