[Greenbuilding] fastening a subfloor over foam over a slab

Benjamin Pratt benjamin.g.pratt at gmail.com
Tue May 31 09:44:29 PDT 2011

Can you use sand over concrete to level the floor? With a sand base,
how are you fastening down the foam?
I was told to put plywood over foam, and screw them both down with
long tapcon screws. But that project has been put on hold

On Tue, May 31, 2011 at 8:29 AM, JOHN SALMEN <terrain at shaw.ca> wrote:
> I have 4 projects done with ply over eps. The first was 3” eps foam over
> concrete slab with 5/8 ply w/ wood and tile flooring over (tile area had a
> 2nd layer of ½” ply over). I visited it after 10 years and it was fine.
> Subsequent floors have been eps over sand base with ply laminated. I’ve not
> used anything less than 3” as there is some reliance on the foam to span any
> voids or deflections. Also it is difficult to lay and set 1” sheets.
> For the concrete slab application the floor was levelled (high spots ground
> out and a levelling mix like quickcrete).
> Polyurethane foam is the most reliable bonding adhesive for eps foam board
> (low expanding type rated for subfloor – enerbond or equiv.) laid in a
> squiggle pattern at 3” o.c.
> Sheets need to be weighted till the adhesive forms its initial set – flat
> weights (16x16 ply square with about 30lbs – conc block or bucket with sand
> works nicely).  I think for 1” weight spacing would be about 3’ o.c.. If you
> cut the sheets to 4’ you can leapfrog the weighting.
> Seams were foamed and then shaved.  Ply is laid with the same adhesive
> pattern (seams staggered over foam). With 5/8 or ¾ t&g ply the end seams are
> foamed and splined (biscuit at 16 o.c. or something equivalent). Ply has to
> be weighted as well and needs to be flat (not deformed). 2 layers of ½ “ are
> nice (no end joining) and for wood floor over - the second layer was foam
> glued and screw fastened a 6” on edge and 10” field or for tile-  at 3” o.c.
> Makes a nice floor – no tile/grout movement (without ditra)
> With 1” foam I would be tempted to use a stucco basecoat adhesive (designed
> for eps foam) with a notched trowel (3/8x3/8) for laying the foam sheets to
> the concrete. Depends on how the concrete was finished.
> Since the floor is tile I would simplify the installation and materials used
> by using 1.5” foam with ½” layer of cemboard laminated over (with seams
> taped). This would be as stiff as the 2x ply and the cemboard installs nice
> and flat (i’ve used this over foam with tile). Both of these assemblies are
> quite stiff in comparison to a normal joist floor so not sure what the need
> for the ditra is.
> From: greenbuilding-bounces at lists.bioenergylists.org
> [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of Alan
> Abrams
> Sent: May-31-11 5:05 AM
> To: listserv Green Building new
> Subject: [Greenbuilding] fastening a subfloor over foam over a slab
> does anyone have a reliable spec for fastening a subfloor over foam, over a
> slab?  my recollection is that JLC or FHB had an article on this, but cannot
> google it up.
> space is very tight in this case; allowing for 1" of xps, two cross plies of
> 1/2" cdx, and finish flooring (tile on Ditra), to stay within code ceiling
> height...
> AA
> Alan Abrams
> Abrams Design Build LLC
> A sustainable approach to beautiful space
> 6411 Orchard Avenue Suite 102
> Takoma Park, MD 20912
> office  301-270-NET- ZERO (301-270-6380)
> fax      301-270-1466
> cell     202-437-8583
> alan at abramsdesignbuild.com
> www.abramsdesignbuild.com
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b e n j a m i n p r a t t

professor art+design
the university of wisconsin stout

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