Repointing External Wall - Experience of Lime Putty or NHL

[Rawler]

Dear Community,

I need to repoint a gable end wall. My house is on a hillside in Derbyshire, but the next building is only about 1.5m away so it's not hugely exposed, although there is potentially a wind tunnel effect (?). I am well versed in the different types of lime mortar, necesary types of sand and have worked with both lime putty and NHL based, but I'm getting conflicting advice between books, local heritage tradesman, AI, and even SPAB etc. My stone is Gritstone, so relatively hardwearing.

On one hand most practictioners and vendors seem to advocate NHL 3.5, but could be due to the ease and speed? Reading "Building with Lime" (Homes and Wingate), it advocates that the lime mortar should always be stronger in the middle of the wall than at the sacrificial pointing (edge). Given the building was built at the end of the 19th Century its likely just (air) lime putty mortar (not NHL). So putting any NHL to repoint will be stronger than the existing lime mortar and could cause the stone to degrade in time. I guess the question is as long as the NHL is weaker than the stone the pointing won't cause the stone to spall, so should be fine? But I can't get away from the fact the using putty based to match the existing will allow the whole wall to work better. Has anyone had any direct, non biased experience of this and seen the effect of NHL versus putty on external semi exposed walls over 20 years? I'd prefer to do it well and once.

Many thanks

 

[stuart45]

When I've got any doubts I take samples to the local suppliers (limebase) and take their advice. Although they compare NHL3.5 to a 1/1/6 mix and NHL2 to a 1/2/9 mix, I've used NHL3.5 without issues on random rubble walls built with Chert for over 20 years without any problems. Larger joints are more likely to get taken out by severe weather, so NHL is a bit easier to use on these jobs. You have to remember that the lump lime delivered to sites years ago was usually hydraulic, so you might find the lime in your building isn't a pure air lime.

 

[Stevers]

You can also 'soften' the NHL3.5 by mixing 50/50 with fresh Hydralime to make something similar to an NHL2 to sort of hedge your bets. The hot lime they used to mix in a hole in the ground in the old days would also have picked up impurities that might well given some degree of initial hydraulic set.

 

[stuart45]

Taken from an old textbook on brickwork. It recommends that chalk lime should not be used for brickwork, as it tends to form a surface crust. Stone or grey chalk for most work, or blue lias for damp condiions.

 

[Stevers]

@stuart45 Is Hydralime not Hydrated Lime, and is Hydrated Lime no good for 'softening' NHL?

It was something I was planning to try for my next project as the stone dust I use with sand seems to quicken the set.

 

[stuart45]

@stuart45 Is Hydralime not Hydrated Lime, and is Hydrated Lime no good for 'softening' NHL?

It was something I was planning to try for my next project as the stone dust I use with sand seems to quicken the set.

Yes, IMHO that would help soften up NHL. The stone dust must be acting as a pozzolan. Hydralime is hydrated lime, normally used with cement mortar for something like a 1/1/6 mix. Years ago I used a 1/2/9 mix for old properties, but since the Smeaton project that mix is not officially a proper mix any more. It's still popular in the USA, where I think it's known as type O mix.

 

[a twig]

NHL3.5 is more popular with trades etc these days mainly because most builders merchants now stock it as their token “lime mortar product” whereas in my experience 15-20 years ago it was nigh on impossible to find NHL in the UK outside of specialist suppliers.

That said in your case I’d be inclined to either go for NHL2 or as suggested above an NHL3.5 softened with hydrated lime - if the gaps between the stones are large I’d go for a mix of 1 NHL3.5, 1 hydrated lime, 2 sharp sand and 4 builders sand