Sheathing grade ply under hardwood

therishel · 07-23-2006, 07:38 PM

I am installing 9/16 cypress over an above-grade concrete slab. My plan is to put down a 6 mil plastic vapor barrier, then two layers of 3/8 ply fastened only to each other, then 15 lb roofing felt, then my cypress. I have purchased 3/8" sheathing grade ply for my sub-floor, but I am concerned about the lack of flatness of the panels. Because I am planning to float the floor, the sub-floor will not be fastened to the slab. I can certainly flatten it out before I fasten the panels to each other, but should I buy luan underlayment instead of sheathing ply? Thanks in advance. -Tom

Floorwizard · 07-25-2006, 01:39 AM

you can float over the 6 mil poly and concrete.

no need for ply.

unless there is something I do not understand

therishel · 07-25-2006, 09:17 AM

Thanks for the response Florcraft. The planks that I am using are solid wood cypress with square edges (ie. no tongue-and-groove). They are 5" and 7" wide. I plan to face-nail them down to the plywood subfloor. Is there a way to float this type of floor over the 6 mil poly securely that I am not aware of? Thanks.

Floorwizard · 07-25-2006, 11:34 PM

Quote:

Because I am planning to float the floor,

sorry, thought you meant float the hardwood.

no, there is no way to float it when it's a solid.

therishel · 08-07-2006, 10:11 AM

I installed the sub-floor this weekend. I ended up going with 3/4" CDX over 15 lb roofing felt and nailing it down using Ramjet powder actuated fasteners. In some places I had to use multiple fasteners to get a solid connection, but I feel pretty good about it. The instructions on the Ramjet say that the fastener must penetrate the slab at least 1". The fasteners come in 1 1/2" and 2" lengths, but not 1 3/4", so I had to choose either 1/4" too short or 1/4" too long. I had already tried drilling holes for TapCons and discovered that my slab is VERY hard, so I decided to go 1/4" short. I think it was a good decision, because, using the highest power charge for the Ramjet that I bought (a .22 cal.), over half of the fasteners did not drive down flush. I had to drive them the rest of the way with a hammer. I used 9 fasteners in a 4 x 4 panel unless something felt loose. In that case I added more until it all felt solid. I put another layer of 15 lb felt over that and am now starting on my layout.

I know this is a rather old-fashioned installation technique, but it is time-tested and, if something does go wrong ten years from now, I know that I can repair it.