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.
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.
