Plywood over concrete: attaching stretchers

[Michael Titus]

Right now I use 1/2 of a two-car garage for my shop, and I'm considering converting the whole garage to a shop and putting in plywood floors over the concrete. I plan to use 2x4 stretchers with insulation between them, covered with polyethylene sheeting, with 3/4" plywood screwed onto the stretchers. Basically the method described in FWW #160. However, I'm only going to be in this house another 5 to 7 years, and I want the stretchers to be easy to remove in case the next owner wants to use the garage as a... garage.

So, nailing the stretchers to the concrete using powder-actuated nails is definitely out. Is there an adhesive I can use to bond the (pressure treated) 2x4s to concrete that won't be difficult to remove in a few years? Something with just enough strength to keep the stretchers from warping and lifting off the concrete, but weak enough to let them be easily pried off? Thanks.

[Jonathan Clement]

I built a free standing cooler in our garage. I fastened the 2x4 sills with tapcon screws after drilling with a hammer drill. I think if removed they will leave small holes that can easily be patched.

[Michael Heffernan]

Michael,
I did this in my garage shop. IMO, you don't need to anchor the 2x4 sleepers to the concrete. Build the frame like you would a 2x4 wall. I built my deck in two sections; half of the space each. Left the 'wall' length about 1/2" short so that I could frame it out and slide it into place. I then leveled and screwed the frame into the stud walls, shimming the edge that I left short for a tight fit. Did the other half and anchored it into the walls and the other half as well. After I filled the stud bays with rigid foam, I screwed the 3/4" ply deck down with exterior torx deck screws. No construction adhesive, as one day I will want to easily dismantle it.
With the weight of the ply and all my machinery and benches, there is no warping or movement. Flat as a pancake.
Make sure that you run some level lines or use a laser level to make sure your framing is level. Garage floors can often be anything but level. I used a variety of thicknesses of plywood squares to level the low spots. Toenail them to the underside of the framing to keep them from shifting.
As for the plastic vapor barrier, you may have one under your garage slab already. If so, you don't want to put another one above the framing which would cause a vapor sandwich. There is various debate as to where the vapor barrier should be placed, if there is not one below the slab. In my previous garage shop, I laid 6 mil poly directly on the slab with a few dabs of construction adhesive and built the sleeper frame on top of that. I didn't have a vapor barrier below that slab.
Hope this helps.

[Jim Neeley]

Michael,

I don't have a "silver bullet" solution but will offer up one solution to keep the stretchers from lifting up off of the floor.

Strength to weight, one of the stronger solutions is a torsion box. It would be possible to make up this flooring with plywood on both sides of the stretchers, with your insulation in between. Built on grade, they will be shape themselves to the natural drainage of your flooring. If you use screws for assembly rather than adhesive, you can disassemble them and rebuild them for your new house.

The upfront cost is higher but, other than a little bit of waste around the edges and a few screw holes, it can be part of your long-term solution.

I did something similar on a small scale when I built my first workbench (3'x7'x3.25" thick), building a 4'x8' torsion box from 1x stockto give me a flat surface on which to build my bench that didn't require kneeling. In the couple of years after the bench was complete I re-used all of the material for jigs and other shop uses.

Jim in Alaska

[Jim Andrew]

Garage floors are usually sloped, to let the water run out the door. The common slope is 2 to 3".

[johnny means]

Keep your sleepers short, say 2 ft. The floor will keep itself flat with it's own weight.

[Michael Dunn]

Garage floors are usually sloped, to let the water run out the door. The common slope is 2 to 3".

Mines slopes to let it run into my crawlspace. I had a few foundation cracks as a result. This is from snow falling from my wife's car and melting.

My response was, "Don't park your car in my shop."

I'd love to build a subfloor like this. Big project though. Everything's has to come out.

[Cameron Handyside]

I can second the "free-floating" floor option. I floated PT 2x4"s over the concrete slab in my shop. I used rigid foam insulation between stringers and laid 3/4" T&G ply over them. I did pocket hole screw the ends to a sill running the perimeter of the shop and limited the 2x4's to no more than 10' long. In the middle of the room, I ran a perpendicular 2x4. It has been down for awhile and is dead flat.

[Don Jarvie]

I did the same thing as Cameron above. You need to find the high point on the floor so you can level the floor off that point. My garage's high point was right down the middle so I shimmed everything level off that.

I used PT 2x4s for the sleepers and OSB for the floor. The OSB provides more traction than plywood and cheaper. I painted it with deck paint.

I also have an island in the middle of the shop so I ran power under the floor to the island for the TS and plugs for power tools. Whole thing cost me 900.00 and 1 weekend to do it.

[Ben Hatcher]

What's the purpose of putting wood over the concrete? Is the concrete in bad shape? What about just laying down sheets of rigid foam and covering that with plywood? You'd get the tool/feet saving properties of the wood floor and the added insulation for a lot less effort.

[Michael Titus]

The concrete floor is in good shape, but I'd prefer to work on a wood surface for my own comfort. The concrete is cold, hard, and damp (the builder did not use a vapor barrier for the garage). Laying plywood on top of insulation without stringers will probably just result in the insulation being crushed: I have a very heavy jointer / planer (550 lbs).

[Cameron Handyside]

Don is right that the OSB is cheaper and better traction. Also, I used T&G which locked the floor together. While its tight, makes it harder to pull up an individual panel for access if needed. I'm really happy with my floor, just things to consider. I also just finished off my ply with deck stain. Goes on easy and touches up quickly.

[Jim Neeley]

How does OSB hold up to moisture? Not that there should vbe a lot with poly sheeting, but..

[Jay Jolliffe]

OSB would not be my choice of plywood to use. Yes it's cheep but you get what you pay for. Never liked the stuff. I've had it delaminate too many times. I'd use 3/4'' t&g Advantec if it's available .

[Don Jarvie]

My floor is 2 years old and has flaked here and there but not anything noticeable. I used 2 coats of deck paint.

I like the OSB because its rougher than plywood so I have some grip while hand planing, etc.