In article ,
basil writes:
On Sun, 10 Apr 2005 09:49:40 +0000 (UTC),
(Andrew Gabriel) wrote:



Raw sand and cement is much less susceptable to this, and it can
be reduced to pretty insignificant levels with the addition of a
waterproofer into the mix. If the mixture is weak as normally
used for renedring and mortar, then it will allow some breathing
(gas permiable so water vapour can pass through), though not as
much as lime.


Good post!

On the subject of waterproofer for sand and cement would wickes's
waterproof PVA be the correct thing? What do you use? I'm missing a
DPC on one small area of internal wall.

No, I don't think that's what the 'waterproof' in waterproof PVA
means. (I think it just means the PVA won't degrade inside damp
sand/cement when the PVA is used as a bonding additive.)

You can buy a waterproof additive for mortar mix. The one I use
is also combined with plasticiser, but you can get it separately.
I think it came from a large B&Q, but I can't remember the make
(may be their own brand).

If you have a measurably damp brick wall, a sand and cement scratch
(base) coat with waterproofer (and I use lime too -- makes the mix
very nice to handle) will protect a gypsum finish coat. I've done
this a number of times, and whilst the underlying wall is still
damp, the gypsum remains bone dry. However it probably isn't a
substitute for tanking, where you have water being forced through
the wall due to being below the water table for example (at least
I've never tried, and I wouldn't expect it to work).

The general sequence for doing this is...
PVA the wall just as you would before any plastering -- if it's
very absorbent, a 1:5 dilution with water soaked in to dry to seal
the surface. Then apply a 1:3 dilution and allow it to get nearly
dry before applying the sand/cement. I use the standard 1:1:6 mix
of cement:lime:sand plus waterproofer (plus plasticiser as that's
mixed with my waterproofer, but the lime acts as a plasticiser
anyway so that's not necessary). You want to apply the gypsum
finish coat after the sand/cement has gone off but before it dries;
this means waiting at least 24 hours, but probably not more than
48 hours. If you leave it longer and the sand/cement does dry out
before skimming, then you'll probably want to PVA it as as though
you were doing a reskim.

--
Andrew Gabriel