Choice of render mix

At the back of our typical north london 1930's pebble-dashed house is a chimney stack that used to service the original coal fired boiler. That boiler was removed decades ago and the chimney remains, finishing somewhere in the space above the kitchen ceiling. It runs up the external face of the back wall (north facing), starting around first floor level and finishing about 5 courses of brick above the eaves. The pebble-dash on it has been patched up over the years and is now falling off in chunks. The base/scratch coat too in places.

I'd like to re-render this and, depending on the adhesion, will later decide whether to take it all back to brick or just patch repair. The underlying brick and mortar seem fairly poor (probably quite porous). Also, I'm considering skipping pebble dashing and just texturing the render.

What render mixes suit the underlying brickwork and hopefully match in general the existing pebble dash colour and, to some extent, texture?

2:7:1 cement:sand:lime seems a starting point. Some mixes mention using 50/50. Does this mean 1/2 sharp and 1/2 builders sand?

Given my situation, is it worth including lime or a waterproofer? Essentially I'm looking for best mix for the situation and my plastering skill level (low!).

Someone mentioned adding airbrick(s) in that chimney too. The interior wall of the chimney backs onto the bathroom, and that suffers a bit from discolored wall paper but isn't damp to touch.

Any advice very much appreciated. Thanks.

Reply to
Dean
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You can use any ratio of sands you want. I think my house was rendered in all sharp. Nice rough texture but not unpaintable. Pebbledash is horrendous to paint as is stucco.

ratio of cement to sad is not critical, and the above seems good. More cement is more waterproof and less porous, but more prone to cracking if the underlying substrate is not sound.

I am not clear why lime is in the mix at all in this case. Generally render is 'showerproof' in that water will penetrate a little. If there is enough cement between the sand grains, its fully waterproof. It dries out fast anyway. The main thing is that it dries out before it freezes or you will get spallation of the surface.

If freezing temps and driving rain are an issue, you can always seal it before painting. That fully stops water getting IN, but may if you have damp masonry from other cases, also stop it getting OUT.

If the chimney is disused, cap it but allow ventilation. stop the rain getting in: allow moisture to evaporate out.

Reply to
The Natural Philosopher

IME, if you paint the brickwork with 4 water:1 pva first, pretty much any mix will hold on well. For small areas I find B&Q general purpose mortar gives a decent finish.

Reply to
stuart noble

1:1:6 is a very common render mix

NT

Reply to
Tabby

If it was designed to be rendered in the first place (seems quite likely if it's pebbledash), then grade B bricks and a less skilled brickie will have built it. Grade A bricks and skilled brickie do the brickwork on show, not the stuff that's invisible. Actually, rough finish and raked out pointing will help the render stick. Musn't be loose bricks though.

Sharp sand or rendering sand will give you a stronger/better render. Builders sand would be easier for a novice to get a polished surface but that's of no significance up on a chimney, and it will wear faster. Adding building sand to sharp sand will help make it more waterproof (depending how much fine sand there is in the sharp sand to start with), but I wouldn't personally go as far as 50/50.

1/2 sharp and 1/2 builders sand is used where you want an authentic 1900's building sand, which wasn't graded like sands are today. More likely to use this for making up a lime mortar (with no cement).

The flue needs to be vented at top and bottom, so a through draft is maintained. Do the venting to the outside if possible (e.g. if chimney is on an outside wall) so the through draft isn't stealing heat from the house. If the top has been blocked off, you can do this by swapping in an air brick near the top of the chimney. If the top is still open, you could leave it open, or you could get one of several types of pot insert which protect against rain, but still leave it ventilated. Protecting agaist rain will increase the life of the chimney, but blocking it off without ventilation will reduce the life of the chimney, and internal decorations.

and if you're using a cement and lime mix, they should be in the same ratio, so although you might change the '6', don't change the '1's. (Most cement and lime mixes don't work well, but 1:1 does.)

I'm not sure I would use a lime mix on a chimney TBH. I would use a mortar plasticiser admix instead with a sand and cement render.

Reply to
Andrew Gabriel

Great. Thank you all for the advice. I'll forgo the lime and builders sand and go for a cement, sharp sand render, 1:4 (?) with some plasticiser.

Regarding poor pointing, shouldn't repoint first then render? If I render over the raked out pointing, will this make the render too keyed-in to the brickwork? Though I like the idea of raking out the bad mortar then doing the repointing and render as a single application.

As far as I can tell the chimney has no ventilation whatsoever. I think it's capped at the top but haven't gone up high enough to check yet. I looked on Google Earth but it wasn't clear ;-) Whatever, I will go for ventilation top and bottom as suggested.

Thanks again.

Reply to
Dean

That's a very strong mix for this purpose, in my opinion. I'd go with a max of 1:5, and possibly even 1:6. I haven't done anything this exposed though.

Too keyed-in? What do you think is going to happen?

One other thought - don't assume any structural integrity of the chimney until you have verified it. i.e. don't assume you can hang off it, support a ladder against it, or anything like that, until you have established it's not just a pile of bricks with no bonding. I noticed when my scaffolders were working near mine, they treated it like it was a pile of children's building blocks, which I guess a good number of the chimneys they deal with are. (Fortunately, mine wasn't bad at all.) Personally, I would only work from scaffolding. I wouldn't dare put a ladder up against a chimney anyway, although I have seen builders do so.

Reply to
Andrew Gabriel

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