How long until new mortar likely to fail if done inappropriately?

[ben81]

Hi everyone, I've been reading up on here the last few days and learnt a lot more about old buildings so thought I'd ask this question about some repointing we've had redone

We bought our Edwardian (1903) house 2 years ago. The back wall was in serious need of repointing due years of being hit by the weather and some seriously atrocious cement DIY jobs by I suspect the previous owner. The gable end and chimney looks to have never had any work done due to difficulty accessing (single story extension coming off it) so had patches of missing or failing mortar (fair enough after 120 years I suppose!) After 2 years of searching for tradesmen who knew the importance of not using cement and being let down by tradesmen not showing up when they said they would we finally found a firm who seemed to know a bit more about old buildings.

They used small masonry chisels to rake out so as to cause minimal harm to the bricks and didnt attempt pointing on seriously rainy days but did some work in rainy conditions. The lime they used was the powdered NHL3.5 pre-mix from Cornish lime. They bought some hessian with them which they only used on a section for a day or two during the recent very cold spell, and they wrapped the chimney in vinyl over christmas to protect from the elements. The new flaunching to the chimney didnt seem to be going off as they expected which they suspected was related to the frost so they hacked it off and did it again.

My question if anyone knows is, with all the rain we've had recently and some of it being put in while the temperature was below 5 degrees, if the mortar is going to fail is it likely to happen over the coming months, when the real frost sets in this month or feb, over the summer etc when things dry out, ie the near future or potentially could it happen in 5 to 10 years or so? I know this is a bit of a how long is a piece of string question but I'm wondering if when it fails does it do so while its setting or during the first seasonal changes as a rule? Theyve said it will outlive me so I can live with the huge amount of money I paid due to scaffolding and labour costs if thats the case, but if its going to start falling out in clumps over the next few years its going to be a very bitter pill to swallow!

 

[stuart45]

You can usually tell with cement and lime mortars if they haven't cured properly and the frost has got at them by the face breaking up.
Ideally, this is not the time of year for lime work. Even with cement mortar, care is needed.

 

[ben81]

Thanks for your reply Stuart. So we'd likely notice fairly soon if the job is likely to fail? That would be reassuring as it will still be under guarantee.

 

[stuart45]

How long is the guarantee?
NHL3.5 does cure a bit quicker than an air lime.

 

[ben81]

2 years I believe but they're saying the pointing will outlive me, I just hope they're right!

The genius who came a few months back and wanted to grind out the chimney with machines and repoint in cement said it would last "8 years minimum" At least he was honest I suppose.

 

[Lacan07]

Why were they doing it in Winter, if you had a choice it would be Spring / Autumn?

 

[Lacan07]

Its a bit difficult to answer your question really, I've been using NHL 3.5 premix and I've never had a bad joint when repointing. The only issue I've had was when replacing whole sections of brick work, every now and again I'd get the mix too wet and it wouldn't suck into the brick. In this instance you can tell its not taken to the brick which wobbles, and the mortar goes crumbly when it dries. Lime likes to be a dry mix, and the moisture is added by either wetting the joint or dipping the brick.The thing about NHL is when it goes off it goes off! Its not nearly as forgiving as Lime putty based mortar

 

[stuart45]

With very porous bricks, idealy, they should be damp in the centre and surface dry. This way there is enough suction to stop the mortar running down the face, but they are not so dry as to suck too much water from the mortar.
When mixing up NHL mortar with a drum mixer, it's best to mix it dry for 5 minutes, then add some water and mix for about 15 to get enough air into the mix. Leave for another 20 to fatten up, then mix for a few more mins.
With a roller pan mixer it can be done a bit quicker.

 

[a twig]

Mixers!? Luxury! Were barrow ‘n’ shovel back in my day etc etc

 

[paulc]

Mixers!? Luxury! Were barrow ‘n’ shovel back in my day etc etc

For small quantities, I use a paddle mixer in the end of an electric drill.

 

[stuart45]

There aren't many labourers nowadays who know the correct method of mixing up by hand. Even the videos by the pros seem to show the DIY method of a volcano.
Another common mistake when mixing up a cement mortar by hand, is to add more water to make it workable. When it's just a bit stiff, it should be turned over for at least 5 minutes to get enough air into the mix. Tiny air bubbles act like ball bearings and make the mix much more workable.

 

[Feltwell]

You can usually tell with cement and lime mortars if they haven't cured properly and the frost has got at them by the face breaking up.
Ideally, this is not the time of year for lime work. Even with cement mortar, care is needed.

I agree with Stuart - I've had pointing fail due to unexpected frost and it was immediately obvious, as soon as it went above freezing the face of the pointing was crumbling.

 

[ben81]

Why were they doing it in Winter, if you had a choice it would be Spring / Autumn?

I think they were trying to get it all in before the ideal cut off period but with it taking longer than anticipated to rake out we got caught out with the cold then the rain. It all looks to have turned out well so far apart from the aforementioned flaunching which they re-did