Tanking how-to?

[Andy Dingley]

I have a shed: it's a long, thin lean-to added onto the back of another shed. When first built, the big shed had a 6' wide trench behind it to cope with a sloping site. Later on someone decided it was a good idea to wall the back of this trench, go up to ceiling height and then put a roof on it. So the lower rear blockwork wall (3-4' high below surface level and 20' long) is thus prone to damp. Floor is concrete, which I expect is too thin, laid on bare earth.

I'd like to fix this, as part of the general shed refurb. Nothing great, just make it dry enough in there to store things without them going soggy.

I'm assuming that I buy something from Perma-gard (or others?) and apply it. Either "paint" in a can, or "cement-based grout" in a dry sack. I'm not planning, nor do I think it justifies, much more than this. So no membranes, inner dry linings, wicking membranes, sumps or pumps.

'm also looking at these Perma-vent tubes. would guess that these things might work in a house (damp, breathing humans and some heating) but that they stop being unidirectional in an unheated shed? Maybe it would be worth trying one here, as the ground exposure is probably making it damper indoors than out anyway.

Any advice? What should I read for a "How to"? Which products would be most suitable for me? Any favourite brands, experience or other advice?

[David WE Roberts]

From reading the description it seems to be just a tube with a permeable membrane. Presumably this allows a gentle flow of moisture from a high temperature/moisture area to a low moisture area without letting any draughts in. There is no suggestion that it has any amazing advanced technology to force water out. So you have to consider the surface area of membrane you would need to allow moisture out to compensate for the moisture coming in through the back wall (and possibly floor). I would assume at least 10% of the surface area of your back wall (depending on how permeable the wall is). It could require quite a lot more. At £15.50 a pop that could be very expensive. And as you say you don't have a temperature gradient (in fact in summer it is likely to be warmer outside - you have a potentially nice wine cellar there).

I am assuming that your main aim is not to keep the shed warm by eliminating heat loss and draughts.

I would have thought that a combination of sealing the floor and back wall plus some good ventilation would be more effective for a shed.

My estimating is demonstrably crap, but if your wall area is roughly 1 metre by 7 metres then one bag of grout should cover it if their 2kilos per square metre coverage is accurate. At £31.80 this is roughly two of the membrane tubes.

Cheers

Dave R