My partners and I were recently hired to apply a
manufactured-stone veneer to the above-grade face of a drive-in
basement foundation. Though we had previously worked with
other, geometric veneers under the same brand name, Cultured
Stone (Owens Corning, 800/255-1727,
www.culturedstone.com), this job was
different — for a couple of reasons.
First, the foundation tucked diagonally into a hillside, with
the grade retained on one side by natural stone boulders. The
veneer selected by the designer was made up of irregular,
"natural" shapes that would match the boulders in the retaining
wall. Second, whereas in the past we had applied manufactured
stone to poured concrete and masonry walls — which
provide ideal backing for masonry veneers — this
foundation was built with Reddiform ICFs (insulating concrete
forms). I was confident that the foam facing would provide
adequate support for the veneer, because integral webs inside
the blocks tie the interior and exterior faces together. But I
wasn't sure whether the standard cement mortar typically used
to install manufactured stone would bond directly to the
polystyrene foam.
However, the project's builder, Todd LaBarge, had extensive
experience with ICF construction and had already researched
direct-applied concrete coating options with various industry
consultants. He was confident we could proceed without worry.
For reference, I checked the Cultured Stone product literature
for specific installation guidelines.
Spec Check
The manufacturer's guidelines include instructions for
application over foam sheathing, but not in a direct bond. The
recommended approach calls for a weather-resistant membrane
such as 15-pound felt to be applied first over the foam board,
followed by a layer of expanded metal lath and a
portland-cement scratch coat. The manufactured stone is then
bonded to the scratch coat in a 1/2- to 3/4-inch-thick mortar
bed and pressed firmly in place to squeeze out excess mortar
and any air pockets. The latter are potential water traps that
can freeze, expand, and pop the concrete veneer off the
wall.
Todd reasoned that while eliminating air pockets is important,
there's no practical reason to install a moisture barrier over
an ICF wall, because it isn't susceptible to fungal decay. And
metal lath, while entirely appropriate over an applied foam
sheathing, is unnecessary as a structural base over ICFs, since
the rough-textured expanded polystyrene foam provides plenty of
"tooth" for the mortar to key into.
By the Numbers
Installation was pretty straightforward. We used a layout
process we developed on the job. The first step was to break
out several boxes and spread the irregular "fieldstone" shapes
on the ground to ensure a good blend of sizes and color.
Layout involved lots of trial and error. We wanted to fit the
stones together tightly to minimize the mortar joints, and we
also wanted to avoid cutting them whenever possible, since
cutting would reveal uncolored concrete edges. Arranging the
stones on the ground in measured plots that represented the
walls' dimensions and window openings saved us a great deal of
time (Figure 1).
Figure 1.Dry layout allowed the installation crew
to plan a natural-looking pattern that involved minimal
cutting. Stones with a slight taper along one edge were chosen
to tuck under the wood siding at the top of the
foundation.
The wood siding above the foundation line provided an overhang
roughly equal to the stones' average thickness of 11/2 inches,
so we began by looking for tapered edges that would tuck under
the siding and worked down from there. The bottom edge of the
veneer would be buried just below grade and could therefore run
random.
Once we had worked out a section of the layout on the ground,
we took each stone in turn and traced its outline on the foam
with a felt pen (Figure 2). We marked both the stone and its
outline with corresponding numbers and stacked the stones by
the wall for installation. As the job progressed and we became
comfortable with the material, we were able to plan the layout
directly on the wall, without working it out on the ground
first.
Figure
2
. Once the
pattern was determined, the layout was traced on the wall and
each stone numbered for reference.
A Solid Bond
To bond the stone, we used ordinary Type N masonry cement, a
medium-compressive-strength (750 psi) mortar made of one part
portland cement, one part lime, and six parts sand, which is
recommended for most exterior above-grade masonry applications
exposed to weather.
In addition to mix water, we used an acrylic latex additive,
Silpro C-21 (SilPro, 800/343-1501,
www.silpro.com), which enhances the
mortar's initial stickiness and ability to bond to both
concrete and polystyrene. It's been our experience that a
masonry wall built with C-21 will break before the mortar bond
will give.
To eliminate trapped air, we troweled mortar directly onto the
wall and completely back-buttered every stone, pressing each
firmly against the wall to force out excess mortar (Figure 3).
Occasional removal of a just-set piece assured us that we were
getting a solid bond.
Figure 3.To ensure a void-free bond, the author
applied mortar to both wall and stone a piece at a time and
removed the squeeze-out. While the abrasion marks seen on the
foam here are not essential for bonding to expanded
polystyrene, roughening the surface of smooth-faced extruded
polystyrene foam would definitely be a good idea.
Figure 4.Periodic jointing of the stone provided
support between courses to prevent slippage before the mortar
set up.
We installed the stone working horizontally from the bottom up,
so that each successive course would be supported by those
below. We jointed the stones as we went along, using a tuck
pointer, then brushed the face clean of excess mortar. Tuck
pointing after every few courses helped prevent the stone from
slipping before the mortar set up (Figure 4).
The completed veneer looks very convincing against the natural
boulders in the retaining wall, and the transformation from
white foam to rustic stone foundation is an aesthetic
winner.
Stephen Minster
co-owns Abednego
Builders in Orleans, Mass., with partners Ryan Haig, Paul
Witter, and Kyle Norman.