I plan on installing 3/4" tongue & groove pre-finished hardwood plank (3 1/2") floor in a first floor family room. This room is roughly 11' - 0" X 17' - 0" and has a 3/4" OSB sub-floor. Joists are 2 X 10, spaced 16" OC. The ground beneath this section of my house is better that 3 feet below the bottom of the joists and is covered with heavy mil clear plastic (and 17 years of dust!). 5 years ago I installed a supplemental header mid-span of the 12 foot long joists to take the "bounce" out of that room, so the floor is well supported.

Because an adjoining tile floor (with its cement backer board) stacks up to approximately an inch, I expect add a 1/4 plywood layer to the OSB subfloor for my hardwood project so that the resultant stack up is close to that of the tile floor.


Q-1: Is there an alternate 1/4" material which is less expensive that would adequately substitute for the plywood?

Q-2: What is the recommended fastening technique for joining the 1/4" plywood (substrate) to the 3/4" OSB?

Q-3: What is the recommended orientation of the 1/4" plywood (substrate) with respect to the 3/4" OSB? Worried about expansion/contraction of the OSB AND the plywood!

Q-4: Should I plan to leave an 1/8" expansion gap between the edges of the 1/4" plywood substrate?

1) Not that I am aware of. You do not need the best grade. Shop around for a deal on that plywood. HD and Lowe's often have the lowest price on that type of product.
2) For 1/4", I would glue and staple.
3) Avoid "marrying" the seams. I usually run my underlayment in the opposing direction as well.
4) That would be a good idea.

Please explain what you mean by "do not marry"?

In construction, to "marry" one board or seam to another is to join them, to place one on top of the other. It is considered poor construction practice to line up seams or joints. By off-setting those seams, it makes for a stronger and more stable application. I hope I explained OK.

That's what I thought. I was also thinking short screws and construction adhesive to join the 1/4" plywood to the 3/4" OSB subfloor. Do you have any improved ideas?

Glue and screws are good, expensive and time consuming, but good. I find that for 1/4" plywood underlayment, staples are very fast and work well. Around here, that is what the lino installers always use for vinyl underlayment. When I staple down underlayment, I staple 2" on the perimeter and a 4" grid in the field.

When stapling, start in the corner, and use a fanning fastening schedule, Although the first sheet, I always start in the middle and fasten it like tightening a head gasket, around in circles.

A good stapler with the ability to bump fire is a plus!!

That's right, Floorguy. One needs to start in the center and work toward the edges so you don't get a bubble in the middle, especially when working with that thin 1/4" material. Or work from one side toward another, as you mentioned.

Which 1/4" substrate should I choose under my 3/4" T&G plank? Heard the Meranti behaves un-predictably with certain glues!

In either case, would I even need to "fasten" the 1/4" substrate at all?!?! When I install the T&G, those staples will/should penetrate the T&G, the 1/4" substrate. and into the 3/4" OSB subfloor. Why fasten the substrate at all????