Install prefinished hardwood over plank subfloor

Dmeyr

We're installing prefinished hardwood and have a diagonal plank subfloor (6" wide t&g planks). Should we install a layer of 3/8 or 1/2 plywood over the top of the diagonal subfloor before laying the hardwood? I know that we're going to need to do some work with the planks so that we make sure the subfloor is as flat as can be for the prefin, but not sure if there is added benefit to using the additional layer of ply. Floor height change isn't a concern for the way these rooms join other areas.

So far our baisc plan is: repair any iffy spots on the planks, add furring strips where we can, and then screw all the planks into the joists. We'll go the shingle route for any areas that we can't get to from below to add furring strips.

joecaption

I would go ahead and add the 1/2 plywood and also screw that down.
Want a really solid floor, remove the old slat boards and install Advantec 3/4" subfloor. That way it would be 1/2 lower, if installed by using constrution adhesive on the joist and ceramic coated decking screws there will be no flex, no squecks and will be rock solid.
The old slats can be cut out along the walls with a Toe Kick saw.

Dmeyr

Is there a particular grade of 1/2" plywood that you would use? Assuming that any type of osb or particle board would not be effective.

joecaption

Look for underlamant rated.
That will mean it's exterier glued, filled voids in the core and filled voids in the top and bottom layer.

Dmeyr

Perfect - thanks so much for the help!

Janetp

Trust me on this one....put down at least a 1/2 inch underlayment or your wood can go off kilter and you can eventually get gaps, possible cracking, and squeeks.It's worth it to spend a little extra now than to be unhappy with it later.

RhodesHardwood

I would just screw down the existing subfloor really well and install to this. We do it all the time when the client can not have the additional height added and have not had issues.

Dmeyr

Yes - we definitely are screwing down the planks, but after remeasuring and scratching heads a bit more we're thinking the 1/2' ply would really help us with a height transition into our kitchen.

But...will we have a problem getting a good tight install on the 3/4" hardwood in the areas where we have shingles under the 1/2" ply? We were already going to use the 2" staples for the hardwood - but in those areas where we have 2 shingles, then 1/2" ply on top, I'm concerned that we won't get a good tight fasten on the hardwood to the subfloor. Any ideas?

Janetp

If it were my project, I would make sure the plywood is securely fastened to the shingles by using plenty of screws, and them you will have the solid foundation we are looking for. As long as that is solid, I would not expect any problems. Just make sure the floor is level. In another home, I had to use a 1/4 ply, taped off all my seams, used a liquid leveler, and then added a 3/8 inch to that. It worked great and for the next 9 years I lived there, not a gap or squeak to be found! THe floors sold the house for me.
Good luck!!