Reclaimed Wood Flooring Over Concrete

Advice on vapor barrier installation and attachment method for an unusual floor assembly. May 19, 2011

I want to lay a longleaf pine floor, using material salvaged from an old warehouse. The wood was milled in 1909, is 3/4" x 5" T and G and in excellent condition as it was used on the warehouse walls. There are no grooves cut into the underside of these boards as is the norm with solid wood flooring. It will go on a concrete slab that I will first seal with a concrete sealer, nail and glue 3/4" subflooring to the slab, and be edge nailed to the sub-flooring. The job site is in a semi-arid zone with three-four months of very hot weather and 40-70% RH’s. The building will not be air-conditioned. Could the lack of grooves cause any problems in terms of stability or cupping?

Forum Responses
(Architectural Woodworking Forum)
From contributor Y:
It wouldn’t hurt although being 100 plus years old it has probably forgot it was part of a tree by now. One thing I would be concerned about is mold during those few months. Even sealing the concrete you could still get some moisture between it and the subfloor just from condensation. Is there any room to put sleepers down to provide some airspace between the concrete and subfloor?

From the original questioner:
No, there is no margin for adding sleepers. I could put down a moisture barrier on the slab, but then I would not be able to glue (and nail) down the subflooring. I don't trust just having nails holding the subflooring down.

From contributor G:
Here's a Knowledge Base article that may be of help.

Why is Wood Flooring Kerfed on the Bottom Face?

From contributor M:
Could you use the moisture barrier between the ply subfloor and the flooring? Why put such a niece floor in a non-ac'd building with such a hot climate? What's the building’s use?

From the original questioner:
The application is a residence; house design is "climate appropriate" to do without a/c (it will be heated).

From contributor S:
If it were me I would lay a vapor diffusion retarder down first and then Tapcon the plywood down. Then put the flooring on top for that. You should never have a problem with condensation gathering under the flooring/plywood because it can still breathe.

From the original questioner:
I agree on the Tapcons being the best solution, when possible. When we started the job in one of the rooms, I put down a moisture barrier and was using tapcons. But it is an old slab and the concrete is extremely hard. I was burning through masonry drill bits every three to six holes. After $120 worth of bits and only five sheets of subflooring laid, I went back to nailing.

From contributor B:
Instead of Tapcons, try drive pins. You hammer drill a 1/4 inch hole through the subfloor and concrete, and drive the pin in. I dare you to try and get it back out, and you won't burn through a ton of bits.

From contributor R:
If you are going the nail-down-the ply route, Ramset is the only way to go.

From Gene Wengert, forum technical advisor:
Perhaps everyone already knows (but just in case) that you do not nail into a concrete pre-stressed floor or other item as it would be extremely serious if you hit a tension cable.

From contributor M:
I disagree Gene. In all my experience as a GC I've never heard of that, you really don't have a choice in most cases. How would you attached the walls of a building to a concrete slab if you didn't penetrate it? The rules typically followed were I'm from is try not to use fasteners or anchors that penetrate deeper than needed, understand where the cables are at if you do need to, and drill carefully and pay attention to the feel of the drill and color of the dust to confirm you are not hitting them. Most tensioned slabs are at least 4" thick, the cables are meant to be in the center but more often than not are slightly lower. Most concrete anchors, especially ones used for the purpose of subflooring and interior walls only need to penetrate 1" or so.

As far as ramset goes - it's great for new construction, especially with metal studs. But old concrete and shooting wood that isn't pressed tightly against the concrete (due to the warped wood or wavy floor) will usually result in a high failure rate - the drive pins are more work but are typically 100% effective. A tip on the drive pins - if you have a bunch like when doing a subfloor, make sure you have a small hand maul for pounding them in because even the heaviest framing hammer will feel inadequate when you get to the 100th one or so, it can be a real workout for the one doing the swinging. I've even started them with a hammer and finished them all of with a ground rod driver on a chipping gun when I had a few hundred to install.

Another tip when drilling concrete is just because a bit is still drilling doesn't mean you shouldn't change it. The sides will wear down and the hole will be smaller than when you started, making anchor installation harder and in the case of Tapcons can cause them to strip out before they are all the way in. I've found Tapcons to be pretty sensitive to hole size especially in really hard or old concrete, 90% of the Tapcons I use go into block and other masonry where I think they perform the best and have much less issues with bits wearing out. Bits will also last much longer in a roto-hammer versus a hammer drill - the higher RPM’s cause more heat that really wears them out fast.