Installing plywood over 1x6 subfloor in preparation for engineered hardwood

Question

I’m prepping to install ½” thick Armstrong Engineered Hardwood 7” tongue and groove planks in the kitchen and family room in our 1964 home. I have several questions about preparing the subfloor and here are some details.

There are 1x6 planks running perpendicular to the joists and fastened with T-nails. There are gaps between each board and the floor squeaks badly, partially due to removing ¾” hardwood flooring from a portion and removing 1/2” OSB from another portion. I’m planning to use ¼ or 3/8” CDX as another subfloor/underlayment to cover the gaps and raise the floor to better match the height of adjoining ¾” hardwoods and some tile.

1) I plan to screw each subfloor plank down to minimize the squeaks (2” #7 or #9 screws).

2) What construction adhesive is best to use under the plywood?

3) I’ve read to only attach the second subfloor to the planks with staples (to decouple), not all the way to the joists. And leave gaps between the sheets and around the perimeter.

4) Do I run the plywood sheets perpendicular to the planks?

5) With the planks and plywood, can I run the 7” Eng HW either direction? Thanks

Answer 1

1. Good. 2-1/2" screws are less likely to strip at the joist connection if that proves to be a problem. You need the screw heads below flush, and sometimes the plywood is hard enough that the screw will strip before it pulls in.

2. If you're asking for brand names, that's off-topic here. As you're indoors, almost any will do. "Heavy duty" adhesives are solvent-based and bond the best. "Project" adhesives are water-based so they don't dissolve foam, but would still be fine. 1/4" beads on a 8" interval would probably do well.

3. This is debatable. I've never concerned myself with decoupling in 30 years of working on homes new and old in Minnesota's wildly varying climate. This site is the first place I've heard of doing so. As long as all layers are stuck down securely, it probably doesn't matter, but I'd be inclined to pinch the original subfloor to the joists with all fasteners to reduce the likelihood of lingering squeaks.

4. Yes, if by "planks" you mean joists. All layers of subfloor are typically run crosswise with respect to joists. Joints are staggered where appropriate.

5. Yes. That's a design decision and doesn't depend on the subflooring direction.