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Mike Holmes is a big fan of OSB/plywood over 1" XPS for basement floors, though his book leaves something to be desired in terms of details. His book says:

...on top of the foam, you want to have 5/8" tongue-and-groove plywood, which should be screwed through the foam and into the concrete floor. This should be done with Tapcon screws, which are designed for concrete... -- Make It Right p165

(Notice it doesn't say anything about how many Tapcons or where to put them.)

The Building Science Corporation also likes plywood over XPS. RR-0309 - Renovating Your Basement says:

...sheets of extruded polystyrene can be installed directly on the slab. The joints between sheets should be sealed with tape or mesh imbedded in mastic. A plywood floor can then be “floated” over the XPS insulation in either of two ways. If sufficient room height is available wood sleepers (2X4 or other dimensional lumber) can be installed to which the flooring is mechanically fastened... If room height is minimal, tongue and groove plywood with the butt ends of the plywood joined can “float” above the XPS insulation.

No mentions of any fasteners (or adhesive), and specifically refers to the plywood as "floating".

To further confuse things, I've also seen suggestions in forums to anchor around the perimeter about 4" in from the edge @ 24" O.C, plus one in the center of the floor. This also seems plausible since it's basically how Dricore is installed (for carpet).

So what do you think? Should this type of subfloor be anchored, and if so, how?


Update: Most of my reading is giving the impression that anchoring is just to control "bounce" and warping. This makes me think you wouldn't need very many screws per sheet, particularly considering that 1/4" tapcons embedded 1" have a pullout value of at least 750lbs, and a shear value of at least 900lbs. But then there's these guys talking about 20 or 32 screws per sheet, which seems like madness. Maybe that's just if you're installing hardwood floors?

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3 Answers 3

up vote 2 down vote accepted

Prescriptive nailing schedules are part and parcel of flooring or wall exteriors. In both above grade situations, the goal is to create a unitized structure. Wall panels properly attached become stressed members that resist lateral racking forces.

In the basement, over a poured concrete slab, no structure enhancement is necessary. Therefore any fastening that produces a secure, trip free (smooth)surface is the goal.

It somewhat depends on what the final surface will be. Floating laminate floors would work over floated ply. Heavy cabinets or a workshop might work better over a more secured surface.

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So maybe start with the Dricore type schedule and then add screws as needed to address any movement or unevenness? –  Brad Mace Apr 13 '13 at 5:26
    
Very reasonable –  HerrBag Apr 13 '13 at 5:47
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I glued 1 inch thick rigid foam board onto the concrete floor which I had previously treated with a 1/16 inch coating of xyprex moisture proofing. Then I glued OSB sheets to the foam board. In both cases I used P-400 foam board glue, which worked very well.

This subfloor will support a variety of laminate products in future when I want to have a more finished surface in this storage room.

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I did this fully floating. No screws. Hardwood on top also floating. All is fine so far. I did not like the idea of punching multiple holes in the vapor barrier with all those screws.

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