Expert: Ted Barnhill (Principal, Design45 LLC) - 2/19/2009

Question
I am doing a drawing section  for a house
addition 21' -6" wide  by 19'-6" long.
The architect said to the person I am working
for, to use roof trusses.
He said to space equally only 4 of them in the
19'-6" length. Isn't that too much spacing?. If
not, on top of the trusses do you need
something else
than the plywood to make the roof strong
between the trusses?

Answer
Emely,

There are many options for roof decking/sheathing, from the industry standard 1/2" (nominal) plywood to tongue-and-groove 2x (or larger) material.  Depending if your 4 trusses include the end bearing walls or not, your decking is likely spanning either 4' or 5'.  For these spans, it may be that plywood will suffice.  The APA rating for 3/4" nominal plywood is often 48/24, so rated for 48" as roof sheathing.  Thicker plywood is made to span up to 60", but I've never seen or specified it.

I recommend you ask the architect what her design calls for.  Keep in mind that the roof sheathing normally is designed to provide structural support in multiple ways: resiting deflection under roof loads, acting as a shear diaphragm, and resisting bending at overhangs.

I hope this helps you complete your drawing.  -Ted