going to build garden block wall and reder. any precautions?

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hi there.
im a general builder, bricklayer by qualification.

im building a garden wall for a friend. its around 7m x 2m and will be build from concrete blocks laid flat..

is there any precautions i should take or anything i should do to help the rendering stage?

i have read a few thread when i searched saying that everything was great for 12 months then the render started to crack and look blown.

looking for any tip at all.
how long should i leave before rendering, what mix for rendering, admixtures, should i allow the scratch coat to fully cure etc etc.

thanks for your time
ian
 
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Raking out the joints in the blockwork as you build, will give a good key for the scratch coat. Wet the wall down and allow to drain off before you scratch.

I think it's always better to allow a newly built block wall to settle/dry out a while before rendering, but that's just my opinion. If there's a lot of money laying out, others might want to complete the job as soon as poss'.

4.1.1, plastering sand, cement, and lime is a good mix for a scratch coat.
5.1.1, plastering sand, cement, and lime is a good mix for the top.

Admixture: I use Evoplast,, a combined waterproofer/plasticiser, easy to understand mixing ratio, 1 part Evoplast to 30 parts water,,, whatever you use to measure it out with. It comes in a 5 litre container,,, other sizes available too.

I always like to let a scratch coat fully cure, whether it's render or roughcast.

Roughcaster.
 
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Just wet the blockwork down with water before scratch-coating Ian, let excess water drain off,, and no need for pva. It makes for a stronger job too if the mortar,, once scratched,, dries out "slowly".

Roughcaster.
 
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perfect, thanks very much roughcaster.
just 2 more Q's for you.

what sort of thickness per coat should i be looking at, 5-7mil each?

and if i was to render the top of the wall( slighty rendered) would this be a bad idea?
was thinking that the water may penertrate it from above and get behing the render on the side and blow it off over time, am i right in thinking this?
thanks
ian
 
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I'd go for 7 to 8 mm per coat Ian. 5ml is a bit thin especially for a scratch coat. By the time you scratch in to it, you'll be more or less scratching right through to the blocks. As long as you've got a good key for the s/coat, that's the main thing. If you're not using copes on the top of the wall, then a rounded rendered top will be the next best thing. As you said if you just render both sides,, water will go down between the wall and the coat,, and in the winter freeze up and then blow the render.

Roughcaster.
 
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thanks alot for the help again rough caster.
i have the wall booked in for later this week so ill post back how it goes :)
ian
 
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You "wouldn't" need weep holes in a typical garden wall. If it was a retaining wall that had a much higher level on the other side of the wall, (soil, slabs, grass etc), or even a wall at the bottom of a steep slope, then weep holes would be essential for sure, in allowing any build up of water to escape through the wall itself..

Roughcaster.
 
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