Expansion Gaps In New Plywood Subfloor Missing

sequoia_2007June 3, 2009

As part of a structural repair to our townhouse we replaced the footings, posts, beams, joists and subfloor in and around our kitchen. We told our contractor we planned to install an engineered hardwood floor and to prepare the subfloor accordingly.

The plywood is 3/4â Hard Sturd-I-Floor attached to 4x6 joist on 16â centers with adhesive and 2-3/8â x .113â ring shank nails. The original 2x6 joist allowed for too much floor bounce due to the long span so we upgraded to 4x6. The upgrade to 4x6 joist improved the floor dramatically.

I was reading about proper subfloor installation and I see a minimum 1/8â expansion gap should be provided along all seams. When I looked at our plywood subfloor I see our contractor butted all the seams tight. See pictures below. Is the 1/8â expansion gap for the plywood subfloor mandatory to avoid problem with our planned engineered hardwood floor?

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HandyMac

The flooring in the picture is tongue and groove design---and is installed correctly.

I have never heard of an expansion gap requirement for subfloor installation. Seems to me that if a sub floor needs to move, only a floating floor could be installed---precluding using anything else save carpet. How would a builder communicate that for an owner 40 years later?

    Bookmark   June 3, 2009 at 9:19AM
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sunnyca_gw

Check your source of info. that told you you needed expansion. Sounds like it's for roofing. If you were using a "Do it yourself" book you might have looked at Plywood & thought it was for floor & you were reading about roof preparation!! If you left space between tongue & groove & someone had heels & walked across the gap they just might leave a good dent. Would be very unstable!

    Bookmark   June 3, 2009 at 11:28PM
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sequoia_2007

The sources of information are three different books on framing. One of them states the procedure may be avoided in dry climates. Maybe this has more to do with open framing that experiences exposure to rain before the roof and walls are built.

Thanks

    Bookmark   June 4, 2009 at 3:20AM
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alphonse

Just another two cents to say it's not an issue with flooring.
BTW, adding depth to joisting, or any beam, increases deflection resistance...width, not so much.

    Bookmark   June 4, 2009 at 6:57AM
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brickeyee

"BTW, adding depth to joisting, or any beam, increases deflection resistance...width, not so much."

The deflection goes directly with width, twice as wide gives 2x stiffness.

It goes by the cube of the height, so even a small increase in beam height has a large affect on stiffness.

A 10% increase in height increases the stiffness by ~33%.

    Bookmark   June 4, 2009 at 9:43AM
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alphonse

Brickeyee, I think we're saying the same thing- a 2x6 is much stiffer than a 2x4, whereas a 4x4 doesn't yield the same result.
I used "depth" to indicate vertical dimension of floor surface to bottom of beam.

    Bookmark   June 4, 2009 at 3:31PM
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brickeyee

The beam deflection equation is pretty easy to find.

I just used real numbers.

    Bookmark   June 4, 2009 at 5:07PM
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alphonse

If we're going to parse, depth = height, not width.

    Bookmark   June 5, 2009 at 12:16AM
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worthy

4x6 joists. That's a new one on me! The only floor sheathing gaps required in the Ontario Building Code relate to a second layer of sheathing as an underlayment for ceramic. (OBC9.30.6.3)

It's true water often swells T&G joints. But not more than a quick run with a belt sander can level off before the finished floor is laid. And maybe if I were in the Seattle/Portland/Vancouver area gaps on non T&G might be a good idea.

    Bookmark   June 5, 2009 at 11:32PM
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brickeyee

"If we're going to parse, depth = height, not width."

You seem to be the only one parsing anything.

The beam equation usually uses w for the width of the beam, and h for the height of the beam since it is normally shown in a side view so that it can be annotated with a shear diagram if required.

If a 2x4 has a strength of 1, a 4x4 has a strength of 2, while a 2x6 has a strength of (5.5/3.5)^3 = ~3.9

    Bookmark   June 6, 2009 at 10:32AM
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alphonse

You got it.

    Bookmark   June 7, 2009 at 2:17AM
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