"Trevor Smith" wrote in message
...
We live in a victorian / edwardian terraced house in Cornwall which has
solid granite "cob" walls, a number of which suffer from penetrating damp.
A
builder has told us that we may need to have them tanked but it appears
that
this is a "trade only" product. Can anyone tell me what it is, how it is
appplied and what the Retail name is. Do Screwfix or e.g. B&Q sell it?
Thanks.



Trevor,

Sorry to de-illusion you but cob is NOT solid granite ! Cob is a mixture of
local earths and straw, usually with a cow dung binder ! Often (but not
always) built up on a stone footing to raise it above the 'rain splash'
height.

There is a good description at:

http://www.endersonbrowns.demon.co.uk/cob/index.html

Tanking consists of lining the inner face of the wall with (usually) a
corrugated bitumastic or plastic membrane that is impermeable, and
plastering over it. The actual wall remains damp. When done woe betide you
if you drill fixings for shelves etc as you puncture the membrane.

There is a good description he

http://www.catrake.demon.co.uk/Basements.htm

If you do have a cob wall and are suffering from penetrating damp damp
problem you need either to stop the rain striking the wall (see later *) or
coat the outside of the wall with a breathable paint that lets trapped
moisture OUT but doesn't let rain IN.

(* Many cottages built with cob were originally thatched with large roof
overhangs to stop normal rainfall hitting the wall. If the cottage is
re-roofed in a different material with less eaves overhang it can cause a
problem with rain penetration)

Good luck,

Andrew Mawson (whose parents had a partially cob Cornish cottage)