[Greenbuilding] fastening a subfloor over foam over a slab

JOHN SALMEN terrain at shaw.ca
Tue May 31 06:29:11 PDT 2011

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
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


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
 <mailto:alan at abramsdesignbuild.com> alan at abramsdesignbuild.com
 <http://www.abramsdesignbuild.com/> www.abramsdesignbuild.com

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