Hi!
I’m looking for some definitive advice on re-pointing my house, specifically regarding the slate DPC level please. House is 1920’s cavity wall, brick and lime mortar. DPC is single layer of double course slate.
The lime mortar is solid (not crumbly or soft), but on the rear elevation we get wind driving rain into the joints and in some areas it has begun to erode. This is a cosmetic issue at present and we have no damp issues internally. As the house is designed to breath, lime mortar, cavity etc., I‘m not sure we would get much damp inside even if the mortar was worse. However, heading it off now seems like a smart idea, rather than letting it erode for another 20 years
Reading through Historic England’s guidance on repointing, it is contrary to the advice I’ve had from a number of general builders. All advised raking the whole lot out and replacing with a modern sand cement mix – one even suggested putting a water repellent on the exterior after repointing. Only one, when pushed, agreed to use a more traditional lime mix, but still insisted on wanting to rake and re-point the whole lot. HE guidance notes that simply patching in the bad areas is sufficient, and recommends sticking to traditional/matching mortar where possible.
I’d say the majority of the original pointing on the rear ~70% is in good condition, 20% is slightly eroded, and 10% is bad (e.g. loose or coming away), so from my perspective and HE advice would be patch the bad 10%, and for cosmetic reasons I could patch the other slightly eroded 20%. Doing the whole lot seems like overkill. As an additional note (from experience on this house), trying to waterproof, excessively insulate, or moisture barrier retrofit this style of older house actually causes more issues, bridging, lack of ventilation, damp, mould etc. so I’m a little confused about the advice I’ve had from more general builders over heritage guidance.
Anyway, hopefully that’s enough back-story!
The main questions I have relate to the slate DPC layer. I’ve attached a couple of pictures to show the issue, as you can see on the front of the house, the DPC layer has a thin bead of lime mortar render over the top (built like this 100 years ago!).
However, on the rear this has come away, and the mortar joint underneath has begun to erode in between the slate layers causing a bit of a ledge in some places (5 ~ 10mm). Obviously, water can then sit on the slate ledges and will cause further degradation as time passes especially in this area as the wind drives it further in.
So, what’s the best course of action here? There’s a lot of advice on what can be done above and below the DPC, but not actually on the slate layer itself. I am loath to try raking the joints between the slate layers out to any depth for fear of damaging the slate, yet I’m concerned that simply rendering over the top of the joint may not have much longevity?
Options seem to be:
1. Leave it as is: the problem is guaranteed to get worse over time although this may be decades from now.
2. Rake out and re-point: Risk of damaging the slate DPC and actually causing more issues.
3. Carefully remove any really bad bits of mortar and re-point and re-render with a breathable lime mortar. Risk of the render failing (and risk of bridging the DPC?).
4. Carefully remove any really bad bits of mortar and re-point with a waterproof mortar. Risk of render failing, should stop any potential bridging but a waterproof mortar would not allow DPC to breath / dry out. Although above and below the DPC layer itself would still be breathable.
As a interesting point of history, these houses were built by and lived in by a community of bricklayers, and although times have changed and modern building techniques and product have advanced, I have a certain level of faith on how these houses were originally constructed (everything was done for a reason) so quite keen to keep things traditional!
Any help or advice will be greatly appreciated!
I’m looking for some definitive advice on re-pointing my house, specifically regarding the slate DPC level please. House is 1920’s cavity wall, brick and lime mortar. DPC is single layer of double course slate.
The lime mortar is solid (not crumbly or soft), but on the rear elevation we get wind driving rain into the joints and in some areas it has begun to erode. This is a cosmetic issue at present and we have no damp issues internally. As the house is designed to breath, lime mortar, cavity etc., I‘m not sure we would get much damp inside even if the mortar was worse. However, heading it off now seems like a smart idea, rather than letting it erode for another 20 years
Reading through Historic England’s guidance on repointing, it is contrary to the advice I’ve had from a number of general builders. All advised raking the whole lot out and replacing with a modern sand cement mix – one even suggested putting a water repellent on the exterior after repointing. Only one, when pushed, agreed to use a more traditional lime mix, but still insisted on wanting to rake and re-point the whole lot. HE guidance notes that simply patching in the bad areas is sufficient, and recommends sticking to traditional/matching mortar where possible.
I’d say the majority of the original pointing on the rear ~70% is in good condition, 20% is slightly eroded, and 10% is bad (e.g. loose or coming away), so from my perspective and HE advice would be patch the bad 10%, and for cosmetic reasons I could patch the other slightly eroded 20%. Doing the whole lot seems like overkill. As an additional note (from experience on this house), trying to waterproof, excessively insulate, or moisture barrier retrofit this style of older house actually causes more issues, bridging, lack of ventilation, damp, mould etc. so I’m a little confused about the advice I’ve had from more general builders over heritage guidance.
Anyway, hopefully that’s enough back-story!
The main questions I have relate to the slate DPC layer. I’ve attached a couple of pictures to show the issue, as you can see on the front of the house, the DPC layer has a thin bead of lime mortar render over the top (built like this 100 years ago!).
However, on the rear this has come away, and the mortar joint underneath has begun to erode in between the slate layers causing a bit of a ledge in some places (5 ~ 10mm). Obviously, water can then sit on the slate ledges and will cause further degradation as time passes especially in this area as the wind drives it further in.
So, what’s the best course of action here? There’s a lot of advice on what can be done above and below the DPC, but not actually on the slate layer itself. I am loath to try raking the joints between the slate layers out to any depth for fear of damaging the slate, yet I’m concerned that simply rendering over the top of the joint may not have much longevity?
Options seem to be:
1. Leave it as is: the problem is guaranteed to get worse over time although this may be decades from now.
2. Rake out and re-point: Risk of damaging the slate DPC and actually causing more issues.
3. Carefully remove any really bad bits of mortar and re-point and re-render with a breathable lime mortar. Risk of the render failing (and risk of bridging the DPC?).
4. Carefully remove any really bad bits of mortar and re-point with a waterproof mortar. Risk of render failing, should stop any potential bridging but a waterproof mortar would not allow DPC to breath / dry out. Although above and below the DPC layer itself would still be breathable.
As a interesting point of history, these houses were built by and lived in by a community of bricklayers, and although times have changed and modern building techniques and product have advanced, I have a certain level of faith on how these houses were originally constructed (everything was done for a reason) so quite keen to keep things traditional!
Any help or advice will be greatly appreciated!
