[Cob] Plaster over plywood and lathe

[Barbara Roemer]

I, too, like the back side of drywall inside, though there's nothing of any
tooth for the plaster to grab.  Wheat or rice paste with lots of sand mixed
in and thinly applied before you plaster results in better adhesion.  Apply
quickly with a wide old paint brush.

Outside, I've used burlap - by it by the roll as it's $ from fabric stores,
or for smaller spaces,cut up bulk coffee bean bags, free at local cafes.  In
straw bale building, we've used blood lathe at the perimeter of window &
door openings where we had to go over flashing and vapor barriers.  It's
very effective at places where two materials with differing properties abut,
such as where cob joins wood or metal.  Burlap is less effective at
resisting cracking at such joints where adjoining materials each have a
different modulus of expansion.  But, that being said, I have a single paned
non-venting skylight where the well of the skylight gets very hot in
summer.  It was never trimmed out nor rocked, and the framing for it was
just a pile of very rough lumber stacked up.  Trying cob and burlap seemed
like it might work. First I dipped the burlap into a slurry of plaster.   I
set it aside,  ready and wet.  Next I made up a very stiff,  heavy straw and
sanded cob mix and smushed that into the huge cracks. Leaving it to dry and
filling in large cracks with several coats, then while the cob was still
setting up, I pressed the wet burlap into the cob so it sort of glued itself
to the cob filler.   Smoothed out the burlap and let that all dry, evened
out with the base plaster, and made a final colored plaster of kaolin and
fine sand.  In 7 years of winter snows piling up on the skylight and
scorching heat gain in summer, not a single crack has developed in that
well, success I attribute to it being carefully burlapped.  Granted, I
wasn't trying to wed wood to metal or metal to rock or cob, but the old
lumber to the sheetrock that surrounds the skylight well.  By the way, for
interior outside corners, bullnose sheetrock edging works really well and
looks soft and attractive with earth plaster.  Repairs are easily made to it
if you chip off a chunk moving in.  Corners here with the bullnose edging
have held up much better than outside corners with nothing but abutting
sheetrock planes.  But you can make rounded corners by just using a coarse
wood rasp to bevel the edge of the sheetrock a bit.

Barbara