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.