Home Improvement--General/flooring substrate for masonry on wood floor
I have about a 3 x 5 foot section I want to lay field stone and mortar in to make a hearth for a faux fireplace. We have a gas stove there and the wife wants it to look like a real fireplace. I will apply moulding and faux stone to the wall, but my chief question deals with the floor. I have cutout the 3x5 section under hardwood flooring. The substrate is now 3/4" plywood over engineered trusses on 2' centers. What do I need to lay down to accept and hold the ~3" flag stone and mortar? Can I mortar it directly to the plywood?
First, if you are able to, stiffen the subfloor by installing one or two 2x4s under it, so your framing spacing becomes about 12". 2' centers is too much spacing for a proper tile/stone installation over 3/4" plywood. Glue the 2x4s to the underside, and screw them through the top through slightly oversized pilot holes and pre-countersink the hole, so the screw heads are slightly below the plywood surface.
To install the stone, set a sheet of 1/4" cement tile board or Hardieboard in a bed of thinset mortar using a 1/4x1/4 notched trowel, held steeply, like 75 degrees or more. Trowel out the mortar tightly, so there is just enough to fully adhere the tile backer board. Thinset mortar is mortar with an acrylic additive that provides adhesion and flexibility.
Set the board in place and nail down at 6" centers using galvanized roofing nails or special tile board screws. This becomes your stone base. It stiffens the floor and provides a much better surface to adhere to.
Then install stone with the same mortar and trowel. Leave a small gap (1/8") at the hardwood flooring, so if the flooring has seasonal movement it won't tend to pop the stone, it may disturb the grout, but that's an easier fix. Alternatively you could rake out the perimeter grout while grouting, and fill the edges with a matching tile caulk (after the grout has set).
Good luck , and let me know if you need anything else.