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The Natural Philosopher[_2_] The Natural Philosopher[_2_] is offline
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Default Choice of render mix

Dean wrote:
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?


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.


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!).

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.




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.


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



Any advice very much appreciated. Thanks.