Hi,
I'm in the early planning stage for a room addition. I know I want to use roof trusses, but I'm torn on whether floor trusses or I-joists are better for the floor. Any opinions? Is one cheaper than the other? Also, would it be better to wait to order the trusses after the foundation is in and I can get exact measurements (since we're matching it to parts of the existing house & measurements won't be exact until we tear off the siding). Thanks for your input!

It depends on the span required, and whether the floor will be over a basement or a crawl space. Over a crawl space, I would think twice about using composite joists or trusses, because of uncertainty about the effect of weather and dampness on them.

Trusses can span greater distances and will cost more than composite joists. Composite joists can span greater distances and will cost more than conventional sawn lumber, however, joists which are structurally sufficient can still exhibit bounciness. I myself would use conventional sawn lumber for the floor, for the stiffness, unless the span is too great and a center girder and columns below are undesirable. If composite joists must be used, they must be carefully designed to avoid the bounce.

Composite joists are usually cut to exact length on site from standard lengths. Trusses can be arranged so that there is some small length tolerance at the ends, so that you needn't wait a long time and hold up construction until they can be fabricated.

Thanks for your response, Richard. It will be over a crawl space. However, since we live in the desert (we average only 6" of rain a year), is that still an issue? The crawl space will have poly down on the ground. The span will only be about 18.5 feet. Does that change your opinion? The dimensional lumber we get here is always anything but straight, which is why I was thinking of the fabricated options.

In a desert atmosphere, with proper ventilation of the crawl space, engineered lumber is likely to be no problem. Be sure your vapor retarder has all joints lapped 6 inches and taped, and is taped to all walls and other penetrations.

A span of 18.5 feet is certainly possible, but I would consult with the composite joist manufacturers about joist stiffness. A joist of a certain size may be adequate to span that distance quite safely, yet still yield an annoyingly bouncy floor. You probably will have to use a deeper joist than structurally necessary, and perhaps with wider top and bottom chords, to achieve sufficient stiffness so that your floor will feel solid.

Definitely glue-nail the subfloor to the joists, and possibly consider a heavier subfloor to help dampen vibrations, such as a 3/4-inch subfloor and then a 1/2-inch underlayment.

Then, check the costs of this method against the cost of using a center girder of laminated veneer lumber, with perhaps one or two posts and the associated footings, and then using either sawn lumber for joists, or smaller composite joists (with the same adjustment for stiffness as the deeper clear-span joists). The increase in joist size will be smaller and less costly, and you may not need the extra floor underlayment.

Another possible adjustment for stiffness with either clear-span or half-span composite joists would be to place the joists on 12-inch centers rather than the customary 16-inch centers.

A good architect would do all of this investigation before selecting a structural system, and if you are designing your own addition, you should do no less. Engineered lumber manufacturers and your local lumber yard people will be happy to help you, as will we here, short of actually designing the system for you. Good luck.

Thanks again. Very helpful!