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Floor truss spacing and sub-floor options for a “stiff” floor : opinions?

mangler66 | Posted in GBA Pro Help on

I am currently building a slab on grade home, with 24″ deep open web floor trusses (24″ selected with the intent of having (2) 20×21″ house long chases to run low velocity HVAC ducts, supply and return and avoid bulkheads).

My architect, who is admittedly old school/conservative, specified spacing at 16″ OC, with 3/4″ ply sub-floor. The house is 30′ wide, and the joists are basically designed to take this full span. However, I have a mid width supporting wall/beam in the design, which basically cuts the span in half (max span from wall to exterior wall is 17′).

So, I am looking at spacing options and potential stiffness compromises. Right now the usable space between trusses is about 12″ (at 16 OC), which will make reaching my main HVAC trunk difficult.

These are the numbers:

Design loads

TC LL 40 psf
TC DL 10 psf
BC LL 0psf
BC DL 5psf

Total 55 psf

Deflections for 24″ on center (assumes 2X6 strong backs +5/8″ ply sub-floor glued and screwed)

Allowable deflection LL L/480 (0.42″)
Calculated vert. deflections LL L/999 (0.12″)
Allowable deflection TL L/180 (1.11″)
Calculated vert. deflection TL L/999 (0.15″)

Deflections for 19″ on center (assumes 2X6 strong backs +5/8″ ply sub-floor glued and screwed)

Allowable deflection LL L/480 (0.42″)
Calculated vert. deflections LL L/999 (0.10″)
Allowable deflection TL L/180 (1.11″)
Calculated vert. deflection TL L/999 (0.12″)

Deflections for 16″ on center (assumes 2X6 strong backs +5/8″ ply sub-floor glued and screwed)

Allowable deflection LL L/480 (0.42″)
Calculated vert. deflections LL L/999 (0.08″)
Allowable deflection TL L/180 (1.11″)
Calculated vert. deflection TL L/999 (0.10″)

Current plans call for 3/4″ ply, which is above what is used in the calculations from what I gather. I am willing to go to 7/8″ OSB with the 24″ on center or even 19″ OC to minimize creaking with hardwood floors, if necessary.

The question, if this was your house, which option would you pick? All seem above code, but I have a code house now, and I would like much stiffer floors in the new build. I also want to avoid wasteful expenses, as going from 16 to 19 to 24″ OC basically takes the number of trusses down from 24 to 20 to 16.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Mai Tai,
    I'd start by getting the trusses designed for the actual span. You may find some saving in just doing that - and trusses designed for the full span may be structurally compromised by having mid-point bearing.

    With 24' deep trusses, the problem his always going to be the subfloor, not the underlying structure. If it was my house I wouldn't go beyond 19.2" and at that spacing i'd use 3/4" plywood or 7/8" OSB.

  2. mangler66 | | #2

    Thanks Malcolm.

    I misspoke, the trusses are designed with the supporting wall in mind, but 24" trusses can span 30' and still meet code without a mid-span supporting wall.

    I was going to split the difference and go with 19 OC, with 7/8" OSB it should be plenty stiff. Glad you seem to have the same train of thought.

    Would it make a difference if I went 1" OSB for the 24OC option? I would have to check to make sure this is wise on the money side (as OSB is going through the roof here), but it would be nice to know if that was an option.

  3. Jon_R | | #3

    Perhaps the wood flooring adds enough stiffness to influence the thickness decision?

    Is it possible to only use 24" where the ducts are and less elsewhere?

  4. Expert Member
    MALCOLM TAYLOR | | #4

    Mai, Tai,
    I think you are making good choice. It's worth remembering that deflection increases very rapidly when you increase the span. So the difference between 19.5" with 7/8" OSB, and 24" with 1" will be noticeable.

    Both OSB and plywood have gone nuts here in BC, especially since the wildfires we've had this summer. Up between 30% and 40% since spring.

  5. Expert Member
    ARMANDO COBO | | #5

    I typically use 18" open web floor trusses @ 24" o.c. with a 24" duct chase for 15"x20" metal trunks. The 18" truss have 480 deflection minimum at any span, but I know that after 22', I need to consider double cords, 19.2" o.c. or a combination of both. If you have 17' span, you should have pretty stiff floors with 3/4" OSB, however, we use 1 1/8" Advantech subfloor. You can always increase the deflection of your trusses, which is fairly inexpensive, just bigger plates.
    We coordinate the schedule between framer and HVAC contractor to install the trunks BEFORE the sheathing is installed, since squared truncks come in 5' sections, so they can slide them from the outside wall. The HVAC contractor will seal and insulate trunks when they come to do their rough-in. We don't have problems with branches, since they are round ducts. I coordinate my truss layout with the duct layout... very important!

  6. mangler66 | | #6

    Malcolm,

    It figures, I am building at one of the worst times in the last 10 years. Lumber is up, labour is up (thanks to a Toronto bubble that has trickled down), OSB is up. 2 years ago I had people chasing me to get the job. These days i have to call back three times to get a quote...

    Armando,

    My truss layout and chases were designed for the HVAC plan. I like the idea of sliding in 5 foot sections at the framing stage. Feels like coordination will be critical there, you really have to keep a handle on it. Perhaps I will just have them screw in the sheathing in key locations. That way I can remove the appropriate pieces when the HVAC contractor graces me with his presence.

    Even at 24" on center, the 1 1/8" Advantech must make the floors feels like concrete. I just don't know how much thickness plus up I will be able to afford at the current prices here. ironically given the price of OSB, 16 OC with 3/4" OSB may turn out to be cheaper.

    Jon,

    I will ask my truss designer if it's possible to do an exceptional 24 OC with all the other 19 or 16 OC. I suspect it will not be an issue, but will make layour a bit tricky. Worst case I can always reinforce under the sub-floor with a screwed and bonded piece of OSB if I am unhappy with deflection at the "large" spacing location.

    Thanks all for the valuable insight.

  7. Expert Member
    MALCOLM TAYLOR | | #7

    Mai Tai,

    Think through your subfloor sheet layout before considering having several different spacings of the trusses. You can end up with a lot of waste, especially trying to incorporate the 19.2".

    The only good thing about construction booms is there are some great deals on tools and trucks when things collapse.

  8. mangler66 | | #8

    After deliberation I likely will be going with 24" OC. The difference in deflection (basically going from 0.08" to 0.12") does not warrant the extra cost to go 16" OC. I like that 24" OC is also an easy layout for the sub-floor sheets (trhanks to Malcolm for pointing this out).

    This brings the cost of my floor truss quote down (16 vs 24 floor trusses), and should be a net cost savings, even if I decide to go crazy on the subloor thickness, (going from 3/4 to 1 1/8 OSB is only 0.50 cents can. a square foot, or roughly 750$ for my build). I probably won't go to that extreme, 7/8 or 1inch osb is probably the sweet spot at 24" OC, glued and nailed. Even at 1 1/8", which is definitely in the overkill range, my net savings are in excess of 1000$ can.

    I will be getting the final price for all options (16,19,24"OC), and I will make the final decision then. But so far I am leaning heavily towards 24" OC.

  9. RobInNorCal | | #9

    Maybe a little late here but I used 16" open web floor trusses on a 24' span with 1 1/8" plywood decking, glued and screwed - you could play basketball on my deck without feeling the floor. It's one of the things visitors comment on after a while - as in, "wow, this place is really solid!".

    Not knocking the OSB alternative, I don't have experience with it - but the plywood is an awesome product, and goes down MUCH more easily than 3/4" T&G. The tongues on the 1 1/8 sheets are tapered so they slide in with just a bit of persuasion (like a good sliding dovetail joint).

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