Pointing below DPC

[RobDee]

Good evening all, my first post here so go easy please

We have an Edwardian building that is a long term renovation project, so far it’s been a case of undoing years of abuse by previous owners and the local council who owned the building for a long time and appear to have been behind the worst of the bodges that we’ve dealt with so far.

Over the last week or so we have been dealing with a damp problem caused by the earth having been built up and bridging the blue brick DPC. This has been excavated back and the levels will be corrected in time, for now I’m leaving the exposed brick work to breathe. Some of the brickwork, both above and below the DPC needs repointing, obviously above DPC I’m using lime mortar in keeping with the original mortar. My question is should I use the same below the DPC or use a cement based mortar for below ground?

 

[malcolm]

It's even more important to use lime below any DPC. Ideally the ground level outside will be lower than inside floor level and you'll have 4 to 6 inches of brick exposed. Lime mortar will let water evaporate before it gets anywhere near getting up the walls. Even below ground use lime. If it lasted for 100 years then the repointing will last you out.

I get very nervous of mentions of DPC on here. It is the go to method for modern buildings to lay a bit of plastic down. Once that fails the modern building probably can't be saved as it has no method to expel water. For older buildings any DPC was intended to be a bit helpful rather than the primary way to stop damp getting up the walls. Letting the walls breathe was still very important.

 

[RobDee]

Hi Malcolm, thanks for the quick reply and I apologise for using DPC in that sense, I realise it isn’t a DPC as found in modern buildings my meaning was in the sense of the distinction between below ground and above ground brickwork. On our property we have two courses of blue brick and then a blue plinth brick course, the internal floor levels is the top of the second course of blue bricks, so I was viewing the blue bricks as a less water permeable layer.
The use of lime mortar below ground level makes sense so I will definitely be using it for the repointing.
I’ve attached a picture showing what I’m dealing with, the plastic down pipe has been removed and the clay drain is being removed, it never worked as it slopes up from the point where the down pipe enters, and it.s badly cracked so all the water that was fed into it simply stayed in place and soaked the ground.

 

[stuart45]

The Building regs still allow 2 courses of Staffordshire blues bedded in a 3/1 sand/OPC mix as a DPC.
A rule of thumb for pointing is to use the same mix as the existing, apart from being a touch weaker.

 

[Cubist]

Malcolm and Stuarts advice is all sound and the only reason for my additional input is to confirm that Lime and Portland mortars are not good bed-fellows as they do not bond well to each other. If the extant mortar is lime based then use the same for any pointing repairs and vice-versa.