Rendering existing brickwork: How to avoid cracking or render blowing


Postby dan32 » Wed Jul 25, 2007 2:24 pm

i am about to try to render side of bungalow on existing brickwork do i dampen down wall first or do i pva seems to be very different opinions also am using sealorend which is waterproofer palsticiser and retarder will this stop cracking or render blowing and would you put it in both coats please advise
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Postby martin497 » Wed Aug 01, 2007 4:33 pm

hello.
Usually you only need to dampen your wall down prior to rendering,
however, if the wall is particualy dusty even after brushing down, or if the existing brickwork had been painted for example you should PVA it.
Last edited by martin497 on Wed Sep 26, 2007 11:05 am, edited 1 time in total.
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Postby martin497 » Wed Aug 01, 2007 6:40 pm

also only use the specified amount of plasticiser etc as stated on the tub.
Last edited by martin497 on Wed Sep 26, 2007 11:06 am, edited 1 time in total.
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Postby aaaplastering » Fri Aug 17, 2007 10:56 pm

Hi i always use pva when rendering, whether painted or un painted, i would highly recommend doing this.
Always use plastersizer, Try to use sand witch does not contain clay.

Only add the specified amount of plasticiser as too much weakens your mix. See our project on mortar mixes. Plasticiser entrains air into the mix which allows it to be more workable. If too much air is in it little air bubbles form, unseen, in the render. These eventually fill with water which freezes when its cold. This action is called freeze thaw and can be seen in our freeze thaw project. It is why render and bricks blow off the face of a wall in the first place. Also, applying PVA to a wall will stop it breathing which will not allow any damp already in the wall, to escape. There is absolutely no point in putting any lime into a wall which has been PVA'd as no moisture can evaporate through the render because it cannot get out of the wall in the first place,
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