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Community and Q&A

Floor Truss System

bdillard | Posted in General Questions on


I’m trying to guess what my basement/1st floor construction might look like.

My house is 36′ wide and about 76′ long.    I’ll have a basement that probably won’t house much more than the mechanical and electrical rooms initially.   I’m not too worried about putting in support columns as necessary – although I would like to have some 16′ x 16′ areas without columns for future rooms.

Any thought – would I need a floor truss below the first floor above the basement about 24″ in height to support l/480 at 16″ OC – can I do anything with cross support or support columns to reduce the height or improve the stiffness?

I’m not an architect or engineer and am probably at least a year away from being ready to see either – this is just a starting point.

Assume an empty basement initially and i don’t know if it matters at all but I’ve got a floorplan for the first floor.

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  1. DCContrarian | | #1

    A rule of thumb is that span is expensive. Yes, they make i-joists that can span 36', but it's probably cheaper to run a beam down the middle and span 18' instead. Or even two beams and span 12'. You can hang joists flush off of a beam so there's no drop, but you will need columns to support the beam.

    Someone who works with this stuff every day like an engineer or architect can probably glance at your plan and rattle off the cheapest option.

    1. Charlie Sullivan | | #2

      Consider asking a company that supplies I-joists to engineer it.

    2. bdillard | | #4

      Thanks - I'm learning from scratch - so would I have two 18' trusses with some sort of center board and a few support columns? Any info you can give would be helpful - I'm not expecting you to give me exact details but a general idea of what it might look like would be great.

  2. Expert Member
    Akos | | #3

    At 36' span you also get into a resonance frequency issue. Even a floor that is L/480 will feel bouncy as the natural frequency is too low.

    18' is easy to span and you end up with a much stiffer floor.

    If you want to keep your basement clear, one option is to split the 76' into three 25' section with steel beams and run your joists in the 76' direction. The shorter 25' is much easier to span with stock engineered lumber. This does not require that much more steel, avoids any columns or the footings needed to support them. Just be careful how you place the steel if you need to run ducting across it. Bellow the joists is the simplest, leaves the space open and less labor, but you do loose a bit of head room.

    1. bdillard | | #6

      Thanks - I apologize if I'm not phrasing this properly. I have no issue with columns halfway through my basement - I'm just not familiar with the structure enough - do I put som sort of truss or a few joists nailed together perpendicular to the 36' truss making them 18' trusses and put a few columns to support that new center joist? Any guess on how far apart is the maximum and minimum for those columns? I'd like a stiff floor above that can support a good amount of weight - e.g. a full floor standing bathtub full of water or a refridgerator or Uncle Larry.

      1. Expert Member
        Akos | | #7

        Generally a smaller wide flange steel beam or LVL is put directly under the joists mid span supported by a number of columns. Your engineer can spec size and spacing of both. The trusses would be built with a hard point for intermediate bearing where the beam would go.

        Going for continuous span (floor joist is the full width of the house) makes for a slightly stiffer floor and a bit less install labor. Half span is also fine and sometimes easier to work with plus stock item at most lumber yard. It can also be handled by a single worker.

        1. bdillard | | #11

          Thanks - again I realize I'm not an engineer but it seems like I could do a single 36' span if the trusses were about 24" in height or I could do 2 12" height spans at 18' if I put a mid span cross beam that is about 12" in height - is that correct?

          1. Doug McEvers | | #17

            I have used open-web floor trusses to clear span 30'. With 2x6 stiffback and t&g glued plywood subfloor, the floor system was solid. It is typically the span less 8 for the depth of the open-web trusses. Cannot speak to any span beyond 30'.

            As Armando has stated, the open-web floor trusses do offer an advantage in running HVAC within the floor system. I added a 2nd story to a rambler and used 26' open web. With a chase from the basement level all of the ductwork for the upper level fit nicely into the floor trusses.

          2. Kyle R | | #18

            I’m not sure the cost, but take a look at

  3. Deleted | | #5


  4. Jonathan Blaney | | #8

    The joist do not have to all run in the same direction. Arrange the posts and beams to fall in walls for your rooms.

  5. Walter Ahlgrim | | #9

    The longer the spans the more it will cost and the taller the floor will be.

    You need to decide if the extra cost for the clear span is worth it for your project and if it is in your budget.

    My guess is the low cost option will be 2 steel beams in the basement with 3 12 foot spans.

    At some point you will need to pay an engineer to run the numbers.


    1. bdillard | | #12

      Thanks. Are the spacings on the column for the usually about 8' apart? I think as a kid that's what they were in the basement but that was long ago.

      1. Bob Irving | | #14

        8’ centers is common for beam columns but as Walter says, it depends entirely on the size of the beam that it’s supporting. If you have a specific floor plan for the area, as we often do, we size the beam and posts to work with the spaces we want. My lumber yard gives me the spans that work for different size beams and we make that decision.

        1. bdillard | | #16


  6. Bob Irving | | #10

    Most “real” lumber yards (not the BigBox stores) will design, size and price your floor truss options including centers of columns, hardware needed if any etc. Floor trusses are great, but it’s not anything you want to guess at.

    1. bdillard | | #13

      Thanks - I follow that but I like to have some knowledge before going there. I don't expect to be an expert but I feel uncomfortable going to a car dealer unless I know as least as much as the sales people. I don't expect to know a fraction of what the mechanic does.

  7. Expert Member
    Armando Cobo | | #15

    Every house I’ve ever designed with a basement had a bearing wood wall, with no more than 18’-20’ span. A longer span will make the floor bouncy, even at l/480, and my preference to use 1 1/8” Advantech floor sheathing. Also, I don’t use steel to lower the carbon footprint. There’s nothing I can’t do with a wood wall, wood columns and LVL beams.
    I like to use 16”-18” open-web trusses at 24” o.c., with a duct chase to run my trunk duct, not having to drop ducts, plumbing or electric conduits below the ceiling.
    With a wall at the middle, or at 16’ and 20’ spans, I would design mechanicals, bathrooms, storage room, bars, and even theaters on the “dark” side of the basement (no windows), and bedrooms and family room on the window side, assuming you have walk-out, view-out wall or window wells.

    1. DCContrarian | | #19

      I was talking to a contractor who is older and wiser than I and he said "get the steel out." For residential construction steel falls into the category of just because you can do it doesn't mean it's a good idea. Unless you need a totally open floor plan often times situating the walls you were going to use anyway as bearing walls means being able to get the steel beams out. Unless you plan to use the basement as a dojo or something I'd totally look into a 2x4 bearing wall running the length instead of beams and columns. Eighteen feet on each side is going to give you plenty of space for most typical uses.

      In addition to the carbon footprint, steel presents a couple of practical problems as the build progresses. It's much more thermally conductive than wood, so it's hard to insulate, especially if the ends are sitting on the foundation walls. It's a chore if you need to run plumbing or electrical through it, or attach framing to it or modify the design. If you go with wood every guy on the site has the tools and expertise to work with it.

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