I just finished laying down a subfloor in my basement. Here is what I did:

over concrete slab (fully sealed with radonloc cement sealer)

2X4 pressure treated sleepers nail gunned into conrete at 24" o/c ( 3 1/2" side lying flat )

1" styrofoam insulation dropped in between sleepers

19/32 tongue and groove plywood screwed to sleepers

I think I may have made 2 mistakes...

1-- I butted the t/g plywood pretty tightly in most spots. I didn't read until later the 1/8" gaps should be maintained.

2-- 24" on center---is that too wide a span? I assumed since its over cement floor and I'm using 2x4s lying flat---so really 3 1/2" wide surface acting as sleeper, the 24 o/c is OK.

Floor feels extremely sturdy with zero noticeable spring to it when walked on.

My concern stems from the fact that mid-stream we decided that we might want to finish with pre-finished hardwood flooring (instead of carpet). Am I OK with 24 o/c spans? What about the lack of 1/8" gaps? Is there a real danger of buckling?


I'll unscrew enitre floor and redo if need be, but that would really be a bummer... approx 300sqft area...

Most radonloc sealer will block nearly all of the moisture normally coming through the concrete floor. There goes most of your worry about buckling. You could just set your circulayrsaw for 19/32 and recut along the seam between each sheet of plywood. You now have a new expansion gap. Your hardwood floor will further stiffen the floor if it is laid perpendicular to your sleepers.

Thanks ballengerb1---after looking over your assurances regarding radonloc I think I am just being paranoid. The basement is bone dry. I have a forced hot air heating system that includes a humidifier to keep the air from getting dry in the winter. I also have central air for summer... so temps and moisture should remain within reasonable ranges year round.

Now for the bathroom area where I have yet to cover the concrete, I suppose I should do the 1/8th inch gap for seams and then go over the plywood with some cement backer board before tiling?

It just seems odd that tongue and groove would require gaps... seems kind of counter-productive to the whole idea.

The tongue and groove tend to swell and not shrink so the spacing helps hold down a cahnce of buckling. Screw and glue 1/2" Hardibacker to you sub floor in the bath. Glue with modified thinset and screw with Hardibacker's special countersinking screws. That isn't the only company out there that makes a good backer. I would not use cement board on the floor but it is fine for walls.

OK... so, just to be clear... I would leave 1/8th inch gap on all seems. I will still push the tongue into groove but leave the 1/8th inch for where the top surface of plywood would meet with each other, correct?

You are correct. The 1/8" gap is between the surface edge of the plywood, not the tongue total 1/8" from the groove.

You might need better support for your bathroom tile, perhaps 16" OC and 3/4" plywood under 1/4" Hardibacker,

Hi dclynch,

I just read your response after the fact, but as luck would have it I did go with 3/4" and 16 O/C for the bath. Thanks...

Ideally Id Like To See 16 On Center, As Far As Tong And Groove Your Fine With That, Did You Glue It Down? You Need To.
As Far As Flooring Id Go With A Full 3/4 Floor And Run It The Opposite Direction For More Support. And Use Stapels For That. Make Sure To Put Down Black Paper Before You Floor It.

As Far As Carpet Goes Instead Of Taking Up The Floor You Can Go Over Top Of It With Chip Board Or Press Board Which Most Use For Carpet Underlayment.
Hope That Helps , Nichols Trim And Carpentry