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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    Our rear garden wall is a mess and I’m looking for tips on what to remedy it with.

    Built in a hurry to satisfy a neighbour, our engineer ruled that it could only be 1 leaf (wythe?) thick to avoid an overturning risk on the retaining concrete below. As a result the builder shuttered our side, so the neighbour has a nicely pointed stone face, but we get all the cement snots and occasional cavities. You can see from the photo there are occasional deeper piers at centres, so the length of the wall is broken up into bays.

    Rebuilding is out of the question. So I’m thinking the best option would be a rough cast lime render to give some frost protection and hide the worst bits on our side. I’ve done some lime re-pointing on historic walls before, but never anything like this.

    So:

    1. Is a lime coat even possible here? The wall’s entirely built of nasty cement but I figure you can cover impermeable cement with breathable lime, just not the other way around, right?

    2. What lime product/mix is appropriate in this case? I’d prefer something ready mixed due to my limited experience and lack of a decent mixer. Is a 20mm coat about right or nowhere near enough? I'm not looking for a flush/straight finish - if anything I'd like to leave some stones exposed but I'm now resigned to the fact that it's not good enough stone to leave on show, so we'll just have a whitewashed rubble wall finish.

    3. I feel like I should try to gouge out some of the excess cement in the joints and repoint flush in lime, but it’s rock hard - is this advisable? What tools could I hire for this? SDS drill or something more like a mini-jackhammer?

    4. My wife has pointed out that even if we achieve this, there's currently no coping at the top due to the wall thickness varying with the stone content. Are there any clever details that could stop the next heavy rainfall washing away my efforts?
      wall.jpg
    • CommentAuthortony
    • CommentTimeJan 18th 2021
     
    Yuk, what a mess, you could (I would) leave the pier exposed , lime is hard to work with but it could be done, can be bought premixed

    pragmatic approach would be two coats or mote realistically a flattening out coat then two coats of high lime render 4 sand, 1 sharp sand, 1 cement 1 lime

    I would do something on top first to protect the wall cut paving slabs set a sideways fall
    •  
      CommentAuthordjh
    • CommentTimeJan 18th 2021
     
    I second what Tony said. I'd suggest using a (feebly?) hydraulic lime so you don't have to wait so long for it to go hard. I think coping stones on the top are definitely a good idea. You might want to try scratching any large flat areas of cement to give a bit better key for the lime.

    If you go for only a 20 mm covering, you'll need to follow the surface fairly closely. It might be easier to flatten it a bit first as Tony suggests, though that will use more material.
  2.  
    What would be the point of lime rendering a wall built out of concrete and stone? I would render the wall with cement render and if the piers are all as 'good' as the photos then I would leave them unrendered and perhaps cleaned up a bit as a feature (Tonys suggestion above). For the top - flaunching (is that the right word in this context?) or cut paving stones (as above) or even ridge tiles. IMO unevenness in the rendering due to undulations in the wall won't matter - it becomes a garden wall with character.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 18th 2021
     
    I sometimes timber clad very rough walls - fix vertical battens to the wall with concrete screws at approx 600mm centres, then horizontal slating battens (25x50), with 8mm shaddow gaps.

    Paint the battens with a watered down grey wood stain to kill the green of the treated slating battens, and give them an immediate weathered look. Super cheap, and visually very effective. Would show off the stone piers.

    Oh, and paint something black over the stone first, so it doesn't show through the batten gaps. Definitely capping to the wall, and exactly as Tony suggested, paving slabs cut to size.

    Just a thought :smile:
    • CommentAuthorjfb
    • CommentTimeJan 19th 2021
     
    I'm with Peter. What is the purpose of using lime here?
    Sure use hydrated lime as a plasticiser with the cement (not sure if that is what you are talking about Tony) but i don't see the benefit of hydraulic lime here.
    Rounded flaunching on top definiteley another option to coping stones

    Whatever render you use you should be fine with two layers - one to fill in holes and provide a flattish scratch coat for the finishing coat. I wouldn't start hacking anything out unless it really sticks out. As said above you will need to make a key for the render in some of the flat cement areas (angle grind some parallel lines). Make sure you use sharp sand for the render.
    • CommentAuthorrevor
    • CommentTimeJan 19th 2021
     
    I rather like green paddy's suggestion must remember that one, you could use the battens as a trellis to grow plants up make a good feature in the garden and very green, economical and quick (apart from the plants growing that is)
  3.  
    Posted By: jfbI'm with Peter. What is the purpose of using lime here?


    Simply that I'd rather not use any more cement than I have to, and all the advice for stone walls is that cement render coats are a nightmare in the long term. We're stuck with the cement 'core' that we have, but surely adding a slightly more flexible & breathable render on the outside would be better than whacking even more cement on top...

    Posted By: GreenPaddyI sometimes timber clad very rough walls


    That's a good idea and one that I've considered since our whole house is timber clad, however I wanted to protect the stonework first regardless, so this is Step 1 if you like and we'll see how it turns out!

    Thanks all for the suggestions and reminder to key the surface.

    Posted By: djhI'd suggest using a (feebly?) hydraulic lime so you don't have to wait so long for it to go hard.


    djh, that seems to contradict the advice here:
    https://www.lime.org.uk/community/types-of-lime/types-of-lime.html

    "Feebly hydraulic lime (NHL 2) – contains less than 12% clay which means it is slower to set (about 20 days in water);
    Moderately hydraulic lime (NHL3.5) – contains 12%-18% clay which gives a slightly faster set (15-20 days in water);
    Eminently hydraulic lime (NHL5) – has up to 25% clay content which means that a much faster set is achieved (hours/days)."

    So am I after Eminently hydraulic (NHL5)?
    •  
      CommentAuthordjh
    • CommentTimeJan 19th 2021
     
    Posted By: Doubting_Thomasdjh, that seems to contradict the advice here:
    https://www.lime.org.uk/community/types-of-lime/types-of-lime.html

    "Feebly hydraulic lime (NHL 2) – contains less than 12% clay which means it is slower to set (about 20 days in water);
    Moderately hydraulic lime (NHL3.5) – contains 12%-18% clay which gives a slightly faster set (15-20 days in water);
    Eminently hydraulic lime (NHL5) – has up to 25% clay content which means that a much faster set is achieved (hours/days)."

    So am I after Eminently hydraulic (NHL5)?

    If you want to use lime I think you basically want to use the weakest hydraulic lime you can get away with. Whether that is 'feebly' or not I don't know. Air/fat lime takes longest and needs reasonable control of moisture and temperature but is most flexible, most breathable and workable for longest. Eminently is strongest, most rigid, sets quickest and doesn't mind at all about moisture/rain. Ask your chosen supplier for advice about the best product to choose.
    • CommentAuthorjfb
    • CommentTimeJan 19th 2021
     
    just use NHL 3.5
  4.  
    Posted By: Doubting_Thomas
    What is the purpose of using lime here?


    Simply that I'd rather not use any more cement than I have to, and all the advice for stone walls is that cement render coats are a nightmare in the long term. We're stuck with the cement 'core' that we have, but surely adding a slightly more flexible & breathable render on the outside would be better than whacking even more cement on top...

    The advice to use lime base render on stone walls assumes that there is either lime or rubble/earth mortar in the build up of the wall. Given that you have a cement mortar which essentially amounts to a concrete with very large aggregate I can't see the problem with a cement render. OK cement doesn't breath like lime but with a cement/stone wall there isn't going to be much breathing of the wall with or without the render (of any sort).
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