It’s funny the different things people notice about a house. For instance, I like to check out the type of floor joists in my friends’ homes. You might think it’s strange, but joists can impact lots of things like floor plan, fire safety, what materials can be installed for flooring surfaces, and even indoor air quality. In this article, I’ll review a few different types of joists, and you can decided if they’re important to you too.
Editor’s note: This article originally ran in 2008. We are re-publishing it with updated information and additional perspective from spending more time on job sites. We hope you’ll add to the conversation in the comments section below.
Joists are horizontal supports that span from wall to wall, wall to beam, or beam to beam. Several popular options include solid lumber, LVL, wood I-beams, steel joists and open web floor trusses. Let’s examine three of these materials.
Solid Lumber Joists
Traditional solid lumber joists are still very common. They were the default for some time, but in the early ’90s builders started using engineered products more and more. Initial costs for solid lumber joists are (usually) cheaper than engineered solutions but this benefit is cancelled out by the costs associated with limited span distances and increased framing members.
Span distances depend on several factors including wood species, board size, spacing and acceptable deflection. If you’d like to learn more, try out this handy calculator that lets you enter all the necessary details.
From an environmental perspective, solid lumber joists are expensive. These 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.
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.
Sistered solid wood joists
Wood I-joists (sometimes called TJI’s) resemble steel I-beams. They are composed of a top and bottom flange (usually a 2×4) with a piece of plywood or OSB fixed perpendicular in between.
The biggest benefit I-joists bring are the long span distances you can achieve. A TJI 16″ deep and spaced 16″ o.c. can span about 26 feet! The second biggest perk is how little I-joists flex, and if you’ve ever installed ceramic tile, you know how important that can be. They also have a higher load carrying capacity than comparable solid joists, and the materials used result in less twisting, shrinking or bowing. If you see a home with an open layout, odds are the builders used TJI’s.
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 or OSB. Many I-joists are pre-scored or have knockouts to make things easier.
The only downside I can see to TJI’s is that plywood and OSB can pose a small health concern because of off-gassing from the adhesive, and this doesn’t bother me at all. Lots of household products can fall into this category like radon produced by granite countertops. I’m no expert, but I seriously doubt that eliminating I-joists will improve indoor air quality very much.
Open Web Floor Trusses
Open web floor trusses are constructed with 2 x 4’s on top and bottom with a sort of “web” in between secured with metal plates. The open space is ideal for running wires, pipes and utilities. I have these in my home, and I use the space for extra storage as you can see in the picture.
These trusses are generally ordered ahead of time and cannot be modified on site (although modifiable trusses do exist). Web trusses are manufactured to eliminate twisting, shrinking, or bowing. They also utilizes less wood than solid joists. Score one for the environment.
The only reason I like TJI’s more is that web trusses will flex more, and I’ve really come to value a floor with limited deflexion.
What do you think? What kind of joists do you have? Any builders want to weigh-in?