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Green Building Bible, Fourth Edition
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    • CommentAuthorlsx
    • CommentTimeSep 28th 2021 edited
     
    Hi

    I feel bad posting about something so old in a forum that's mainly about new things, but I could do with some help.

    I have a victorian terrace that has a coal cellar underneath the front garden. The ceiling of this is provided by a large piece of stone that runs from the front door of the house almost to the garden gate. There is then a approx 20mm gap (that's about 100mm deep) between this piece of stone and another piece of stone that joins onto the pavement. Hopefully the photo will explain things better than I have.

    The gap used to be filled with a stone-like filler material that had failed (probably 30+ years old) and was allowing rain water to run into the coal cellar. I've removed the remainder of the filler and am now looking for something to refill the gap with. The filler was very much like the local Huddersfield stone in appearance and very hard, but you can tell it's not stone when you look at side-on.

    I was thinking I could use some NHL3 or NHL5 lime mortar, but I'm not sure how long that would last because it's in such an exposed location. I don't want to use anything cement based, because it needs to be flexible enough to avoid cracking when it gets cold/hot outside.

    Thanks for any advice.
      File 28-09-2021, 16 50 19.jpg
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2021
     
    I'd be tempted to fill the gap with a compressed rubber gasket (Compriband or similar), then maybe some sealant for good measure and finally your choice of finishing hard surface. But I'm no expert; others will have better suggestions I'm sure.
    • CommentAuthorowlman
    • CommentTimeSep 28th 2021
     
    I'm guessing that lifting the slab is too onerous a job? Does the slab cover other parts of the old coal cellar too if so how are the other parts sealed? or; Is the rest OK and just this is the vulnerable bit.
    • CommentAuthorlsx
    • CommentTimeSep 28th 2021 edited
     
    The slab is approx 3m (from the house to the gate) x 1m wide and to keep it simple I've not told you about the wall sitting on one side of it or the porch sitting on the other end of it. Lifting it would be a pain.

    The coal cellar is brick lined on the inside. On the outside the bricks are protected by a 3" skin of random stone and lime mortar. It generally works well, it's mostly water tight apart from the bit under the gate.

    I've attached another picture for a side on view of the slab. The complication with it all is that the slab has earth/soil next to it and so while the joint under the gate is stone to stone to brickwork (underneath), the joint at the soil side of the slab is stone to brickwork (wall) to earth (underneath) and earth (to the side).
      IMG_3568.jpg
    • CommentAuthorvord
    • CommentTimeSep 28th 2021
     
    It seems odd to have that sort of gap if water can fall right down to the cellar should the mortar fail. My temptation would be lime. NHL 3.5 will last for 50 years. If it's exposed to rain maybe NHL 5. If you grind down a table knife to about 6 to 8mm or smaller than the gap you can use it to press the lime into the full depth of the slab by packing it sideways.
    • CommentAuthorlsx
    • CommentTimeSep 28th 2021 edited
     
    Posted By: vordIt seems odd to have that sort of gap if water can fall right down to the cellar should the mortar fail. My temptation would be lime. NHL 3.5 will last for 50 years. If it's exposed to rain maybe NHL 5. If you grind down a table knife to about 6 to 8mm or smaller than the gap you can use it to press the lime into the full depth of the slab by packing it sideways.


    I totally agree re 'seems odd', a big weakness really. All the neighbours have one-piece slabs that go under their gates and butt up to the pavement with no joins. For some reason I'm the odd one out.

    I've done quite a few interior brickwork repairs with NHL lime so was hoping someone might suggest that. I was thinking I might have to do a couple of inches at a time though rather the full depth in one go.

    Any ideas what I could cover the work with to keep the rain off until the lime has set? I've currently got the gap covered with some dpc and gaffer tape but that only keeps the worst out.
    • CommentAuthorEd Davies
    • CommentTimeSep 28th 2021
     
    Is there any tar-like material which could be used either to fill the gap completely or for a cm or so at the top to protect lime below? Ideally matching the pavement to be less visually obtrusive.
    •  
      CommentAuthordjh
    • CommentTimeSep 28th 2021
     
    I like the tar (bitumen?) as a top coat. My worry about lime, especially NHL, is that it can crack or separate from the edge due to thermal cycling, frost etc and won't maintain a watertight boundary.
    • CommentAuthorlsx
    • CommentTimeSep 28th 2021 edited
     
    I like the sound of bitumen too, but working with something like that is way beyond my skill level. I'll take a look.
    • CommentAuthorlineweight
    • CommentTimeSep 28th 2021
     
    I might start thinking of something like making two grooves in the stone, one each side of the gap (and parallel with it) and then using some sort of inverted U-shaped strip of metal (maybe even lead), with each of its downward facing legs bedded into some kind of flexible sealant in the grooves.

    Then water would have to find its way around quite a few corners to get to the gap, and you wouldn't be 100% reliant on a perfect seal.

    Possibly I would start doing this and then realise softness/hardness of the stone made it impractical to form such grooves.

    But trying to just fill a vertical gap with "stuff" seems like a losing battle to me.
    • CommentAuthorlsx
    • CommentTimeSep 28th 2021
     
    Posted By: lineweightI might start thinking of something like making two grooves in the stone, one each side of the gap (and parallel with it) and then using some sort of inverted U-shaped strip of metal (maybe even lead), with each of its downward facing legs bedded into some kind of flexible sealant in the grooves.

    Then water would have to find its way around quite a few corners to get to the gap, and you wouldn't be 100% reliant on a perfect seal.

    Possibly I would start doing this and then realise softness/hardness of the stone made it impractical to form such grooves.

    But trying to just fill a vertical gap with "stuff" seems like a losing battle to me.


    I was actually trying to think of some way of building a 'french drain like' device into the bottom of the gap that would run into the garden but I don't think it's doable in 20mm.

    I didn't think it was relevant to say earlier, but I have actually got access to the bottom of the gap, because I've removed the top course of stone/brick from underneath the slab. Even with access to the underside though, I don't think I've got enough space to make a groove in the sides of the gap, even with a dremel.
  1.  
    Could you put a half-pipe/ gutter under the gap and run it off somewhere e.g. to the garden?
    As belt & braces approach as well as lime mortar & molten tar to seal the top inch.
    • CommentAuthorlineweight
    • CommentTimeSep 29th 2021 edited
     
    Posted By: lsx
    Posted By: lineweightI might start thinking of something like making two grooves in the stone, one each side of the gap (and parallel with it) and then using some sort of inverted U-shaped strip of metal (maybe even lead), with each of its downward facing legs bedded into some kind of flexible sealant in the grooves.

    Then water would have to find its way around quite a few corners to get to the gap, and you wouldn't be 100% reliant on a perfect seal.

    Possibly I would start doing this and then realise softness/hardness of the stone made it impractical to form such grooves.

    But trying to just fill a vertical gap with "stuff" seems like a losing battle to me.


    I was actually trying to think of some way of building a 'french drain like' device into the bottom of the gap that would run into the garden but I don't think it's doable in 20mm.

    I didn't think it was relevant to say earlier, but I have actually got access to the bottom of the gap, because I've removed the top course of stone/brick from underneath the slab. Even with access to the underside though, I don't think I've got enough space to make a groove in the sides of the gap, even with a dremel.


    I was thinking the grooves would be in the horizontal surfaces each side of the gap - rather than in the vertical sides of the gap.
    • CommentAuthorlsx
    • CommentTimeSep 29th 2021
     

    I was thinking the grooves would be in the horizontal surfaces each side of the gap - rather than in the vertical sides of the gap.


    Ah I get you. I like your idea.

    I have a multitool with carbide blades that can create cuts approx 2mm wide in the stone, so that might work.

    I guess you are suggesting something like:

    Fill the gap upto maybe 5mm from the top
    Take a metal strip and form into a U, run this at an angle so water runs off it into the garden.

    I've done a drawing in paint, is this close to what you are thinking?

    I suppose the only downside is I've never worked with lead like this before.

    I like what Dominic suggested re running a half-pipe at the bottom of the gap, but that would do nothing to stop water running down into the gap if the filler in the gap fails. It would also probably need a hole drilling through the side of the coal cellar (just under the slab) to run the water out to the garden.
      Strip in top.jpg
    • CommentAuthorlineweight
    • CommentTimeSep 29th 2021 edited
     
    Pretty much yes. I was thinking of something like the image attached below and just relying on the water running off but you could also make it a kind of drainage channel like you've shown.

    Don't take this as any kind of official advice! It's something I would experiment with a bit and might well find there is some reason it wouldn't work.

    I think you might find it tricky to shape lead into the form you've shown. For my version you'd need to put something in the gap to stop it sagging there.

    Maybe a rigid pre-made profile from aluminium or stainless steel would be easier than lead.
      Screenshot 2021-09-29 at 11.43.49.jpg
    • CommentAuthorowlman
    • CommentTimeSep 29th 2021 edited
     
    You could try this;-

    https://www.metals4u.co.uk/materials/aluminium/aluminium-t-section?p=2

    bedded into this:-

    https://www.roofingsuperstore.co.uk/product/black-jack-bitumen-trowel-mastic-5-litres

    Perhaps with the Tee section also sealed onto the flagstone top with a waterproof sealant.

    IMO forget lime mortar
    •  
      CommentAuthordjh
    • CommentTimeSep 29th 2021
     
    Or a U-channel and just cut or grind the sides down?

    https://metalsshop.co.uk/product-eng-713-30-x-15-x-2-mm-Aluminium-Channel.html
  2.  
    I would close the underside of the gap with something and then put 10mm or 20mm thick layer of adhesive silicone sealant at the bottom of the crack, once this is set I would then fill the remainder of the crack with a hard filling (e.g.sand and cement mix)

    As to why it is different to the rest of the houses - I would speculate that damage happened at some time in the past, possibly at build time and the option to get a replacement slab was not there so the choice was made to do it in 2 parts. After all it is quite a chunky slab of stone to replace.
    • CommentAuthorlineweight
    • CommentTimeSep 29th 2021
     
    No hard filling is going to remain attached to both sides of the gap so there will be small cracks and it will just provide a water storage zone, albeit with less volume of water than an open gap.

    So then you are relying on the flexible filling to hopefully stay attached to both sides, which seems optimistic. But if that is the strategy then surely it makes sense to fill the gap with hard filling first, to provide a backing for the flexible sealant, and then put the flexible sealant in as the top layer. This also would make it easier to replace the sealant in the future.
    • CommentAuthorArtiglio
    • CommentTimeSep 29th 2021
     
    At the risk of being a broken record, a foam like material to fill and seal the bottom half of the joint , then fill rest with black CT1 / Stickslikes##t or similar construction adhesive. Gaffer tape each side gun it in and smooth it off then remove tape once its cured.
    I have a sandstone slab forming a roof over a lower flat entrance on a property, the slab had cracked just outside the upper doorway. The cracks were ground out and normal silicon sealant gunned in , cut flush once cured and floor paint over that. It’s lasted 18 years so far.
    • CommentAuthorEd Davies
    • CommentTimeSep 29th 2021
     
    Posted By: owlmanbedded into this:-

    https://www.roofingsuperstore.co.uk/product/black-jack-bitumen-trowel-mastic-5-litres
    Ahah, that's the sort of tar-like stuff I was thinking of to fill the top of the gap. Bookmarked, thanks.
    • CommentAuthorowlman
    • CommentTimeSep 29th 2021 edited
     
    Sure Ed; it's fibre added and sticks like you know what, but once skinned over stays flexible underneath. Personally I wouldn't mess about except hot air gun dry, clean and prime the stone, gun a small amount of foam in the bottom to stop the stuff falling through and then just pack the whole gap with it.
    • CommentAuthorlsx
    • CommentTimeOct 2nd 2021 edited
     
    A huge thanks for all your replies.

    I've ordered some blackjack so will have to wait until mid-week before I get to have a go. Hopefully by then the weather won't be so bad anyway.

    Re priming the sides of the gap, won't this stop the blackjack sticking to it?
    • CommentAuthorowlman
    • CommentTimeOct 2nd 2021
     
    Try and get into the gap with something e.g. a small pointing trowel, to scrape the two sides of the stone clean of any organic matter, moss and the like, to provide a clean key. You could even use a thin wire wheel on a drill or angle grinder. Vac it out and then use a hot air gun if possible to get it nice and dry. You can then inspect and choose to bitumen prime or not, the brush on primer gives a key to subsequent materials.
    I just noticed on your second picture that there is an uneven difference of levels of the two stones. This may need a bit of judicious grinding out before any of the above IMO.
    • CommentAuthorlsx
    • CommentTimeOct 2nd 2021 edited
     
    You are right re a difference in the levels (the slab is slightly higher than the section under the gate), I'm just going to have to fill to the highest point I can.

    I'm going to mock up the situation with some random pieces of stone I have lying around, some bricks and some sand and see how the mastic behaves when there's no pressure on.

    I'd like to know how hard it is to clean the mastic off places I don't want it and how difficult it is to remove before doing it for real.

    It probably adds nothing at all but here's what it used to look like before.
      IMG_1731.jpg
  3.  
    Posted By: owlmanyour second picture that there is an uneven difference of levels of the two stones.


    I think that’s a plank of wood over the gap temporarily. Can see the outline of it in the earlier picture.
    • CommentAuthorowlman
    • CommentTimeOct 3rd 2021
     
    Posted By: lsx
    I'd like to know how hard it is to clean the mastic off places I don't want it and how difficult it is to remove before doing it for real.




    Mask off the bits you don't want contaminating. Masking tape and some polythene sheet.
    • CommentAuthorlsx
    • CommentTime5 days ago edited
     
    So here's the update finally....

    I've had a test go in an old storage box, lined it with dpc, found some pieces of stone and used building sand to form a trench/gap between the stone.

    I used some 902 primer on the faces of the stone either side of the gap.
      IMG_3632.jpg
    • CommentAuthorlsx
    • CommentTime5 days ago edited
     
    A few hours later (once the primer was dry) I used a small edging trowel to push some 903 mastic into the gap.

    Obviously it's a test so I wasn't as careful as I could be with keeping the mastic away from the top of the stone.

    It's worked well except the mastic's consistency is sticky like porridge so it's hard to get anything like a smooth finish on the top of it. If you lift the trowel upwards in an attempt to break contact with the mastic, you end up lifting a whole lot of mastic with it. So to finish I used a window packer to tamp the surface.

    Hopefully the top will be dry by tomorrow....
      IMG_3639.jpg
    • CommentAuthorlineweight
    • CommentTime2 days ago edited
     
    Two things I notice looking at the data sheet for that stuff

    https://res-2.cloudinary.com/diq3xoegw/image/upload/v1/datasheets/roofing/bitumen/blackjack_903_bitumen_trowel_mastic_v1.1

    - They say you should incorporate a woven mesh reinforcement that overlaps the gap, if you are filling a large crack

    - They are very vague about the life expectancy - the amount of time the mass of material will remain flexible is only stated as "long periods" or "many months". They have chosen not to say "many years" or give any actual figure at all.
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