Cob: Cob failure problems

[John Fordice]

Cob failure experiences,

COB WALL FINISHES
On the building in Berkeley we have applied no finish & after three
winters there has been only minimal erosion of the wall facing the
direction of storm driven rain.  What was a more or less smooth trimmed
surface on that wall when new has eroded to a rough surface with the
larger aggregate and ends of embedded straw showing & sticking out on
the surface.  I intend to keep an eye on it & if the erosion from the
rain continues, will apply a wear layer of a mud plaster on that part of
the wall.  In the past, I've done finish plasters on cob with a basic
earth and sand mix the same as the cob & with about 30% horse shit mixed
in.  The addition of the manure makes the plaster really creamy in
consistency & it goes on like a dream.

A conversation with a cobber from Scotland said that cob will erode on
the surface the first couple of winters & then stabilize after the fines
are washed away from the exposed surface.

PROTECTION OF BENCHES
On an  unroofed bench at another project in Berkeley , we used the same
cob / manure mix to finish the bench & it didn't do anything to protect
it from rain intrusion & there was a subsequent failure after the bench
got saturated.

On the two benches at the Berkeley building which are also exposed to
the weather, we had two failures:
The first winter, the seat portion of the benches  ( which were finished
with the same plaster & both the cob and the finish plaster had lime
(10%) added into the mix )  had chunks fall out of the overhanging front
part of the seat.

After this failure, we rebuilt the seats of both benches & embedded two
layers of plastic garden netting in the cob to support the overhanging
front part.  I didn't pay proper attention to the mix ratios & we got
some cracking in these rebuilt seats.  After the rebuilt seats dried, we
sealed them with three coats of thinned lindseed oil.  This worked well
to cause the water to run off the horizontal surfaces.  Unfortunately
though, the cracks let in sufficient rain water to cause the base of the
back wall of one of the benches to become saturated & the back of that
bench fell off !!!

A conversation with a paint chemist said that adding bees wax after the
linseed oil will prevent the surface from being able to absorb any
subsequent applications of linseed oil sealant.  This is assumedly
important because, linseed oil being organic will deteriorate ( from
oxidization if I recall correctly ) % will need periodic renewal.  

The how to make a successful  cob bench moral of this story: 
1.  Pay attention to the mix & make it a clay lean as you can to prevent cracking.
2.  Adding lime into the mix body doesn't do much good ( if any ).
3.  Linseed oil thinned with mineral spirits ( three coats: 1 straight,
second 50% thinned, final 100% thinned ) shed water quite well.
4.  AddING bees wax may be counter productive.

There's lots to learn & that's what makes cob so much fun !!!

john fordice