Garage Slab Tips
Thank you for the article "Better Garage Floors" (5/11), which
contains most of the advice I've presented to clients for the
last 20 years on making better concrete. I offer one more point
of advice and a couple of lessons from experience.
The photo of concrete delivery shows it coming directly out of
the chute of a ready-mix truck, which is excellent. One of the
best ways to end up with cracks throughout hardened concrete is
to use a pea-gravel mix that can be pumped through a hose. The
huge portion of sand in a pea-gravel mix requires a lot of
extra water just to wet all the sand grains. As this water
evaporates, the concrete cracks much more than a standard rock
On the last few concrete placing operations I supervised, I was
the wheelbarrow jockey. On the last placement, a couple of
helpers and I even moved the concrete in five-gallon buckets.
It takes a couple of workers to lug a concrete pump hose around
anyway; even though I had to pay about 30 minutes of standby
time for the concrete truck, I saved about 200 bucks on the
pump and ended up with better concrete for less cost.
I've also cured several slabs using 0.3-mil plastic sheeting
(typically sold in 12-foot-by-400-foot rolls for paint
masking). This ultrathin plastic clings to wet surfaces easily
because it doesn't have enough rigidity to stand up on its own.
Almost as soon as we spread it out, enough vapor has condensed
on the bottom of the sheeting to weight it down onto the slab.
Overlaps need to be weighted down to resist breezes, but that's
easy because the plastic doesn't fight back. Where surface
mottling is acceptable, this is a very effective and economical
Thor Matteson, S.E.
Wood Foundation Skeptic
In a lifetime in this business, I have never seen a wood
foundation and cannot believe that anyone would ever want one
under his house. Let's start with the obvious lack of
resistance to exterior soil pressure vs. CMU or poured
foundations; there is no way a framed wall is even comparable.
Eventual water infiltration is all but a foregone conclusion.
Your 6-mil vapor barrier is shredded by the backfill, and while
you may be fortunate enough not to have any penetrations
through the foam board, eventually moisture is going to find
its way in somewhere. When this happens on concrete, maybe you
get a damp wall and a little efflorescence. When it happens in
a wood wall, you get rot, and probably termites or carpenter
ants. I've seen insects consume pressure-treated 6x6 posts
above grade, so I am sure they would make fast work of plywood
and 2x8s buried in wet ground. The best lesson learned from
this article would be to hire a mason, unless you like the idea
of a biodegradable foundation.
Morristown, N.J.Author Terry Shields responds: I
understand your concerns and would probably share them if I
lived in an area where pressure-treated wood foundations (PWFs)
were uncommon. I worked on my first wood foundation in 1991 and
have built many since then. In addition to my own jobs, there
have been plenty of other PWFs built in this area to validate
their performance. I do not know the exact strength of wood
foundations vs. concrete, but it's important to note that these
structures are being built with professional engineering input.
As for moisture management, I agree that it's critical; it's
important to create positive drainage and manage moisture for
any foundation, and we take great care at backfill not to
"shred" the poly barrier on the outside.
Although wood foundations may not be suited for every
situation, some of the warmest, driest, most comfortable
basements I've been in are inside PWFs.