Roof Trusses
Definition
Load Load not Load
Transferred Transferred Transferred
Truss Types
• Trusses are categorised into 3 groups
depending on the shape of the top chord
1. Triangular Roof Trusses
2. Crescent roof Trusses
3. Other Types
Triangular Roof Trusses
• Simple Triangular geometric shape
• Web Bracing
• Straight Top Chord
Triangular Roof Trusses
Crescent Roof Trusses
•Top Chord is manufactured with a curved top chord
•The Harbour bridge is a good example
Other Types
• Top Chords may be parallel – such as floor
joist trusses
• Or they may be nearly parallel – such as
bridges
Terminolgy
Roof Truss Members
Roof Truss Panel Points
Roof Truss Stress Types
Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
•Lighter
•Larger Spans
•Allow for easy access for
services
Disadvantages
•Cannot be site modified
Parallel Chord Trusses
Top Chord & Bottom Chord are parallel
Used as Rafters
Advantages
•Lighter
•Larger Spans
•Allow for easy access for
services
Disadvantages
•Cannot be site modified
•Generally Deeper
Parallel Chord Trusses
Truss Manufacture
• Designed by Structural Engineer
• No Site Modification or repair without
engineer supervision
• Trusses manufactured in controlled factory
conditions to ensure design is strictly followed
Truss Manufacture
• Nailing Plates
– Claw Type, only suitable
for use with a press
– Knuckle type, can be
nailed with a hammer
or pressed
Truss Manufacture
• Member Sizes must be specified by engineer
• Trusses Manufactured in factory in controlled
environment
• Members are assembled and cut in jigs and
presses
Camber
Trusses are manufactured with camber in the
bottom chord
1. To allow for calculated deflection while
dead loads such as Roof Covering &
Ceiling Linings
2. Bottom chord should not be supported
between supports, unless specifically
designed
Transfer of Loads
Click to show load flow on
correctly installed trusses
Tensile Load to
Counteract
Compressive Load
Compression
Load
Internal Wall
No load
Min 12 Clear
in this
Area
Transfer of Loads
Click to show load flow on
incorrectly installed trusses
Bottom Chord is not
designed to take
horizontal load and will
fail
Bottom Chord
Load transmits bearing on
Horizontally to wall. Internal Wall
Support to Trusses
• Top Plates Based on AS 1684 Span Tables
• As no internal support walls, spans are large
• Loads imposed on top plates are greater than
conventional roofs
Top Plates
• Option1
• Using Nominal
Thickness Top Plates
(i.e. 90 x 35)
• Place Studs directly
under trusses
Top Plates
• If Trusses are not
placed directly over
studs
• Top Plates may be
overloaded and
deflect and/or fail
Top Plates
• Option 2
• Increase Top Plate
as per 1684
Top Plates & Girder Truss
• It is always good practice to have studs as per
concentrated loads beneath a girder truss
Lintels
• Similarly Lintels should be sized according to
AS 1684
• As the spans are larger than a conventional
roofs , large members may be required
• An options may be to use C & Z metal lintels
Z Lintels
Lifting Roof Trusses
• Never lift by the Apex
• This will damage the roof trusses
Lifting of Trusses
Note That lifting is done at Panel Points
Bundle Lifting
Ensure that trusses are
tied or banded together
at base
Storage of Trusses
• Trusses should be inspected on delivery
• No site repairs with out design engineers
supervision
• Stored flat on timber dunnage
Carrying of Trusses
Installation of Trusses
Erection of Trusses
Erection of Trusses
• Note – All Trusses need to Temporarily Braced
during Installation
The purpose of temporary bracing is to
hold the trusses plumb & true until
permanent bracing is installed
Erection Tolerances
Erection Tolerances
Complex Roofs
• Trusses can be made to suit just about any
roof shape
Complex Roofs
• Trusses can be made to suit just about any
roof shape
Truss Layout
Trussed Hip Roofs
• Jack End Truss
Hip Truss
Truncated Girder Truss
• Placed at position determined by Engineer
• Will take load of Jack Truss, Hip Truss &
Creeper Truss
• Designed to take more load than other trusses
Truncated Truss
•Placed between girder truss and gathering point
•Important to be placed at specified spacing at they
will increase in height to match the hip end
•Made to similar specification to standard truss as
they take no extra load
Truss Layout – Scotch Valley
Scotch Valley
Saddle or Valley Truss
• Used to form diminishing Minor Roof
• Edges form valley
End Truss Support