Rendering

essexandy

The Lake Governor
I've been asked by Church to put together a guide on the use of sand & cement for external rendering, what follows is a mix of information gleaned from British Standards and nearly thirty years of personal experience.

Rendering is generally made up of two coats of sand, cement and lime, except in areas of severe exposure where three coats of render should be used. If you don't want to get a copy of the British Standards for rendering I believe there is a map of the UK giving the areas considered to be of sheltered, moderate and severe exposure in the Catnic catalogue. The map shows that most of us live and work in areas considered to be of sheltered or moderate exposure.
Sand
All sand used for external rendering should be washed and uncontaminated. For general rendering you should use as coarse a sand as it is practicable to use for the for the desired finish, the sand sold by B&Q's and Wickes as plastering sand (at least in Essex) is good and other suppliers often refer to it as "fine sharp". For some pattered and banded finishes it may be preferable to use a finer sand. The sand should be neither to dry or to wet prior to mixing, I would recommend always sheeting up the sand as this stops it getting to wet and also keeps the cats off.
Cement
For the scratch coat/ coats I use either Ordinary Portland except where there are soluble salts in the background and here you should use sulphate resisting cement. For the top coat I use either Ordinary Portland or Snowcrete (white cement).
Lime
Hydrated lime is used in external rendering.
Fibres
I use fibres in both my scratch and top coats except in the top coat for dashing.
Admixtures
I always use a three in one admixture for my scratch coat (plasticiser, waterproofer and retarder) and a small amount of liquid febmix in the top coat.
Scratch coat
Before you start you need to determine the strength/density of the background so that you can use the appropriate strength mix. If the background is high suction you should dampen but not soak it before applying the scratch coat, once applied the render must be kept damp for at least three days to allow the cement to fully set and cure. If allowed to dry to quickly the render will never set and harden correctly. Personally I don’t use lime in the scratch coat as it doesn’t work well with some of the admixtures I use.

Top coat
I never put a top coat on until the scratch has been allowed to cure for a minimum of five days in the summer or seven days in the winter. Once you have finished an area of top coat this should not be allowed to dry out to quickly and should be kept damp the same as the scratch coat was.
Mixes and spec. suitable for moderate and sheltered exposure
For two coat work the finished thickness should be between 16mm-20mm

Background Scratch coat Topcoat
Strong/moderate 1:3 to 4 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

Moderate/weak 1:5 to 6 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

EML 1st. 1:3 @ 3mm to 6mm Dashing 1:1:5 to 6
2nd. 1:3 to 4 @ 10mm to 14mm Float finish 1:1:6 to 8
Patterned “

Mixes and spec. suitable for areas of severe exposure
Three coat work should have a finished thickness of not less than 20mm
Background Scratch coat Topcoat
Strong/moderate 1st. 1:3 to 4 @ 8mm to 12mm Dashing 1:1:4
2nd. 1:3 to 4 @ 8mm to 12mm Float finish 1:1:5
Patterned “

Moderate/weak 1st. 1:5 to 6 @ 8mm to 12mm Dashing 1:1:6
2nd. 1:5 to 6 @ 8mm to 12mm Float finish 1:1:6
Patterned “

EML undercoat as per moderate Dashing 1:1/2:4 to 41/2
exposure Float finish 1:1:5 to 6

The mix of each successive coat should never be richer in cement than the mix used for the coat to which it is applied.
Strong mixes have a high drying shrinkage and are consequently more likely to cracking, their use should be restricted to strong backgrounds such as first coats on EML and abrasion-resistant applications such as plinths. Mixes such as 1:1:5 to 6 or 1:2:8 to 9 are less liable to cracking and crazing than stronger mixes.
There is so much to external rendering and so many ways to go wrong with serious consequences that in my opinion it should not be attempted by the inexperienced on anything but a small scale.
 

beddy

Well-Known Member
definately mate. nice one, just one question. why is there 2 ratios under scratch coat? is it one top one for dashing, second one for float finish?
 

Chris W

Active Member
2 scratch coats for three coat work...

like a splatter coat on eml, followed by a scratch, followed by a float coat...

i think... that is it andy? right?
 

essexandy

The Lake Governor
steve cov said:
definately mate. nice one, just one question. why is there 2 ratios under scratch coat? is it one top one for dashing, second one for float finish?
Yes Steve just read across from the scratch to the finish to get the correct ratios.

Chris W said:
2 scratch coats for three coat work...

like a splatter coat on eml, followed by a scratch, followed by a float coat...

i think... that is it andy? right?

With three coat work both the scratch coats are trowel applied to a decent thickness Chris not just a thin coat for adhesion purposes like a splatter coat.
I know I've left as many questions as given answers but it's a job to know where to stop.
 

plasterjfe

Private Member
nice on andy i think if i ever had to go back from polymers to sand and cement i will print this off and stick it on the mixer
 

merlin

Private Member
you essex boys defo know your rendering, been travelling through essex lately and some of the renders especially the pargetting is stunning.
 
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richardbrown

Private Member
merlin said:
you essex boys defo know your rendering, been travelling through essex lately and some of the renders especially the pargetting is stunning.
dont come down to plymouth mate ... we just bang it on then wipe a bit of skirting over it then a quick sponge
 

essexandy

The Lake Governor
richardbrown said:
merlin said:
you essex boys defo know your rendering, been travelling through essex lately and some of the renders especially the pargetting is stunning.
dont come down to plymouth mate ... we just bang it on then wipe a bit of skirting over it then a quick sponge
Funny you should say that Richard I'm coming down your way for a break soon and thought I might call in and pester you for a while.
There are still as many rough clueless barstewards in Essex as anywhere else Merlin.
 

merlin

Private Member
i know mate . seems to be alot of pargetting your way tho, some very good, some really good free hand stuff, not just the stamping.
 

peck

New Member
Hi. Thanks, i found the guide very useful. I followed it to the letter and the small area i had to do turned out really nicely. My name s greg, I m a bricklayer by trade but do all my own skimming, not as fast as most plasterers, but i aim to get a nice finish. Not so clued up on rendering though, hence the questions. Thanks for the help, top site, wish there was a brick/stonework equivalent. thanks again
regards
Greg
 
I've been asked by Church to put together a guide on the use of sand & cement for external rendering, what follows is a mix of information gleaned from British Standards and nearly thirty years of personal experience.

Rendering is generally made up of two coats of sand, cement and lime, except in areas of severe exposure where three coats of render should be used. If you don't want to get a copy of the British Standards for rendering I believe there is a map of the UK giving the areas considered to be of sheltered, moderate and severe exposure in the Catnic catalogue. The map shows that most of us live and work in areas considered to be of sheltered or moderate exposure.
Sand
All sand used for external rendering should be washed and uncontaminated. For general rendering you should use as coarse a sand as it is practicable to use for the for the desired finish, the sand sold by B&Q's and Wickes as plastering sand (at least in Essex) is good and other suppliers often refer to it as "fine sharp". For some pattered and banded finishes it may be preferable to use a finer sand. The sand should be neither to dry or to wet prior to mixing, I would recommend always sheeting up the sand as this stops it getting to wet and also keeps the cats off.
Cement
For the scratch coat/ coats I use either Ordinary Portland except where there are soluble salts in the background and here you should use sulphate resisting cement. For the top coat I use either Ordinary Portland or Snowcrete (white cement).
Lime
Hydrated lime is used in external rendering.
Fibres
I use fibres in both my scratch and top coats except in the top coat for dashing.
Admixtures
I always use a three in one admixture for my scratch coat (plasticiser, waterproofer and retarder) and a small amount of liquid febmix in the top coat.
Scratch coat
Before you start you need to determine the strength/density of the background so that you can use the appropriate strength mix. If the background is high suction you should dampen but not soak it before applying the scratch coat, once applied the render must be kept damp for at least three days to allow the cement to fully set and cure. If allowed to dry to quickly the render will never set and harden correctly. Personally I don’t use lime in the scratch coat as it doesn’t work well with some of the admixtures I use.

Top coat
I never put a top coat on until the scratch has been allowed to cure for a minimum of five days in the summer or seven days in the winter. Once you have finished an area of top coat this should not be allowed to dry out to quickly and should be kept damp the same as the scratch coat was.
Mixes and spec. suitable for moderate and sheltered exposure
For two coat work the finished thickness should be between 16mm-20mm

Background Scratch coat Topcoat
Strong/moderate 1:3 to 4 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

Moderate/weak 1:5 to 6 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

EML 1st. 1:3 @ 3mm to 6mm Dashing 1:1:5 to 6
2nd. 1:3 to 4 @ 10mm to 14mm Float finish 1:1:6 to 8
Patterned “

Mixes and spec. suitable for areas of severe exposure
Three coat work should have a finished thickness of not less than 20mm
Background Scratch coat Topcoat
Strong/moderate 1st. 1:3 to 4 @ 8mm to 12mm Dashing 1:1:4
2nd. 1:3 to 4 @ 8mm to 12mm Float finish 1:1:5
Patterned “

Moderate/weak 1st. 1:5 to 6 @ 8mm to 12mm Dashing 1:1:6
2nd. 1:5 to 6 @ 8mm to 12mm Float finish 1:1:6
Patterned “

EML undercoat as per moderate Dashing 1:1/2:4 to 41/2
exposure Float finish 1:1:5 to 6

The mix of each successive coat should never be richer in cement than the mix used for the coat to which it is applied.
Strong mixes have a high drying shrinkage and are consequently more likely to cracking, their use should be restricted to strong backgrounds such as first coats on EML and abrasion-resistant applications such as plinths. Mixes such as 1:1:5 to 6 or 1:2:8 to 9 are less liable to cracking and crazing than stronger mixes.
There is so much to external rendering and so many ways to go wrong with serious consequences that in my opinion it should not be attempted by the inexperienced on anything but a small scale.
Thank you very much, a good write up. That all I wanted to know in the first place. Thanks again.
 

Rigsby

TPF Special Forces
Well done Andy.

I did GCSE English back in the 90's and for the exam I wrote a story titled, How to pebble dash a wall and get away with it. I filled 5 A4 pages!

Got a B for it though!
 

Saquach

Member
Would u not recommend it in the top coat then I've always used it as I imagiond it would waterproof that coat too
 

KTJ

New Member
I've been asked by Church to put together a guide on the use of sand & cement for external rendering, what follows is a mix of information gleaned from British Standards and nearly thirty years of personal experience.

Rendering is generally made up of two coats of sand, cement and lime, except in areas of severe exposure where three coats of render should be used. If you don't want to get a copy of the British Standards for rendering I believe there is a map of the UK giving the areas considered to be of sheltered, moderate and severe exposure in the Catnic catalogue. The map shows that most of us live and work in areas considered to be of sheltered or moderate exposure.
Sand
All sand used for external rendering should be washed and uncontaminated. For general rendering you should use as coarse a sand as it is practicable to use for the for the desired finish, the sand sold by B&Q's and Wickes as plastering sand (at least in Essex) is good and other suppliers often refer to it as "fine sharp". For some pattered and banded finishes it may be preferable to use a finer sand. The sand should be neither to dry or to wet prior to mixing, I would recommend always sheeting up the sand as this stops it getting to wet and also keeps the cats off.
Cement
For the scratch coat/ coats I use either Ordinary Portland except where there are soluble salts in the background and here you should use sulphate resisting cement. For the top coat I use either Ordinary Portland or Snowcrete (white cement).
Lime
Hydrated lime is used in external rendering.
Fibres
I use fibres in both my scratch and top coats except in the top coat for dashing.
Admixtures
I always use a three in one admixture for my scratch coat (plasticiser, waterproofer and retarder) and a small amount of liquid febmix in the top coat.
Scratch coat
Before you start you need to determine the strength/density of the background so that you can use the appropriate strength mix. If the background is high suction you should dampen but not soak it before applying the scratch coat, once applied the render must be kept damp for at least three days to allow the cement to fully set and cure. If allowed to dry to quickly the render will never set and harden correctly. Personally I don’t use lime in the scratch coat as it doesn’t work well with some of the admixtures I use.

Top coat
I never put a top coat on until the scratch has been allowed to cure for a minimum of five days in the summer or seven days in the winter. Once you have finished an area of top coat this should not be allowed to dry out to quickly and should be kept damp the same as the scratch coat was.
Mixes and spec. suitable for moderate and sheltered exposure
For two coat work the finished thickness should be between 16mm-20mm

Background Scratch coat Topcoat
Strong/moderate 1:3 to 4 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

Moderate/weak 1:5 to 6 @ 8mm to 12mm Dashing 1:1:5 to 6
1:5 to 6 @ 8mm to 12mm Float finish 1:2:8 to 9
Patterned “

EML 1st. 1:3 @ 3mm to 6mm Dashing 1:1:5 to 6
2nd. 1:3 to 4 @ 10mm to 14mm Float finish 1:1:6 to 8
Patterned “

Mixes and spec. suitable for areas of severe exposure
Three coat work should have a finished thickness of not less than 20mm
Background Scratch coat Topcoat
Strong/moderate 1st. 1:3 to 4 @ 8mm to 12mm Dashing 1:1:4
2nd. 1:3 to 4 @ 8mm to 12mm Float finish 1:1:5
Patterned “

Moderate/weak 1st. 1:5 to 6 @ 8mm to 12mm Dashing 1:1:6
2nd. 1:5 to 6 @ 8mm to 12mm Float finish 1:1:6
Patterned “

EML undercoat as per moderate Dashing 1:1/2:4 to 41/2
exposure Float finish 1:1:5 to 6

The mix of each successive coat should never be richer in cement than the mix used for the coat to which it is applied.
Strong mixes have a high drying shrinkage and are consequently more likely to cracking, their use should be restricted to strong backgrounds such as first coats on EML and abrasion-resistant applications such as plinths. Mixes such as 1:1:5 to 6 or 1:2:8 to 9 are less liable to cracking and crazing than stronger mixes.
There is so much to external rendering and so many ways to go wrong with serious consequences that in my opinion it should not be attempted by the inexperienced on anything but a small scale.
 
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