Selecting TJI Floor Joist Sizes

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Sizing Trus Joist TJI’s

Trus Joist TJIs 225x300 Selecting TJI Floor Joist SizesI’ve received dozens of emails asking me how you size TJI’s for a new home. Sizing Trus Joist brand TJI’s isn’t really that difficult once you understand the issues. Most people think that joists are sized based on minimum design loads. That is ultimately true, however, loading doesn’t usually govern the size you’ll select for your new home. Deflections are almost always the governing criteria in determining joist sizes in residential and even commercial applications.

First you’ll need to download the most current Trus Joist TJI Manual. The design manual contains all the span tables for different sizes and orientations of joists. The manual also includes span tables based on different deflection criteria. Also included in the manual are extremely important construction and safety details that need to be followed for a safely framed floor system.

Using TJI Span Tables

TJI Span Tables 231x300 Selecting TJI Floor Joist SizesThe following screen shot comes from the Trus Joist TJI Manual. It shows the standard span tables for TJI’s. The tables look overwhelming but they are actually pretty simple to use. They are organized by two main tables, one for live load deflections of L/360 and one for L/480. Within each of those two tables the information is divided again into to sections, one section for a dead load of 10 pounds per square foot (psf) and one for 20 psf.

So the question is: How on earth do I use these tables? Minimum codes typically require a live load of 40 psf and a dead load of 10 psf. These minimum design loads work well as a minimum safe loading capacity. However, those actually loads don’t typically govern the design. The minimum deflection criteria of L/360 will usually control.

L/360 means that if you have a joist spanning 10 feet you can expect it to deflect 0.33 inches at mid-span based on 40 psf live loading. I don’t know about you but a deflection of almost 3/8 of an inch while I’m walking across the floor seems too “bouncy”. That’s why the tables are also written for L/480. For the same 10 foot span you can then expect a deflection of 1/4 inch.

So my recommendation is to select your floor joists based on L/480 deflection criteria. Now depending on the room you’ll need to decide if you want to account for 10 psf dead loading or the higher 20 psf. Again for my house I went with the higher dead load of 20 psf because I wanted a really nice stiff floor that wouldn’t “bounce” as we walked around.

Example Joist Selection

So here’s how you’d select a typical TJI floor joist from the span tables. Let’s assume we’re going to have a 15 foot span (fairly typically with a 30 ft wide house). Let’s also assume we want a very stiff floor so I’ll choose L/480 deflection criteria and 20 psf dead loading. I’ll also assume we’re going to frame the joists at 16 inches on center.

TJI Span Tables L over 4801 Selecting TJI Floor Joist Sizes

I’ve highlighted several key columns in the L/480 Span Table as you can see above. First off I’ve decided I want to try and use 11-7/8 inch deep joists (most standard size). Secondly I’m using the columns for 20 psf dead load fround on the right side of the table. Now all I did was pick the lightest (I’ll explain this below) TJI in the 11-7/8 inch depth that has an allowable span greater than my design of 15 ft. As you can see I’ve highlighted the TJI 110 (left column) that has an allowable span of 17′-8″ for a dead load of 20 psf and a deflection criteria of L/480.

The second column in the table identifies the TJI size for a given depth. The sizes range from lightest (110) to heaviest 560. Once you’ve selected a size be sure to check with our local supplier. Some of the sizes in the span tables are not always available. If that’s the situation, just pick the next biggest size and go with that.

Follow Standard Construction Details

Be sure you follow all of the recommended construction details in the design manual. The details ensure safety during and after construction. The details also help you build the floor to eliminate floor noises and performance issues.

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26 Comments »

  • alan chancey says:

    how do you attach to concrete stem wall. it would be helpfull to have a picture for the want to build your own home person.i will soon be using this product.

  • Mark says:

    So, I am remodeling a garage (20′ ceiling) to add a second floor. My plan is to lag bolt 2×12′s to the existing studs in the walls and run TJI in hangers across the span. Is this ok? Anything I should be looking at?

  • Russ says:

    The helpful TJI advice and related links much appreciated. Thanks!

  • elias rivera says:

    I have a 24′x 24′ gabel roof garage with loft for storage. loft is made from 2″x 8″ joist spaning across 24′, 24″oc with 1/2″ plywood floor. Rafters are 2″x 6″ 24″ oc, set on 2″x 8″ ridge board, notched on to walls. the roof has 7-12 pitch. the joist sit on top of 2″x 4″ walls nailed to rafters. Walls are 8′ made of 2″x4″ 16″oc with double top plate. I would like to use the loft as a lounge possably adding a dormer on one side of the roof. What can I do to stengthen the joist and floor. I have thougth of sistering 2″x10″ to the 2″x8″ and adding one between ea. to make it 12″oc. Would this be enough. I’ve been told also to attach 2″x4″ to the joist and raffters about 6′ in, but that would kill the dormer. Could i just do it on one side and leave the other side open.

    • Todd says:

      Is there a center support in the middle of the 24′ span? If not you’ll be hard pressed to span 24′ without using some serious TJI’s. You’d likely be better off removing the old joints one at a time and replacing with a properly sized TJI. However, the depth of them will be a concern. you may need to create a ledger and drop them down which ultimately drops your garage ceiling height down.

      Any photos?

  • elias rivera says:

    No center support. There is four to five inches clearance over the top of the overhead door. The ledger (to the wall?),sounds like it would work, how would I attach it, and what dimension. I have photos. I’ve subscribed via e-mail.

    • Todd says:

      First off let me say you really need to work with a local building supplier or structural engineer to select the proper joist. Without knowing you entire situation and local building code requirements I can not give you a final solution.

      However, just for talking purposes, if you look at the table you will see that you’ll need something like a TJI 230, 14″ deep, spaced at 12 inches on center. So then the challenge becomes how do you get them in place without the roof falling down! Honestly you’d be better off with a center column/beam arrangement.

  • Fernando says:

    My first floor is made up of 2×10 joist’s, that are 13′ long. The floor has some bounce in it, and I was hoping to eliminate some, if not all, of the bounce. One idea would be to glue and screw some 3/4″ plywood to every other joist, and then, add bracing every 4″. What do you think?

    Thank You

    • Todd says:

      That approach will definitely stiffen the floor. I’d install the plywood on every joist but just do the middle 8′ of the joists. This is where bending stresses are highest. Bridging will also help out quite a bit. This helps engage more than one joist when a load is applied to it.

      Good luck.

      • Fernando says:

        Todd,

        Thank you for the reply.

        Should I just add the plywood to one side of the joist, or both? Should I jack the joist, before screwing and gluing the plywood? What type of plywood should I be using?

        Thank You

        • Todd says:

          From your description it should like you’re trying to fix “bouncing” of the floor. This is due to what we refer to as live load. So no jacking is required. Me personally I’d add plywood to both sides to keep things symetrical. I’d use 1/2″ thick CDX plywood.

          • Fernando says:

            When adding the plywood, to both sides of every other joist, should I just glue and screw both pieces to the middle of the joist, or should I stagger the pieces, on each side of the joist? Should I still add the plywood to every other joist? Do you think 3′ is a good space, for the bridging, or should it be more or less?

          • Todd says:

            I would do every joist so the floor has an even distribution of stiffness. Not sure what you mean by staggering, I’d just use an 8′ long piece at mid-span, both sides of the web.

            3′ is more than enough, 4′ spacing willl likely work well as well.

      • The strongest option is 1 or maybe 2 rows of bridge blocking. A 9 1/8″ plywood strip, even on both sides will not stiffen enough to stop live load deflection at length.

  • Larry says:

    Hi Todd,
    First off, thanks for putting this website together! Very helpful.
    I am a home designer and have a question about TJI sizes/ spacing vs. cost. In your experience, is it less expensive to go with a tighter spacing with a lighter joist, or wider spacing with a heavier joist?
    Regards, Larry

    • Todd says:

      Typically the best solution is one that uses 16″ o.c. spacing. That spacing works well with 3/4″ sub-floors and finished flooring and it requires less labor than a 12″ o.c. spacing. Obviously there are times when that won’t work but that’s typically my preference.

  • Earl says:

    I would like to confirm what floor joist will yield a strong floor. I will be 24 foot outside to outside of 2×6 exterior walls of a 42 foot long home and want to keep the first floor open by using TJI joists running right to left. I will have a 12/12 pitch roof so when I get upstairs there will be a vertical wall from center outwards, at 6 foot running from front to back and supporting a stick framed 2×8 rafter…or a 12 foot room the length of the house…so the load from the roof will essentially bear at 6 foot from the downstairs exterior walls inward if I am giving a proper mental image. I am in upstate NY and want the most rigid floor I can get within reason. I don’t want my daughter running back and forth and watching the floor/ceiling move before my eyes. I like to think of this as a heavy duty request/inquiry.

    • Todd says:

      Earl – With a configuration like that you’ll need to confirm sizing with a TJI rep. Most distributors have staff that are trained to use their design software and confirm sizing. Taking a quick look at the charts you’re going to need a 14″ or 16″ deep joist. Spacing and model will depend on how much loading that upper wall framing adds to the span.

      The deeper 14″ or 16″ joists will need to be accounted for in floor to floor heights for stair design and details.

  • Frank says:

    Hi I have TJI 560 series 16 inch and 16oc spanning 21 ft I have a 16 inch rim boards. Do I need squash or web fillers at the top plates at the load ends. My tji are all cut tight to the rim board If I need them how would you apply in place. Glue then screw .this is for a second floor floor joist
    Thanks

    • Todd says:

      Frank – You’d have to look at their standard details. Typically we only see squash blocks at interior load bearing locations but it’s certainly possible. Installing them wouldn’t be a big deal, typically a vertical 2×4 on each side of the flange, tight to the plate below and tight to the sub-floor above would be used.

  • Todd,
    I am looking at a cantilevered radius on the second floor. The projection at the widest point is about 36″. I am familiar with a doubled LVL at 1 1/2 times the cantilevered length inside the wall. I will turn the LVL beam at a 45 degree angle to the corner where the radius projects. Should I use LVL’s as floor joists? It doesn’t appear that a 14″ TGI will support the span.
    Craig

  • Adam says:

    I am thinking of remodeling my garage that is 23×25 with 17ft ceiling by adding a second floor for mostly storage…if I sandwich two 2×12 and lag bolt them to studs in the wall and use TJI in hangers would that be okay…and would it be beneficial to add 4×4 posts along the wall for support too…

    • Todd says:

      Adam – Spans over 20′ are what I would consider a pretty significant span. Regardless of whether you’re using it for storage or finished space, you need to have someone evaluate that floor and size the joists properly. I’d work with your local lumber yard and have someone design the floor system. 2×12′s are not likely to work unless you install a beam at mid-span.

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