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General: repointing chimney with nhl

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  1.  
    • CommentAuthorrsk1
    • CommentTimeNov 18th 2025
     
    I'm planning to repoint the chimney on my Victorian property whilst i have scaffolding up. Thinking of an nhl 3.5 mortar but I'm aware that this is the wrong time of year for lime mortars. I'll be able to keep it covered with hessian for a week, during which time the temp will likely dip below 5 degrees or get very close. After that we're heading into Winter so almost certainly getting colder. In other words, marginal conditions at best to or nhl. So what should i do? Risk an nhl mortar or would a weak cement mortar be better on balance?
    • CommentAuthorGreenPaddy
    • CommentTimeNov 19th 2025
     
    A couple of questions that might help clarity to give better answers...

    - is the chimney passsing through the slates/tiles, or is it part of the gable wall, so no break from ground to pots?
    - is it a brick or stone build?
    - if brick, what is the percieved benefit of nhl over cement-based for masonry that is not part of the thermal envelope, where internal/external vapour movement might be important?
    • CommentAuthorrsk1
    • CommentTimeNov 19th 2025 edited
     
    Brick chimney emerging from slate roof, ie: no gable
    Percieved benefits of nhl versus cement based mortars: softer and more porous mortar means better durability of pointing and less likelihood of damage to brickwork because of better moisture management
    •  
      CommentAuthorfostertom
    • CommentTimeNov 20th 2025
     
    rsk1, your last comment is suffering from GBF's current glitch - making all comments into private Whispers by default. For some reason, I can see what you Whispered privately to yourself, but most other users AFAIK can't see it. Edit the post, deleting your name from the Whisper box.
    • CommentAuthorLF
    • CommentTimeNov 20th 2025
     
    Greenpaddy's advice seems wise.

    I am a novice but we did my son's chimney earlier this year with Lime(Summer)
    We read that if mortar should be softer than the brick, I seem to remember.

    He used stainless helibars and resin on some parts to make it stronger.
    He had 3 bricks come down with the TV arial.
    I think we were not that happy with the inside that had all the smoke going through it.
    • CommentAuthortony
    • CommentTimeNov 22nd 2025
     
    I much prefer lime putty to nhl
    •  
      CommentAuthordjh
    • CommentTimeNov 23rd 2025
     
    Posted By: tonyI much prefer lime putty to nhl
    Do you prefer it at this time of year?
    • CommentAuthorArtiglio
    • CommentTimeNov 23rd 2025
     
    When i did my 1870 building, the stacks were in poor condition, I was advised to use nhl 3.5 rather than a lime putty mortar, the reasoning was that it would have better resistance to weather on what is the most exposed part of the building.
    It was also suggested i add double boiled linseed oil to the mix to improve water shedding and use a masonry water repellant. All this based on many of the bricks being pretty worn. The top 8 courses were taken down and rebuilt using new replacements for the worst bricks.
    As with any such job, i’d expect to be long gone before anyone will know if it was the right approach.
    I’d hazard a guess that TV aerials having been affixed to the stack had done more damage than the weather. Plus seeing as there probably had’nt been open fires in the place since the seventies another major source of damage is no longer a factor either.
  9.  
    rsk1 Do you need to repoint the bricks or will render be acceptable? if the bricks are in poor condition then rendering may be a better option. I have just had 165 m2 of limestone wall rendered with
    Baumit Sanova MonoTrass
    which according to the spec. is a dehumidifying plaster based on lime and natural pozzolan, single-layer, water-repellent, for manual and mechanical application, suitable for the renovation of old buildings, for internal and external use.?
    It set OK in about 2-3 days and I am impressed with the results - hopefully long term is equally impressive.

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