Tuesday, June 17, 2008

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.

Istockhouseplans is proud to present you with a load of plans with details that will facilitate green building. Feel free to browse our catalog and contact us with any questions.

Tuesday, June 3, 2008

Advanced Framing Techniques, Studs

Dr. Istockhouseplans answers your questions about studs.

"Dr., I'm not comfortable placing my studs at 24" o.c. Won't the wall fall down?"

No, as a matter of fact, it won't. Satisfying the seismic and wind requirements is the job of the sheathing, not the studs. The studs take the direct downward weight of the house and give you something to attach your sheathing to. The sheathing needs to be able to withstand the wind and movement. If you think about a SIPS panel, how many studs are there inside? None. How much sheathing is there? On every wall. 24" o.c. spacing also allows for more insulation inside the walls.

"But doesn't 24" o.c. studs cause wavy walls?"

Again, no. Wavy walls are caused by you not allowing 1/8"* between your sheathing panels. When you butt that OSB right up against each other, where does it go when it gets wet and expands? Out to the sides? No, the panels push against each other and buckle at every 4' increment. This causes the wavy look. Space your sheathing panels per manufacturer's recommendation. Scroll down to read item #1 on extremehowto.com. (*please read the sheathing for the proper spacing).

"What about my drywall, won't it break if someone pushes on it?"

If you are installing drywall horizontally against the studs you shouldn't have any problem at all. If it concerns you then you can bump up to 5/8" sheetrock instead of 1/2" but keep in mind that the code allows 1/2" gyp on a 24" o.c. wall.

"But all my savings from the studs will be eaten up by having to buy 5/8" sheetrock. What's the point?

The point is that even if the wall costs you the same (which if it is then your drywaller is upcharging you quite a bit - shop around), you have built a better wall which will differentiate yourself from all the other builders out there. Your wall will be more insulated and save your homebuyer in energy bills. Saving energy is good for the environment. Saving the environment makes you look like a hero.

"I don't think the code will let me build a wall like that."

Please stop the excuses. We wouldn't be encouraging you to do something illegal. If you have an IRC code book handy, open up to 602.3.1 (page 123) and reference table 602.3(5) (page 127). You'll note that a 2x6 wall with studs spaced 24" o.c. is allowed to support a second story, ceiling, and roof AND be 10' tall. How about that? Granted, a 3 story home will need 16" o.c. spacing on the first floor, but you can still use 24" o.c. spacing for floors two and three. Also note how interior non-bearing walls may be built with 2x3 studs at 24" o.c. Understandably this is not a good idea for walls with doors in them unless you do fat trim work or build your own frames.

"I still don't think it's a good idea."

Well that's your issue. We can't make you do anything, only educate you as much as possible. If you don't want to then be prepared to be left in the dust by builders whose homes sell quicker and cost them less to build. But don't say we didn't warn you. If you have any further questions or concerns please feel free to contact us. Stay tuned next time for floor issues.