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Andrew Gabriel Andrew Gabriel is offline
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Default Choice of render mix

In article ,
Tabby writes:
On Sep 2, 1:59*pm, 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


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.

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?


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).

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.


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.

Any advice very much appreciated. *Thanks.

1:1:6 is a very common render mix


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.

--
Andrew Gabriel
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