Advanced Framing Techniques, Floor Joists

Dr. Istockhouseplans continues his thoughts on 24" o.c. framing, floor joist style.

"Dr., why would I space my floor joists at 24" o.c.? Won't the floor bounce more?"

Floor bounce is based on several factors, not just spacing of the joists. Your span is the first factor. Obviously a 16' span will have more bounce than a 12' span. Second is the spacing of the joists. You have voiced your concern about 24" o.c. spacing versus the more conventional 16" o.c. spacing. Third factor is joist size. 2x12 joists will have less bounce over the same span than 2x10 joists will. Fourth is the subfloor material which makes a difference as well. A 1-1/8" thick subfloor feels less bouncy than 3/4" pwd. Tongue and groove pwd helps. Finally, how you attach the floor to the joists impacts the feel of the floor. Nails have less resistance than gluing and screwing the plywood down. By gluing and screwing you actually create one cohesive floor system that moves together.

So why don't we answer your question thusly: no. More accurately, we need to know what else you intend to do with the floor. Are you using 2x12 joists? You should be able to get a 14' span without a problem. Gluing and screwing the plywood may not increase your span-ability but it will make your floor feel stiffer. If this is a second floor then the gypsum board that you apply to the ceiling on the first floor will also help stiffen the joists.

The type of joist you use will have the biggest impact on how you can design the house. There is solid sawn (i.e. 2x12) engineered (I-joists) and open-web floor trusses. Of the three the open-web floor trusses give you the greatest span. They cost more, but the big advantage is that your subcontractors don't need to drill holes through every joist. The open webs facilitate easier running of wiring, piping, and even duct-work thereby saving you money in labor. You also may be able to span up to 20' with a 12" tall member. I-joists are also able to span slightly farther than solid-sawn and also come with knockouts for utilities.

Why would you want to go to 24" o.c. spacing? On the first floor over a foundation the concept is the same as studs. With further spacing of joists there is more room for insulation and less thermal bridging to an unfinished basement or crawl space. You also use less lumber. For the space between a first and second floor there may not be a huge advantage except using less lumber. However I highly recommend using the open-web floor trusses between floors so that ductwork can be run any direction. By using these (@24" o.c.) you can span up to 20' or more thereby reducing the need for bearing walls. It really is a beautiful thing.

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