On Thu, 20 Mar 2003 19:06:12 GMT,
(Jim) wrote:

I'm building an 8' x 16' greenhouse. The foundation will be cinder
block between posts. I'm going to put a 2 x 8 bottom plate on top of
the cinder blocks. I want to avoid pressure treated lumber because of
the chemicles used. Anybody have a recommendation for a preservative
to use on the wood?

Thanks,

Jim

Jim,

Without knowing where you are located, what sort of use you will make
of your greenhouse, (seasonal, drainage, type of floor, etc) it is
pretty difficult to guess what would work. Basically, pressure
treated wood has the clear advantage that it works and will prevent or
diminish rot for quite some time. If you are growing food in soil
into which the materials in the wood might leach, that can be a good
reason to either not use it or move the food-growing beds. A
greenhouse is just about the worst possible combination of heat,
light, humidity, and enriched diet for wood decay. Anything that you
can put on the wood to accomplish the same ends as pressure treatment
is likely to (1) be at least as toxic, (2) offer more exposure because
you will have to apply it, (3) leach even more rapidly because it is
on the surface, (4) fail more quickly because it is on the surface.
You could begin by using some of the more rot resistant woods, cypress
for example. Being careful to avoid a way for water to wick to the
wood or pool on it will help. Unfortunately, even with careful
flashing, the weeping of GH sheathing will keep the sill plate wet
much of the time, not to mention your own watering. If you GH is
framed in aluminum, you might consider sinking bolts into concrete
filled holes in the block and attaching the metal to the block without
a sillplate.