plywood subfloor sizing help!

dug · December 02, 2009, 04:07:57 PM

So I am installing plywood tongue and groove subfloor now and after measuring a few sheets have discovered that instead of 48 in. they are 47 1/2. I know from my previous recent post that I am supposed to leave a gap (thanks Mt. Don) but to keep it at 48 the tongue wouldn't even be engaged (in the groove). I called HD (I had all my materials delivered) and talked to someone at the 'pro' desk and was informed that it is all this size. True? If so, My alternatives seem to be to leave it short an inch or inch and a half at each end, which doesn't seem the best to me, or take it back and get cdx ply (200 mile round trip). Seems odd to me that they are sized this way.

MountainDon · December 02, 2009, 04:36:55 PM

All T&G sheet goods I've seen are like that. It surprises just about everyone I think. As you lay the sheets down, starting at one corner, you do end up short at the other long side. When you reach the far side you will likely have to piece in a strip.

The long 8 ft. dimension falls across the joists, and you stagger the sheets so there are never 4 corners meeting at a single point. The short dimension falls on a joist and is nailed down.

You do not want to make the T&G joint slack as that will defeat the purpose. Square edged sheet goods are not advised for subflooring as the material may give or flex along the 8 foot side as people walk about. Square edges on the long sides would only be supported at each joist. The T&G locks the sheet edges to prevent that.

G/L

One reason I designed our cabin 15.75' wide was to avoid having to worry about the fill in strip at one edge.

December 02, 2009, 04:53:08 PM

thanks Mt. Don, I wish I would have known that, then I would have done the same as you. Seems very odd to me as most buildings are on 2 ft. increments (or so I thought). Guess I don't make the rules though. One more question- I measured the panel lengthwise and it was exactly 8 ft. If I leave an 1/8 in. gap then I would have to cut that much off, or maybe 1/4 every other piece. I didn't think you would have to do this but I don't see another way.

MountainDon · December 02, 2009, 04:57:29 PM

Lengthwise mine worked out pretty good but IIRC I did trim a sheet or two a little here and there. That was easy using the guidebar on the circular saw.

Jens · December 02, 2009, 05:08:44 PM

trimming of the sheets in length is usually due to layout errors on the framing, or even bowed boards. If you have a big floor to do, it can help to make sre that you measure the distance from the end of the sheet to the center of the joist, and move it into proper place before nailing. Often the joists tend to drift. You want to leave about a nail thick gap on all sides, especially important with OSB, plywood seems a bit more forgiving. Sometimes the tongues are longer than the depth of the grooves, so that you don't have to think

December 02, 2009, 05:25:11 PM

Quote from: dug on December 02, 2009, 04:53:08 PM
thanks Mt. Don, I wish I would have known that, then I would have done the same as you. Seems very odd to me as most buildings are on 2 ft. increments (or so I thought). Guess I don't make the rules though. One more question- I measured the panel lengthwise and it was exactly 8 ft. If I leave an 1/8 in. gap then I would have to cut that much off, or maybe 1/4 every other piece. I didn't think you would have to do this but I don't see another way.

I would make the complete run lengthwise and if there is any left over use a chalk line and set the depth of cut on your circular saw and make the cut. Normally on wider structures the contractor has had to deal with this issue before and will cut a half sheet to run the length and when he arrives at the other side uses the other half sheet to finish out by reversing so he has a tounge to use.

dug · December 02, 2009, 07:55:56 PM

Quotetrimming of the sheets in length is usually due to layout errors on the framing, or even bowed boards. If you have a big floor to do, it can help to make sre that you measure the distance from the end of the sheet to the center of the joist, and move it into proper place before nailing. Often the joists tend to drift. You want to leave about a nail thick gap on all sides,

The thing is I was real careful to make sure they (the joists) were dead on. They are also attached on a center beam and lots of blocking, so they line up right wherever I measure. With exactly 8 ft. lengths and either 3 or 4 breaks (depending weather or not I start with a half sheet) I will gain 3/8 to 1/2 inch over the 30 ft. run (leaving a 1/8 gap). No big deal- but I will have to trim a sheet or two.

MountainDon · December 02, 2009, 08:36:39 PM

Same here dug, those 1/8 inches add up.

Jens · December 02, 2009, 11:27:03 PM

Quote from: dug on December 02, 2009, 07:55:56 PM

The thing is I was real careful to make sure they (the joists) were dead on. They are also attached on a center beam and lots of blocking, so they line up right wherever I measure. With exactly 8 ft. lengths and either 3 or 4 breaks (depending weather or not I start with a half sheet) I will gain 3/8 to 1/2 inch over the 30 ft. run (leaving a 1/8 gap). No big deal- but I will have to trim a sheet or two.

I have not had that problem, and in much longer runs. Lucky maybe. It is amazing how quickly a framing layout can get off, and will be almost unnoticeable. Can't forget to subtract the 3/4" from your first layout mark either, that'll get ya from the start.