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Dave321 09-16-2009 03:22 AM

Subfloor repair
OK, I just joined and this is my first post so excuse me if I fall out of line.

I have a house that needs some repairs do to water damage in one of the bathrooms. The bathroom had a soft spot in the floor and the hallway out side of the bathroom has a raised spot approximately 1 foot in diameter and an inch or so high in the middle. The bathroom had linoleum flooring and most of the house was carpeted. Since the carpet was old and musty smelling, I removed all of the carpet and pad in the house. Once the carpet and pad were removed I found 3/4" partical board on top with a layer of 1/2" plywood nailed directly to the floor joist (underneath the partical board). This joists are 2x12's, 16" OC and are on top of a cement block foundation with a crawl space underneath.

It appears that the leak from the bathroom migrated across the hallway (underneath the partical board) and slightly into the den area where I can see indications of the partical board having water stains.

So I am now in the process of cutting out all layers of subflooring in the bathroom since much of it was rotted due to water damage. I removed the sink / vanity and the toilet and have most of the flooring out (I can see through the crawl space down to the ground). Some of the damaged plywood goes to the wall studs. My plan is to use a jigsaw from underneath the bottom layer of plywood to cut the bottom layer right up against the base plate of the wall stud. I want to get as much bad wood out as possible. I am going to have to do further investigation under the bathtub and of the subfloor layers in adjacent rooms.

For the bathroom, I am going to have a few edges of the subfloor that have been cut out which run paralled to the floor joist which will need support. For the supports I was thinking of using 2x12's along the cut seam in between 2x12 blocking supports. I can probably use 2x6's for these but I figure the bigger the better. Since this is in a bathroom (which always has potential leaks) should I use treated pine, regular pine or something else? Can I just use deck screws (3"-3 1/2" screwed through existing joist into the ends of blocking pieces) to attach the blocking pieces to the existing joists? Can I use deck screws to attach the supporting joist (which will run underneath the cut seam) at the ends to the blocking pieces or should I use joist hangers? The house has valted ceilings and the surrounding walls don't appear to bear any load other than the wall itself.

When replacing the bathroom subfloor, I was thinking of using 3/4" TG plywood to lay on top of the floor joist. My other thought was to use 3/4" treated plywood on top of the floor joist but these tend to warp and I am afraid that the floor won't be as level as needed. What is the consensus here?

Since I plan on putting ceramic tile in the bathroom (eventually), I was going to place 1/2" cement board (I know there are several types) on top of the 3/4" plywood or should I put another smaller layer of plywood on top of the 3/4" layer and then use 1/4" cement board?

I was going to use deck screws to secure the 3/4" plywood to the floor joist and deck screws to secure additional layers making sure they were long enough to go through the floor joist as well.

I was going to use flooring adhesive between floor joist and bottom sub floor but what if the subfloor ever needs to be replaced? It seems like it would just tear up the top of the floor joist.

Back to the flooring in the rest of the house. I am going to have to remove several pieces of the partical board due to water damage. I plan on putting in laminate flooring eventually. I talked to the guy at the flooring store and he recommended that I replace all of the partical board with plywood. I am willing to do this but wanted to make sure that I wasn't waisting my time and $. Since the plywood underneath the partical board is 1/2", he said that replacing the partical board with 1/2" plywood would give a real solid floor. I wanted to use TG plywood to provide a nice level floor for the laminate flooring but this doesn't come in 1/2" at the local stores and the 3/4" TG plywood isn't much more than the 1/2" plywood as I recall. So, do you think adding a layer of 3/4" TG over the 1/2" plywood is way overkill? Again, I was going to use deck screws to attach the second layer of plywood to the first going through the floor joist as well. Then there is the question of flooring adhesive. Is this definitely the way to go in the long run. If it ever has to come up, it will probably make a mess of the bottom 1/2" plywood.

By the way, the water damage in the bathroom appears to have come from the shower. I plan on cutting out the wall behind the shower to investigate the plumbing before the water to the house is turned back on.

Sorry for the long post. I was trying to get as much detail as possible and I am sure I managed to leave out some important details or ramble on without getting to the point. I will be glad to provide photo's as needed.

Thanks in advance!

Scuba_Dave 09-16-2009 07:48 AM

Particle board or OSB?
Photo's always help

I use just plain 3/4" plywood for sub floor
T&G will help keep edges inline with each other
But it can be a PIA to install if you haven't done it before
Make sure the new sub floor is supported near the walls where you cut
Blocking if perpindicular to the wall, another joist if parallel
When I did my last bathroom I put in new level joists & went to 12", small bathroom - maybe 4 joists in all were added
I left the old joists in place

I've never used treatewd wood inside, with the exception against concrete
You can seal sub floors with different methods - Redguard is one

3/4" on top of 1/2" is fine, sturdier floor
Does the crawl space have plastic against the ground to prevent water vapor from coming up?

Dave321 09-16-2009 12:52 PM

Thanks for the speed reply Scuba_Dave.

I'm 99% sure it is partical board. It will crumble into saw dust if you pound chunks of it a few times with a hammer.

If I go with two layers of plywood for the bathroom subfloor, does it make the most sense to lay the thickest layer down first?

As far as installing the blocking supports and new support joists. Is it ok to use deck screws for the blocking supports and joist hangers for the new support joists?

I recently had a 6 mil vapor barrior installed. Although, after inspecting the work, I did notice that there were uncovered areas in between the various concrete supports in the middle of the crawl space. The crawl space has a sump pump which is piped into the sewer line but the lot drainage isn't optimum. Because there was some fungus growth visible on the under side of some of the plywood subfloor and a few joists (in the area of the bathroom), I had it sprayed with a product that kills fungus and prevents any new growth. All of the floor joists look great. I'll have to further investigate the remaining subfloor panels for any deterioration. Probably more than you needed to know.

I can post some pictures after doing some more work at the house this weekend.

mvesr 09-16-2009 08:59 PM

Repair subfloor
I have a particle board subfloor also. I want to put down a lock and fold hardwood floor over it. There is some degrading of the particle board. I really don't want to cut it all up. Can I install the floor over this or do I need to put plywood down over the particle board?

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