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External render

i'm thinking of rendering an external entry wall on my property.can you tell me the ideal mix/ratios of materials and also what type of sand do is best to use? also,what additives do i use? [i.e. plasticiser/waterproofer etc.......] and how much of this do i need to use?

6 Answers from MyBuilder Plasterers

Best Answer

For the scratch coat as we call it the trade , the mix should be 4:1 , the best way to achieve this is gauge the sand in 12 litre bucket brought from any mechant ( builders bucket). Add mortar plasticiser preferabley febmix.When mixing the mortar you should looking in the mixer at the blades .The mortar should fall from the blades like dog ears. Sounds funny but this is the best consistancy. Bead with scratch coat using either plastic or stainless steel stop or bell beads to get the finish. The Best sand is Plastering sand from any mechant. Once mix starts to pick up scratch up with a scarifier. Thickness around 10 mm. Amount of plasticer to use approx a cup full for 1 mix . Most importantly around any windows or heads of doors use Hessian Tape , which will lstop any cracks on the corners of the heads . A problem i always see on rendering thats not been applied correctly. This MUST be done before scratch coat is appied !
For Top coat the mixing process is exactly the same. Except NO mortar plasticiser, Replace this with Waterproofer, Around a cup full with water . If you are near the coast maybe half a cup more .Due to nature sea salts in the air, which can corrupt the mortar mix over time.
Hope this helps ,
R Carrigan


Answered 26th May 2011

Hi Kem_53, as for the sand plastering sand is good but i was brought up on building sand and it's been used for decade's so either or. The mix ratio should be kept at 4 to 1,using a plasticiser will make the mortar mix workable (and less like mud). your additives can be picked up from any building merhcants or b&q. You'll need to put an undercoat on around 10-15mm thick and score it with a scarifier. This will make a key for the top coat also 10-15mm. To bridge the gap to the floor either use a bell cast bead or a render stop pending on your choice. If your going to come to a stop on your wall render stops will give you a nice clean edge. If your going for a flat finish and your inexperienced i'd recommend a 3 in 1 additive which has a retarder that will slow the setting time so you can give it your finish.

Good luck with it mate.
Dave Williams Plastering


Answered 19th May 2011

Hi Kem_53
Personally you cant beat getting in the right trade for the job! But if you can do it then why not:)

You will need a plastering sand which you can buy from any Builders merchants,its slightly coarse compared to the building sand and you want a mix of 4 sand to 1 cement.The sand needs to be the correct grade as you want it to be loam free and not a sharp sand.
You can buy the additive from quite a few suppliers but i use Wykamol and if its only a couple 5 litre tubs needed you will have to pay post.Dont use a plasticiser in the mix as its not needed and its not going to help with your waterproof coating.Mix ratios will be on the back of the product and you can also ask for a product Data sheet so you know all about the product your using ie: Mix,Whether its harmfull and precautions etc.

Last thing when rendering outside DONT take the coat straight onto the outside base/ground as this will bridge the damp course! So either leave it short of the base or stop it level with your Damp proof course which is 2 courses above external ground.Usually!!Should hopefully be visible but if not then as a rule if your internal level is higher than external then it may be higher and look at 2 courses above internal.

Hope this helps a little! Good luck and hope all goes well.

Scott Cannon.
Cannon Preservation.


Answered 18th May 2011

not being rude ,but if you do not know the mix ,do not attempt to do this yourself,smooth finish render is a highly skilled job do yourself a favour and put this job on the website and get it done properly


Answered 18th May 2011

as above leave to to the professionals!


Answered 19th May 2011

I use 3 parts plastering sand 1 part building sand 1 part cement and trowel full of lime.
Make sure the surface is dust free and sound, if it’s not mix up a bucket of water and a cup of pva and a few trowels full of cement, stir it to make a “gravy” and brush it all over with an old soft broom “ gives good reach” but make sure you cover the ground but don’t bother with sheets just lay some scaffold boards below or ply or hardboard off cuts, “ if the render falls on a clean surface you can put it back in the mixer as you go along”
Apply the render “without waterproof additive because if you do there will be no suction for the second coat and it tends to slide and bag down”
Scratch the first coat with a scratcher, “I use an Afro comb, works for me”
Apply the second coat with the same mix but this time with the waterproof additive, no need to use too much, the amount will be on the back of product container “ sika” is a top brand.
Both coats should be about 10mm thick.
As the render begins to firm up use a plastic float in a circular motion, “ don’t use a small float, broader the better, don’t tilt the float keep it flat to the surface. You can cheat and use a damp sponge instead of a float, I even use one myself on “very uneven surfaces”
Don’t try and render the whole side of your house as your first project.
Finally, I’ve seen top spreads render large flanks which look great but after it’s had its coats of paint and the sun is shining on it you can still see some blemishes. It really is skilled work and I bet most spreads would secretly admit it’s not their favourite job.
Oh yeah, as a p.s. do it on overcast mild days, hot sunny days suck the water out like nobody’s business and it’s more comfy working than on a hot or cold day.


Answered 23rd Jan 2018

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