Hi,

I'm new to the site, but I have had quite a bit of experience installing floors.

I am preparing to put down a hardwood floor over an existing 1/2" subfloor which is installed on 1/2" furring strips over a relatively thin (3" - 4") layer of concrete.

My first step is going to be to add an additional 5/8" or 3/4" subfloor in order to bring up the finished floor an existing tile floor in the next room (it's an old house).

When I removed the old carpet in the room I noticed that there is an area of the subfloor, alongside an inside wall, that has water stains on it that change from day to day. They extend outward from the wall about 4". The wood is just barely damp to the touch. Since we've had very little rain lately, I've checked for leaks in the plumbing etc. and can't find a source for the water stains. I'm thinking that it might be condensation that is being drawn up thru the concrete and/or from the adjacent tiled bathroom floor. This wall is 6 feet from any outside wall.

Since the water staining never reaches into the room by more than 4" and it's almost never wider than a foot or two I have decided that I'll just have to live with it. The effected area is roughly 6' wide.

My question is this. Should I put some kind of wood waterproofing material on this section before I cover it with the new subfloor? Is there one type that anyone could suggest?

My 2nd question is this. Since I already have a solid 1/2" subfloor can I fasten the next layer using 2" nails & a finishing gun? It's a very small room (10 x 10) or do I have to screw it down?

Thanks for any idea's or suggestions.

Mike

Mike,
If you install the wood floor over the moisture affected area, you just bought it. You need to get a moisture meter and check that area in relation to the surrounding areas, and also find the source of the moisture prior to your installation.

Believe me I've tried but there's no way to trace its source. Where this 10' wall is showing the water staining is about 6' in from an outside wall, in a room with an outside dimension of about 16' It's weird. It's not there every day. I haven't been able to ascertain any type of pattern as to when it will show up.

I have a feeling it has something to do with the adjacent room, which is ceramic tile over concrete/mud. It might be that the tile is gathering condensation from the concrete/mud (which is only about 3-4" thick) and where the tile & subfloor meet the subfloor might be wicking that condensation away.

I think I'll just have to put a good layer of poly plastic along that area & hope for the best.

Wood that is above 19% moisture content is going to rot. Check the wood sub-floor before you put plastic down which will trap moisture that is coming from somewhere. You need a moisture meter to get the percentage of water content.
If your floor is damp you will be replacing it sooner or later.
Dry rot needs moisture to grow. If the sub-floor is dry there will be no dry rot.

It's amazing how moisture moves.

How high up is the stain?

Assuming you only noticed it when you removed the carpet I am guessing the stain is where the wall meets the floor?

If so you could run a DPM under the floor as mentioned to protect the floor. But you then need to float the floor as you cannot nail through this , so you are better to use a quality engineered board as they are more stable when floated.

Ideally though get a survey done to find the source of the damp and see if it can be fixed.

I would recommend finding a home inspector or maybe a flooring inspector weho had a FLIR infrared imaging camera.

http://www.nachi.org/ir-cameras-inspecting-moisture.htm