Floor Underlayment for new flooring on exisitng slab-on-grade

[ajk1]

1. Is normal 3/4" thick plywood underlay sheathing (for flooring that will be placed on top of it) dimensionally stable in a room environment that varies from 20% relative humidity in the winter to 80% relative humidity on humid summer days? Room temperature varies from about 67 deg. F to 73 deg. F. These RH and temperature measurements have been made in the room over the course of a year. By dimensionally stable, I mean that it will not warp, expand or shrink significantly in this environment.

2. Is exterior grade plywood more dimensionally stable the interior grade?

3. Is there any other material that could be used for this purpose (flooring underlayment sheathing), that would be more stable than plywood in this environment?

The area where this plywood will be put is a portion of the total floor srea; this portion was originally located under a wooden stage area. That stage has now been permanently demolished exposing the top of the original concrete slab-on-grade. It is this area where we now want to place the plywood to level it up with the the top of the old flooring in the remainder of the room; then we will place new flooring in the entire room. The entire room is about 30 feet by 80 feet; the area where the plywood infill sheathing would be placed is about 20 x 20 feet.

 

[SlideRuleEra]

ajk1 - IMHO, plywood would not be the best choice for underlayment for this application. The reason has to do with difficulty anchoring the plywood underlayment to the existing concrete. As I understand the description, what is needed is a 3/4" thick "filler" between the existing concrete and the bottom of the new finish flooring, suggest that you take a look at a cementitious self-leveling underlayment. Here is a typical product:

http://www.custombuildingproducts.com/TDS/TDS-108.pdf

As for your other questions, plywood is reasonably dimensionally stable, giving good results for various applications such as traditional subfloors and concrete forms. However, for these uses the plywood is securely anchored at fairly close intervals (say, nailed to joists spaced at 16"). Even so, plywood sheets should have a small gap between sheets (1/8" to 1/4" gaps are typical) to allow for expansion. Without a gap problems are almost guaranteed.

For a given wood species, the only real difference between exterior and interior plywood is the glue. Exterior plywood uses glue that is more water resistant. Dimensional stability is determined by the properties of the wood, not the glue. Therefore I would expect both exterior and interior plywood to be comparable in dimensional stability.

Additionally, the existing concrete floor may not be very flat; using a self-leveling underlayment will solve this problem... if it exists.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[ajk1]

SlideRuleara - thanks for your advice. My thinking in fact parallels yours, including the idea of using a cementitious material for the fill, but it is good to get your advice on this. I was aware of the need for the gap between plywood because it is in the Ontario Building Code where they say 2 mm minimum gap. By the way, I am also from the slide rule era! Thanks again. Very helpful and much appreciated.