Flooring Load Calculation

Discussion in 'Remodel Forum & Blog' started by Dan87, Sep 30, 2019.

  1. Dan87

    Dan87 New Member

    Nov 3, 2015
    Appliance Sales, Electrical Contracting, Plumbing
    Boyertown, PA

    I'm remodeling my house and have a flooring load question. The floor joists are just under 3" wide, true 8" tall, and span varying lengths depending on location. The spacing varies between 19" - 20" on center. Maximum span is in living room where they span 14', in the dining they span 11', and kitchen is varying depending on location, but nothing more than 14'. I cannot verify the species of wood. A few spots have cut out joists areas that have been headered out (prior to me owning the home). We ripped out the carpeting and linoleum that was covering the hardwood flooring beneath. The hardwood is 3" wide planks and is just over 3/4" thick; the hardwood is nailed directly the floor joists (house was built in 1920 time frame). The hardwood is in rough shape and has cut outs and patches in areas so refinishing is not happening. I plan to tighten up the hardwood flooring with deck screws to eliminate noise and stiffen up some areas, and then apply a 1/4" underlayment overtop stapled down with a crown stapler as specified to smooth out any slight imperfections that do exist in the subfloor, which there are some. I've priced out some vinyl plank flooring options to put over the 1/4" underlayment, some with pre-attached foam backs, some need foam underlayment added. The flooring I would like to go with is 2.89 lbs/sqft. Adding this weight to the original 3/4" hardwood and the 1/4" underlayment, I'm coming up with just about 6.75 lb/sqft in dead load. I looked at the various charts available online to determine load rating but this is beyond me. Should my current construction support this load or is it too much? I can easily go with a lighter flooring option but I prefer the one I chose. Any input would be greatly appreciated. Pictures for reference.

    Attached Files:

    Last edited: Oct 1, 2019
  2. Dana

    Dana In the trades

    Jan 14, 2009
    The structural capacity of clear old-growth stuff used in the 1920s is considerably higher than current stuff. But for estimation purposes when referring to charts, your 2.5"x8" joists are nearly twice as thick as milled 2x lumber, so from a total capacity point of view the 20" o.c. spacing is like having milled 2 x 8 joists 10-12" on center, only stronger, due to the true 8" depth. With old grown timber from the 1920s it's probably pretty close to the capacity of milled 2 x 10s 12" o.c..

    But at the VERY least it's going to have more capacity than 2x8 12" o.c.. A 2.5" wide joist every 20" is 0.125" per inch of joist spacing. That's the same ratio as 1.5" wide joists 12" on center. Take a look at TABLE R502.3.1(2) in the IRC at 12" joist spacing, under the 10 psf dead loading section.

    Even without additional beefing up milled 2x8s 12" on center is totally good for the 11' span at 10 psf dead loading, unless it's the lightest and least-structural southern pine species in common use, and even then it's close. With the true 8"depth rather than 7.25", you should be good to go.

    For the 14' span, looking at the 2x8 column it could be marginal at 12" o.c.-equivalent if it's a crummier species, but looking at the 2x10 column it's still pretty solid, and given the old-growth lumber you're probably in good shape. Try jumping up and down in the middle of the room with the 14' span. If it feels pretty bouncy in the middle it may need some beefing up, but if it feels pretty stout it doesn't.

    If suspiciously bouncy, if instead of screws to fix the squeaks in that room, adding a sistered-on 2x4 or 2x6 on each joist, laying a heavy bead of construction adhesive on the top edge of the 2x lumber as it is installed to glue to the floor planks should increase the load capacity of the joists enough to get you there.
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  4. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    The boxed joist section needs some beefing up. The short board is not doing anything but perhaps something for the floor board to be nailed too. Whatever the reason the main joist were cut and boxed off, (I'm not a carpenter and others may correct me), but the 2 x used to transfer the load to the two adjacent joist should be a double header. Depending on the static load above it the two outer joist are doing the work of four joist. Just to be sure everything is true, tie a string across the bottom of the joist from one end to the other and look for sags in the joist. For a 100 year old home the wood looks good, no cracks or knots. Nothing like old timber.

    FYI... the cross bracing between the joist is for minimizing floor bounce.

    Nice work with the wiring but the holes are not right but because of the size of the joist you're probably OK.

    Drilling Floor Joists:

    • The diameter of holes shall not be greater than 1/3 the depth of the floor joists. ...
    • Holes cannot be closer than 2″ to the bottom or top of a joist (or to another hole or notch in the joist)
  5. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    I was thinking that is a trap door/removable panel for lowering items into the basement. However, now that I say it, I don't know if it would big enough to be useful for lowering a freezer to the basement.
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