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Best way to cover gap in raised floor due to scant-face OSB?

Jeff Cooper | Posted in General Questions on

The floor for my one-story house is 48’x20′, made of 3/4″ T&G OSB nailed and glued to 11-7/8″ i-joists 16″ oc, which are nailed to an 11-7/8″ rimboard and rest upon a 2×6 sill plate on a 4-course CMU stem wall.

As I am about to install the OSB, I have just learned that virtually all of the tongue-and-groove OSB panels sold these days, including mine, are effectively only 47-1/2″ wide. With my 20′ wide floor, my five rows of panels will therefore come up 2-1/2″ short of covering the full width. From the discussions I’ve read, builders typically deal with this by filling the gap with narrow strips of OSB that generally don’t have a tongue, but don’t need one or blocking because they’re underneath the wall, where no loads will be imposed between the joists. If their solution is appropriate, with my double wall 7″ thick, I could make my final strip a few inches wider than 2-1/2″ by reducing the width of the first row of panels.

I gather that the advantage of increasing the width of the rows of OSB attached to the rimboards is that they will provide a stronger tie-in for the rimboards to resist lateral (outward) loads.

If my final strip needs to be wider than 7″, so that it extends outside the wall, I expect I would have to buy 6 full sheets of OSB to get 48′ of tongue, rip strips from one sheet and make tongues, or install the final strip without tongues and install blocking. I would not, however, want to make the final strip so wide that I would have to reduce the width of my first row of panels so much that they would no longer brace my i-joists sufficiently to make it safe to stand on the second row of panels while I install them. From what I’ve read, when I use my pneumatic nailer, I should use my body weight to ensure full contact between the OSB and the i-joists. Since I can’t stand on the first row of panels, I’ll nail them by hand, and then once they brace the i-joists, I’ll be able to stand on the second and subsequent rows. I considered bracing the i-joists with 1x4s, but that would require almost as many nails as installing the first row of OSB, and it would waste a lot of 1x4s.

What do you suggest?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jeff,
    Q. "What do you suggest?"

    A. I suggest that you are overthinking.

    If you want a good tie-in to your rim joists, make sure that the strip of OSB that ties to the rim joists is wide enough that you can sleep at night without worrying. If one of your OSB seams lacks a tongue, don't worry. (To some extent, worries about whether or not your subfloor has T&G joints depends on what type of finish flooring you'll be installing, and whether or not you intend to install underlayment between your subfloor and your finish flooring.)

  2. Tim R | | #2

    Hi,
    Look at the code. The floor is a diaphragm that transfers loads. In an unblocked diaphragm if you have pieces that are less than 2 ft wide you need to block the joints. This should be a note on the plans that you submitted for the building permit.

  3. Expert Member
    Malcolm Taylor | | #3

    Jeff,
    As Martin said: don't worry about it, and as Tim said: just throw in a row of blocking.

    Start your subfloor with full sheets, use you nailer for everything, and nail a row of 2"x4's on the flat to bridge the gap between the last sheet and the small strips you need to piece in. Cutting and adding the blocking shouldn't add more than 1/2 hour to the job.

  4. Jeff Cooper | | #4

    Thank you, Martin, Tim, and Malcolm.

    In the 2012 IRC, Chapter 5, Floors, does say that unsupported edges need blocking, but I did not find anything addressing a minimal width as it relates to blocking or providing lateral hold on the rimboard. If you know where I would find that, Tim, I would be greatly interested to read it.

    Malcolm, your suggestion sounds good, and I really appreciate your attention to efficiency. Forgive me for continuing to worry about it, but with a 1-1/2" rimboard, a 2-1/2" strip of OSB would reach only 1" onto a 2x4, and I understand I need to keep nails at least 3/8" from the OSB edges, so those nails would be at most 5/8" from the edges of the 2x4s. Would I be better off with a wider strip of OSB that would place the nails farther from the edges of the 2x4s to minimize the likelihood of splitting them? Also, are you saying that I can use the nailer on the first row without using my body weight to press the OSB against the i-joists? (BTW, Malcolm, I did post the results of my anchor-bolt-holder experiment in that previous thread, but I didn't get the usual notification of my post, so you may not have either, but it's there.)

  5. Expert Member
    Malcolm Taylor | | #5

    Jeff,
    First off thanks for posting the update, I'll try and find it and read it.

    It's a suckers game to anticipate the exact amount of gap you will have left over that distance. If you get all your sheets tight enough to end up with exactly 2 1/2" you are a better framer than me - so any solution needs to take some variation into account. Install the subfloor until you get to that last strip, set your circular saw depth gauge to the thickness of the ply and cut off enough from the last sheets to sit on the middle of the 2x blocking.

    Never hand nail unless you are forced to. Run you glue lines on the I joists, set your sheets for the first row by nailing off the corners on the rim joist, then standing on the sheet nail off the joists on the far end. You can then work your way back nailing off the field using your weight to keep things snug.

  6. Jeff Cooper | | #6

    Thanks again, Malcolm. The thread on anchor-bolt-holders, with the update I posted on 9/19, is here:
    https://www.greenbuildingadvisor.com/community/forum/general-questions/98564/homemade-anchor-bolt-holders

  7. GBA Editor
    Martin Holladay | | #7

    I'm grateful to Tim Rudolph and Malcolm Taylor, whose answers did a better job than mine of focusing on (a) code issues and (b) the need for blocking under joints that lack a tongue-and-groove seam.

  8. Tim R | | #8

    Hi, The blocking size is called out in the diaphragm tables in the referenced American Wood Council document SDPWS 2015 for the latest edition Special Design Provisions for Wind and Seismic. Free at ACW.org They have the design tables for diaphragm and shearwall design. They have shear capacity tables that use either unblocked, 2x blocked or 3x blocked diaphragms. It is likely the small house is designed as an unblocked diaphragm. The footnotes give the minimum panel sizes that don't require blocking, any thing smaller requires blocking. Blocked diaphragms deflect 2-3x less than unblocked and deflection equals damage.
    The lap of the panel on to the rim should be the full width of the rim - 1.25 rim is common you can buy thicker. But you need to make sure the nails are in tolerance of the supporting material, best in the center of the engineered rim.

  9. Jeff Cooper | | #9

    Thank you for the reference, Tim; it goes into more depth than I had read on this topic. Other documents from the AWC specify 24" as the minimum width required for the span ratings to be valid, but.if I make my last row narrow enough that It would lie entirely underneath my wall, it will not bear any loads. That turns out to be by far the less wasteful solution, even with the blocking I will need. I thought I had I figured out a cut schedule to minimize the panels needed for a wider last row, but it wasn't actually feasible.

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