Help! Rendering sagging/falling off

dmcm · 4 Nov 2007

Hi there,

Any advice on this would be very welcome,

I am rendering a brick garden wall. I have coated the wall with a watered down mixture of cement sand and PVA additive, that has created a nice rough surface for the render to bond to.

I am using a mix of 1:1/2:4, cement/lime/sharp sand. I am applying the render with a metal plastering trowel. The render goes on fine, but after a bit it seems to sag in areas like its pulling away from the wall, one of the sections just peeled off. I have tried dryer and wetter mixes, but have the same problem. I noticed as I was smoothing out the render the trowel was bringing the water to the surface, thought that might be the problem so tried using a wooden float, but again the render started to bulge and fall off. I am applying the render approx 1cm thick.

marshman · 5 Nov 2007

are you using a plasterzier or waterproofer in your mix

dmcm · 5 Nov 2007

No plasterizer or waterproofer, just some lime.

roughcaster · 5 Nov 2007

There is no question about it, the mix you have put on your wall is dead, and that is the cause of it slumping (sagging).There is no AIR in it.Making it wetter or dryer will not solve your problem.You will have to use a plasticiser at the very least in your mix, which will make it more"buttery", and preferably use a cement mixer.If you are mixing by hand,then you will have to work it very hard with the shovel.With a plasticiser in your mix,you will not need anywhere near as much water.You can always add water, but you can't take it out...A steel trowel will always bring water to the surface...Mixing mortar correctly is a skill in itself,and is always the key to a successful job.

Roughcaster.

dmcm · 5 Nov 2007

Thanks for that roughcaster!!

I'm curious; what did people do before plasticiser?

roughcaster · 5 Nov 2007

dmcm,
Another point I forgot to mention.It is always better to apply new cement render onto a proper scratch-coat.I wouldn't do it any other way.

Roughcaster.

diyisfree · 5 Nov 2007

Apply your render in two coats.

As roughcaster says, first a scratch coat, let that go off overnight, then following day second coat (final floating coat) DONT try to put it on too thick (max 8 to 10mm)

Leave it to stiffen then rub up with either a poly float or your wooden one. The more you keep messing with it, the more likely it will drop off the wall.

dont forget to make the second coat a weaker mix.

noseall · 5 Nov 2007

also, wetting the wall helps.

a lot.

dmcm · 5 Nov 2007

Thanks alot for all the advice everyone!

I tried again today with some plasticiser and the result was much improved; the render is still on the wall!!

roughcaster · 5 Nov 2007

Thanks for your feedback dmcm.

Roughcaster.

Micilin · 5 Nov 2007

dmcm said:
Thanks for that roughcaster!!

I'm curious; what did people do before plasticiser?

Fairy Liquid

csrt · 27 May 2008

yes you can use fairy liquid, but lime was used before the fairy liquid in the olden days, another tip for who ever wants to listen, add a small amount to plaster, bonding or browning that has gone off,and it will give you more time to use it, only for small jobs/areas or bedding beads, etc (not with multi finish) with this you,ll have to buy a new bag. cheers

diynot

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Help! Rendering sagging/falling off

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