The following sections will give you an idea of what a truss system typically
looks like in practice.
Structural Sheathing
- Shingles? Plywood is needed to provide a nail base. 5/8" FRT CDX plywood is
typical.
- Standing seam? Metal deck provides a suitable substrate and allows higher truss
spacing. 22ga or 20ga B or F deck suits most situations.
Truss Spacing
The optimal truss spacing depends on the capabilities of the sheating that will
go on top of them. While it is possible to add additional structural elements
under plywood to increase the possible truss spacing, this tends to not be worth
the trouble in practice.
- Plywood sheathing typically requires support about every 2 feet
- Metal deck can span 4-5 feet efficiently
Ceiling
Typically, the ceiling below trusses is either dropped acoustical ceiling or
drywall on a flexible resilient channel or a floating furring channel system.
It is important to note that such a furring channel system cannot act as bottom
chord bracing because it needs to allow the drywall to float independently of
the trusses. Should the connection be rigid, deflection of the trusses durring
loading can damage the drywall, causing aesthetic and fire barrier issues.
Connections
Because the loads and details of each individual connection situation can vary
greatly, the truss to bearing and truss to truss connections should be specified
by the truss engineer. In general, structural steel and cold formed steel are
easy to connect to and concrete and bond beams are slightly less preferable but
still practical; however, connections to wood and connections through built-up
roofing should be avoided.
Bracing
Permanent lateral bracing will be required to develop the full design strength
of the trusses in all but the lightest and simplest of applications. This
bracing will almost always be present on the bottom chord, sometimes on the
webs, and occasionally on the top chord if the sheathing is unable to provide
adequate bracing for the top chord. The bracing requirements of the truss
members should be clearly indicated in the truss
engineering. Additionally, temporary bracing serves a critical role durring the
installation--you can find out more about the industry's recommendations for temporary bracing
at the
CFSEI website.
Shear Resistance
The handling of horizontal forces within the roof system is closely intertwined
with the truss system and we recommend that the truss system be used to transfer
these forces wherever practical. These force should include the horizontal
forces placed on the trusses by wind and other loads and may also include
diaphragm forces the trusses need to pass from the sheathing to shear resistance
elements below. These forces and their locations should be included in the
contract documents so the truss engineer can account for them accordingly.
Catwalks
Catwalks and other open areas of moderate width are generally fairly easy to
include within trusses. However, one should be careful when locating bearings
near a catwalk! A bearing at each edge of a catwalk is good but an asymetric
bearing near just one side of the catwalk or a bearing in the middle of the
catwalk without bearings at the edges will create forces within the trusses that
are very difficult to deal with.
Budgeting
Now that you have an idea about what your system will look like, you can get an
idea about the cost using our budget tool: