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Choice of render mix



 
 
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  #1  
Old September 2nd 10, 01:59 PM posted to uk.d-i-y
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Posts: 18
Default 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.

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  #2  
Old September 2nd 10, 02:55 PM posted to uk.d-i-y
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Posts: 18,600
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.

  #3  
Old September 2nd 10, 03:08 PM posted to uk.d-i-y
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Posts: 4,906
Default Choice of render mix

On 02/09/2010 14:55, The Natural Philosopher wrote:
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.


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.
  #4  
Old September 2nd 10, 10:55 PM posted to uk.d-i-y
external usenet poster
 
Posts: 1,532
Default Choice of render mix

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
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.


1:1:6 is a very common render mix


NT
  #5  
Old September 3rd 10, 10:38 AM posted to uk.d-i-y
external usenet poster
 
Posts: 9,161
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
[email address is not usable -- followup in the newsgroup]
  #6  
Old September 3rd 10, 01:19 PM posted to uk.d-i-y
external usenet poster
 
Posts: 18
Default Choice of render mix

On Fri, 3 Sep 2010 09:38:38 +0000 (UTC), Andrew Gabriel wrote:

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.


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.



  #7  
Old September 6th 10, 04:19 PM posted to uk.d-i-y
external usenet poster
 
Posts: 9,161
Default Choice of render mix

In article ,
Dean writes:
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.


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.

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


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

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.


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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
 




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