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?
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'
There is a good description at:
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 here:
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)
Andrew Mawson (whose parents had a partially cob Cornish cottage)
well cob it aint then! It's granite blocks held together with some sort of
brown muck, which the locals here refer to as "cob", to make up a 20" thick
solid wall (no cavity). Thanks for web link on the tanking. The stuff our
builder was talking about is a type of cement based product that is brushed
on underneath the final plaster skim. I think he said one of the products
was Vandex but I guess there are alternatives.
I know exactly what you mean (same area)
one of my walls was extremely damp due to
wooden wedges being left in the wall
I rendered over the granite, thinly and
roughly for grip, painted on a bitumen
coating, rendered & plastered to a finish
Tanking is a system of creating a waterproof Tank on the inside of the
walls. Our cottage is tanked at the back as its dug in the Hill. Vandex
seems to be the main one in use. Its best left to the pros. Its basicly
layers of waterproof concreate of differnt types and waterproofness appled
to the walls then plastered over.
Trevor - before you go off down the tanking route.......
Remember granite itself is totally waterproof, not porous like brick or soft
stone. The only damp part of granite walls is the joints - rainwater runs down
the face of the wall rather than soaking in like it does with softer materials,
and this quickly leaches out the pointing (what you call the cob). I think
you'll find you'll make a really big difference to the internal dampness if you
rake out and repoint the wall properly outside.
This link from the granite city of Aberdeen tells you how to repoint properly:
Best of luck
Exactly the same principles apply, OldScrawn - the mortar must be softer than
the stone. With the softer limestones, particularly the Oxfordshire ragstone
used for rubble walling, definitely no cement. But you see it so often, cement
mortar roughly smeared over the face of the stone, finished with a dustpan-type
brush. It harbours moisture which freezes and spalls (flakes) the face of the
stone. Don't be fooled into thinking that because soft mortar soaks up water it
must encourage dampness - it dries out almost as quickly as it gets wet.
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