Chimney stack repointing: lime and cement mix??

[lizziecupcakes]

Hi all, I spoke to a roofer about repointing my chimney stack mortar. It’s an 1906 Edwardian house.

He said he would use a lime, cement and sand mix. Is this right?

I thought the cement counteracts the breathability of lime?

Would love to hear your experiences and recommendations.

Thank you

 

[Stevers]

My experience of repointing anything originally built with lime mortar is that NHL mortar will stay put, and that's not been the case for me with cement mortars even with lime.

 

[lizziecupcakes]

thanks @Stevers
what kind of trouble have the cement mortars with lime caused you?

 

[stuart45]

The vast majority of roofers will only be using cement mortars when repointing a chimney stack. Probably a 1/1/6 mix.

 

[Stevers]

This was almost exclusively in garden walls built of stone so comparable in exposure to chimneys. The 1:1:6 pointing clearly wasn't stable and after a while started to crack and fall out. Building from scratch in 1:1:6 has worked well for me, it's just 1:1:6 pointing of anything built in lime mortar seems a bit short term. In my experience NHL is a more reliable alternative that is just as easy to use.

 

[stuart45]

Technically, the correct mix for repointing is using the same as the original, maybe just a touch weaker. This allows equal movement throughout the wall. There are exceptions, such as on a new build when the original mix was too weak and allowed the weather to wreck it.
The problem is knowing what the original mix was. Houses built around the time of yours were generally lime mortar, as cement mortar was virtually unworkable. However builders were starting to gauge small quantities of cement into the mix to help with it hardening. By the 20's and 30's a 1/2/9 or 1/3/12 mix was quite common. In the 50's a 1/1/6 mix became the most common, and an equal amount of cement and lime meant it was classed as a cement mortar. Later on the lime was largely replaced with plasticizer. It's only really workable with a soft building sand.

 

[lizziecupcakes]

The vast majority of roofers will only be using cement mortars when repointing a chimney stack. Probably a 1/1/6 mix.

My roofer has said he will use a lime + cement + sand mix when I enquired about lime repointing. Wdyt? thank you

 

[lizziecupcakes]

This was almost exclusively in garden walls built of stone so comparable in exposure to chimneys. The 1:1:6 pointing clearly wasn't stable and after a while started to crack and fall out. Building from scratch in 1:1:6 has worked well for me, it's just 1:1:6 pointing of anything built in lime mortar seems a bit short term. In my experience NHL is a more reliable alternative that is just as easy to use.

thanks for this.
when you say "1:1:6 pointing of anything built in lime mortar seems a bit short term" - how long is short term? thank you

 

[Stevers]

My stone garden wall repointing in 1:1:6 was in about 2010, it was starting to separate off within a couple of years, but I only got round to repointing it in NHL in 2021. It was the last bit of NHL repointing on that 60' run of random rubble wall and it's all good so far, some of it going back to 2016.

I've been here 27 years and the work I do here needs to last, and not do further damage, or I'll be in a vicious cycle of repairs. My philosophy is to leave this house (previously the subject of benign neglect) in a healthier state than I found it - whilst still period correct - apart that is from the plumbing, electrics, heating...

 

[stuart45]

My roofer has said he will use a lime + cement + sand mix when I enquired about lime repointing. Wdyt? thank you

It's quite likely he's unfamiliar with using lime mortar. Roofers don't normally do a lot of pointing, usually restricted to chimneys where it's not clearly on display. When lime is first done, it needs protecting from the elements, and kept damp, and the average roofer isn't going to be coming back all week looking after it. They normally want to finish the job, get paid and off to the next one. Using a cement mortar gives them more confidence to leave it.
Mixing up hydraulic lime needs more care as well.