I was told, if you use floor trusses then you have to also use roof trusses. It's this true? If so, why?
I can't think of a reason the two structures would have anything in common. The structure of a house often contains Laminated Veneer Lumber beams, floor trusses, I-Joists, roof trusses as well as the usual dimensioned lumber joists and rafters. What they do have in common with all other "engineered" structural elements is that they must be designed by an engineer.
The person who told you that might have meant that if the floor trusses were spanning a very large distance and the roof was also spanning that distance it would need to be a roof truss. Such a condition would be unusual in a residence.
Agree completely with renovator. Long span trusses provide design opportunities. In 1960, wooden trusses in residences were not widely used in this area. I worked on a house which had the roof trusses covered both sides solid with plywood, stained and varnished. The interior had no permanent partitions except for the baths, just dividers similar to Lincoln logs. Not my preference, but another available choice. I like to be able to make choices, but most things are now dictated by some law.
"I was told, if you use floor trusses then you have to also use roof trusses."
More of a practical problem.
If the floor trusses provide a large clear span, the only way to roof it may end up being with roof trusses.
Watch out for long spans and 'bouncy' floors.
Deflection is normally set as a fraction of the span.
1/360 is common, with hard tile and stone floors reqauireing smaller def;ection based on the size of the stone or tile.
Natural stone and large tile may require a 1/720 maximum deflection (or smaller) to avoid cracking the flooring.
The first I-joists had 1/360 deflection out to lengths long enough to create very 'bouncy' floors that still met the code.
That is why smaller deflections quickly became available.
This post was edited by brickeyee on Thu, Feb 7, 13 at 16:18