Floor Joists: Solid Lumber, I-Joists and Open Web Floor Trusses
When constructing a house, builders need to decide what type of floor joists to install. Joists are horizontal supports that span from wall to wall, wall to beam or beam to beam. There are several different options including solid lumber, laminated veneer lumber (LVL), wood I-beams, open web floor trusses or steel floor joists. I didn’t even realize engineered alternatives existed until Jocie and I purchased our town home. We have open web floors joists. Here is a comparison of several floor joist options.
Solid Lumber Joists
Traditional solid lumber joists are still very common. They have been the default for some time but engineered options are being used more and more. Initial costs are cheaper than engineered solutions but incorporate certain quality issues such as limited span distances and environmental strain (which have additional associated costs). One major advantage is that solid sawn joists will last longer during a fire. This is important when you or a fireman is depending on them to hold weight.
From an environmental perspective, solid lumber joists are expensive. Solid lumber joists are contiguous boards created from old growth trees. A supply which is quickly being exhausted. Using younger trees often results in warped or bowed wood. Alternatives like I-beams use less wood and are composed of several different pieces.
I-Beam Joists
Wood I-joists resemble steel I-beams. They are composed of a top and bottom flange with a piece of plywood or OSB fixed perpendicular in between. It’s important to follow manufacturers guidelines when notching or cutting a hole for things like utilities. Cut-outs should have rounded corners to reduce stress and should only be done in the plywood / OSB. Many I-joists are pre-scored or have knockouts to make things easier.
I-joists can also span larger distances. A joist 16 inches deep and spaced 16 inches apart can span about 26 feet. They have a higher load carrying capacity than comparable solid joists. And the materials used result in less twisting, shrinking or bowing.
Open Web Floor Trusses
Open web floor trusses are constructed with 2 x 4’s on top and bottom with wood web material in between secured with metal plates. The open space is ideal for running wires, pipes and utilities. These trusses are generally ordered ahead of time and cannot be modified on site (although modifiable trusses do exist). Again, these are manufactured to eliminate twisting, shrinking, or bowing. It’s also another option that utilizes less wood than solid joists.
Personally, I’ve never been able to tell the difference between my floor and a solid joist floor. It feels just as sturdy to me. Someday, I plan to run speaker wire and install recessed lights and the open trusses will make this much easier.
What do you think? What kind of joists do you have? Any builders want to weigh-in?
June 3rd, 2008 at 4:40 pm
Ethan,
Every new home we build uses some type of composite floor beam system. There are so many advantages to use them that the cost never really plays into the decision. The big deal really is span and deflections. Typically floor sizing isn’t governed by load condition. The size of a beam that works for code governed loads typically isn’t as large as a beam sized to produce minimal deflections and vibrations. Now that people are installing beautiful tile floors and granite counter tops the floors need to deflect less. The stiffness of a 2×12 regular floor joist can be less than half the stiffness of a 11.25″ deep TJI. The fire issue is certainly a problem but we find that most people are finishing basements today so a layer of drywall will certainly improve the fire rating drastically. Great post!
June 3rd, 2008 at 5:38 pm
@Todd, I knew you were knowledgeable in this area and had hoped you’d offer your two cents. From what I can tell the savings with solid joists are canceled out in the long run of building a house. I installed ceramic in my kitchen and the open web joists are stiff enough that I’m not worried about anything cracking. Thanks for your thoughts.
June 3rd, 2008 at 8:03 pm
The cost difference really isn’t a big deal. Especially when you look at the cost savings in additional framing members that can be eliminated with longer spans. Plus when you start trying to get conventional lumber to meet the same deflection criteria you get in trouble really fast. Then there’s the bonus of using younger growth trees from multiple sources. As far as I’m concerned it’s the only way to go in modern wood framed construction.
August 15th, 2008 at 11:12 am
Off-gassing in tighter homes from poisonous adhesives and plastics are a real problem. Most current OSB I joists and other like products do not belong in a healthy home. Also it took 2 hours for the fire department to get to me for a garage fire, that’s a problem. I say real wood joists or perhaps steel.
August 15th, 2008 at 12:00 pm
@S Bauer, I think it’s a trade off. I-joists use less wood and therefore benefit the enviornment / building costs. It’s true that adhesives can give off toxins but typically not harmful levels. Also some plants can be used to eliminate those toxins. Your right to be concerned with burn rates but if it takes 2 hours, I’m not sure how much it will matter. Hopefully your fire dept. isn’t normally that slow. Thanks for the comment.