Rafters that are not supported at the top or somewhere along the span by a beam create an outward force on the wall they rest on. This force is frequently offset by ceiling joists joining the walls on the opposite side of the roof.
The chart below shows a common size of ceiling joist needed for different spans.
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Ceiling Joist Span Chart 24” O. C. 20 PSF uninhabitable Species & Grade Maximum Ceiling Joist Spans
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The previous chart shows a common size of ceiling joist needed for different spans.
The bottom chord of roof trusses acts as a ceiling joist and provides support for the ceiling sheathing, and support to keep the walls from pushing out.
Attic areas above ceiling joists must be made accessible if there is a clear height of 30" or more.
This requires framing an attic opening. The opening must not be less than 20" by 30" and have at least 30" clear space above. The attic access should be framed similar to an opening in floor joists as illustrated on “Step 5-Frame Openings in Joists" in Chapter 3.
Trusses spread ready to roll
Gable-end truss sitting on trusses.
When trusses are delivered in stacks, they should be set on the roof to allow for easy spreading. The gable ends should be on top because they go up first. The direction of the ridge is important so they can be spread and tilted up easily.
When spreading the trusses, place them on your layout marks so that when you roll them, you will have minimum moving of the trusses. They are easier to move lying down.
