In the middle of a large great room where I am installing a nail down hardwood floor, there is a raised hearth fireplace situated on the outside wall. The sub floor has a pretty good sized low spot across the width of the front of the hearth. It starts from about the middle of the room, where a beam is located in the basement, and slopes towards the hearth. The low spot is due to a framing issue that would have taken a great deal of reconstruction to correct.

I have been able to minimize the low spot using shingles to shim it, with 3 thick at the lowest point. The hearth is undercut enough to accept the flooring with the shimming and tuck nicely just under the brick. Here is my problem, since the hearth is raised, I won't be able to use a flooring nailer the last few rows. I do have a palm nailer that accepts cleats that will get me closer but I will still have a couple of rows, maybe three, that I won't be able to nail unless from the top. Normally, I would glue the last couple of rows but with shingle shimming that won't work. I would prefer not to top nail for cosmetic reasons mostly but also because of the shimming shingles underneath, especially with finish nails. I was thinking it might be acceptable to use a layer of 5/16' plywood, instead of the shingles, under the last couple of rows and then glue to that?

Would I glue and screw the plywood? What type of glue would be best, subfloor, carpenters glue, or poly construction adhesive?. I use Loctite PL for gluing the last couple of rows.

Thanks for any insight offered.

We shim larger areas with plywood quite often. Screws and subfloor adhesive is what we use to fasten the plywood and we always top nail the hardwood. the only time we glue the flooring is under a toe kick where we simply can't nail.

Thanks for the reply Willy. Is there any reason you choose to top nail instead of gluing? I usually glue my last couple of finishing rows just to avoid any top nails where they will be visible. This is with pre-finished flooring. Is this case, because my last rows will be ending at a focal point, (the fireplace hearth) I'd prefer to glue over top nailing. Additionally, I don't think 15 ga 2" finish nails have the holding power that cleats do. The flooring will tuck slightly under the brick with room for expansion. Which brings me to another question/concern: expansion.

A lot is made about leaving room for expansion at the perimeters, especially across the width. I've left 1/2 to 3/4 at the width, with approx 3/8" at the hearth behind the brick, which is across the width. The flooring manufacturer suggested 3/8" as sufficient. I've left about 1/4-3/8" at the sides along the length of the boards, less at a couple of doorway transitions to tile. Whether top nailed, glued, or both, it seems that the last rows aren't going to allow much if any movement. So where does expansion occur? Some say (Ken) the flooring moves as a whole unit. Others say, only the last couple of rows in a climate controlled environment. I've also read that most movement will occur in the direction of the tongue. I know there are regional differences too, I'm in the Midwest, 4 seasons. Most say not too worry too much if the wood is properly acclimated and the climate is controlled. Any experiences related here would be appreciated.