Softus said:
I'm notching up those reported failures in injected DPCs!
I know absolutely nothing about the injected DPC systems. The only dealings I have had is providing alternative solutions where they have "failed". For all I know, the vast majority of these treatments work quite effectively. I just haven't seen a working example, yet.
Softus said:
My own understanding of the purpose to provide a continuous layer that moisture can't breach - the role of the DPC in the wall is, I would reason, less important in preventing moisture rising through the bricks and more important in preventing wicking between the bricks and their covering.
Dependant on the mix, dry mortar can be just as absorbent as the bricks themselves. In fact, it is standard practice to use a mortar with approximately the same absorption as the bricks. Therefore above DPC with standard bricks a fairly week mix (5:1 - 6:1) is the norm, whereas below the dpc, nowadays it's standard practice to use engineering grade bricks (such as staffordshire blues). These are much less absobent than stocks, and are laid with a much less permiable mortar (such as 3:1). Footings laid like this are almost as good as a DPC for preventing the rise of the water.
Softus said:
what do you call pretty a impressive height? And without meaning to denegrate your good character, can I press you on this next point: have you actually seen an example of water having risen to an impressive height
The job to which I previously refered, had a space of about 4ft below the floor, the thickness of the floor itself was about another 9" (allowing for the joists), and while I was there, the wall was dripping wet upto a hight of about 2.5 ft above this. That makes at least 7ft rise from the base of the wall. The oversite itself, although generally damp, had no standing water at all. There was also considerable staining of the walls that tallied with the neighbours claims of 5ft above ground level during the winter, that would be a total rise of, going on 10ft!
Softus said:
Moreover, do those pores in a brick connect in the same way as in a sponge? I'm not convinced that they do. Even if they did, here's a question: if you took two sponges, one on top of the other, and separated them with a layer of morter, and if the bottom one was sodden, how long would it take for the top sponge to be equally sodden?
There are a few problems with the sponge analogy. First of all, sponges tend to have much larger pores than bricks. This makes them less efficient than bricks when it comes to capillary action. To get sponges to work well you need to squeeze the air out of them and allow the partial vacuum to suck the water in. Sponges are also far softer than bricks. As a result, any water in them will cause them to deform (therefore squeezing the water out of the lower parts of the sponge). Bricks do not suffer from this, so even when the water has risen to impressive heights, it still doesn't squeeze the water out of the bottom. When it comes to soaking up water, bricks (and regular mortar) are far better than sponges.
Once you've been impressed with the roasting dish experiment, you may want to try it with your actual house wall. I wouldn't suggest you do this if you have a solid wall construction, but if you have a cavity wall, just setup a hose so that it is gently trickling onto the outside wall, just above the DPC for a few hours. Ideally on a dry calm day. If you don't beleive in rising damp, you'll be seriously impressed. As you observe the damp patch rising up your wall, imagine how much more effective it would be, if the entire bottom course of bricks were exposed to a constant source of water.
For all this, I still think that rising damp is very rare. A small crack in the DPC will only allow wicking at a very low rate, as this is rapidly dissipated through the rest of the wall and evaporation takes over, it's affects are kept very localised and generally quite minimal. I have yet to see an effective DPC spontaneously fail. IMHO it just doesn't happen. But then I have no experience of these new fangled injection systems. Perhaps they are prone to "fail", I just don't know.