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Forever_Remodeling 10-05-2010 12:29 PM

Remodeling my 1st floor powder room. Gutted everything. The floor is 3/8" plywood subfloor. Wall plates etc built on top of that. Then there is a layer of 3/4" particle board. On top of that was 1/4" plywood underlayment that had the vinyl flooring on. I pulled the underlayment off easily. When cutting out the 3/4" particle board with my sawzall, I cut into the 3/8" plywood in a small area. The cut is between and parallel to the floor joists (16" oc). Do I need to worry about this since I will be putting down 3/4" plywood over top of it anyways? My guess is that the 3/8" plywood subfloor is simply a "spacer" to help the vinyl flooring match up to adjoining hardwood floors?

budro 10-05-2010 01:24 PM

make sure the 3/4" plywood you put down covering the two joists on each side of the cut is from one piece and you will be fine. squirt a bead of liquid nail in the cut to repair the damage. budro

remmons 10-06-2010 02:46 PM


Originally Posted by budro (Post 49183)
make sure the 3/4" plywood you put down covering the two joists on each side of the cut is from one piece and you will be fine. squirt a bead of liquid nail in the cut to repair the damage. budro


Hopefully, you did not cut into the floor joist. If you did, you will have to call a Home Improvement Center or lumber yard for what's called a Joist Repair.

Forever_Remodeling 10-28-2010 07:49 AM

Thanks! Next question. The 3/8" subfloor that I mentioned goes under the wall plates. On one side of the room, the joist is located back under the wall plate. The next joist out is about 14"- 15" OC. But, from the edge of the bottom wall plate to the joist (the area where the 3/4" plywood would sit) is only about a 10" - 11" span. Is there any concern when I glue and screw in the 3/4" plywood since I won't be hitting a joist along this edge of the wall? I can't add any cross bracing, etc in this joist bay because ductwork is running in it. If I glue and screw directly into the 3/8" plywood, does this give sufficient support? The 3/8" is pretty solid in this area.

The plan is to glue and screw 3/4" plywood on top of the 3/8" plywood floor that runs under the walls. On top of this to add 1/4" cement board for tile. The powder room is about 4-1/2' x 5-1/2' in size. Goofy size. I cross braced in most places (especially under the toilet) to help solidify the floor, but my concern is a lack of a joist to screw the edge of the 3/4" plywood along the wall as mentioned above.

Jaz 10-28-2010 03:54 PM

You will not get much "bit" into 3/8" ply. for the new 3/4". Might be better to remove some of that 3/8", add blocking, put it back, then install the new sub.

Normally the new top ply (underlayment) is fastened only into the subfloor. In this case you should pretend the 3/8" is not there, except I would still add some screws between the joists.

Don't forget the thin set mortar under the concrete backer, tape joints etc.


Forever_Remodeling 10-29-2010 09:35 AM

Thanks! Do I need to leave any sort of "expansion joint" for the 3/4" plywood? I will be using only 2 small sections (roughly 2'x3') with a seam down the middle of the room....this will run perpendicular to the 3/8" plywood. I did notice that the 3/8" does not have any expansion joint where it is butted against each other, so am not sure if the same holds true for the 3/4".

Lastly, when I install the 1/4" cement board, does the cement board also need to be screwed into the joists or just into the 3/4" plywood? I also will use thinest mortar between the 3/4" plywood and cement board, and stagger the joints --- although there shouldn't be no more than 1 or so.

Jaz 10-29-2010 02:10 PM

Yes leave about 1/8". Be sure to run the grain across the joists and it's best to hit at least 3 joists. So about 32" min. with 16" o.c.


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