plywood for basement subfloor

14 years ago

I am finishing my 1600sqft basement. My basement is dry most of times/yrs. I added a new interior French drain system for 100yrs storm. I didnÃ¯Â¿Â½t see any water when I dig down for the sump pump basin. The outside surface water was also addressed 2 yrs ago. So I assume the basement is safe for finished floor.

I am planning to put 1" XPS foam boards on top of the concrete floor, then cover them with plywood subfloor and then laminate/wood/ceramic tile (bathroom) floor.

I need your advice on the plywood subfloor first:

1. WhatÃ¯Â¿Â½s the thickness I should use, 15/32", 19/32" or 23/32"?

2. Should I use the sanded plywood or the regular plywood is just fine?

3. I should use the subfloor plywood but not sheathing plywood?

4. At homedepot there is a subfloor plywood called DryPly(http://www.homedepot.com/Lumber-Composites-Plywood-Sheathing-Subfloor-Plywood/h_d1/N-5yc1vZbu0m/R-202522977/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051) which has a water-repellent coating. I should use it or avoid it?

5. With the T&G, can I install the plywood as a floating subfloor without Tapcon to fix them with the concrete floor?

6. Where should I leave some gaps for plywood expansion or no need them at all?

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Comments (6)

rmtdoug
14 years ago
Hi James, I did something similar 15 years ago when I converted my garage with concrete floor into a woodworking shop. I first laid 6 mil plastic on the floor and sealed any seams with construction adhesive. I did not seal the outer edges of the plastic at the walls so any moisture would move into the walls instead of pooling in the floor.

I then laid 2x4 stringers on the flat (1 1/2" thick) on 16" centers and nailed them into the concrete and insulated with rigid foam between the stringers. Over this, I screwed 3/4" exterior rated OSB T&G. I planned to put a finish floor down but ran out of funds, so I painted the OSB with grey floor paint. 15 years later, there is no sign of moisture damage and the shop stays at 45-50% humidity year around here in the rainy NW coast. I havent even worn through the floor paint yet. OSB is amazingly durable stuff.

So, if your moisture problems are taken care of, I think you should be okay with basic exterior grade T&G suitable for whatever finish floor you will lay. No need to gap it as that is built into the product.
brickeyee
14 years ago
Below grade in contact with masonry is pressure treated land.

Below grade masonry is ALWAYS wet.
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rmtdoug
14 years ago
Even pressure treated wood will not stand a chance if there is liquid moisture. The wood will rot eventually. That's why I qualified my response with "if your moisture problems are taken care of."

Wet is a relative term. As I understand it, as long as any interior surface does not fall below the dew point so that water vapor condenses, no mold can grow or rot can get started. I have 100 year old untreated wood on my basement subfloor that is as sound as the day it was laid. That's because no moisture has ever condensed there and the basement has never leaked.
james_007
Original Author
14 years ago
You are right. Water and moisture issue should be addressed first. I will put a 10mil plastic vapor barrier on top of the concrete floor. Seal it around the wall, and then put the 1" foam. So the AdvanTech floor should be ok then.
brickeyee
14 years ago
"Even pressure treated wood will not stand a chance if there is liquid moisture. The wood will rot eventually. "

The correct pressure treat would be fine.

Pressure treat is used for direct earth contact, and even bulkheads and dock piers all the time.
rmtdoug
14 years ago
Brikeyee, although I agree in principle with what you are saying, I have seen ground contact pressure treated wood rot completely through in 10-15 years under the right conditions, i.e, continual damp/dry cycles in a temperate climate. I would never trust PS wood exclusively. For the garden variety ground contact wood one buys at Home Depot, as soon as you cut it, you loose all benefit of treatment where it's cut.