Blown render: repair

Hi. Part of my outside wall has developed large cracks and has render that has blown, i.e. separated from the brickwork. I'm pretty sure it's not subsidance. I want to remove the old render and re render it. The question is, is there some way of tackling the area where new render meets old to avoid visible shrinkage in this area? Also, how would you treat the red bricks before applying render to them? PVA?
23:01 Wed 11th May 2011
 
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I know things change so this might not be the modern spec method but when I've had to render onto brick in the past I would apply two coats of PVA, let the first one dry then apply a second coat just before you apply the base coat of render (containing waterproofer) so that the render is applied to the PVA while it is still wet, let it dry then the next day apply the top coat of render. Like I say though there may be better products around now.
This won't answer you question fully but we had the side of our house rendered and Tyrolean effect sprayed on to match a new extension we had built at the same time about 24 years ago. The existing house brickwork was then 55 years old and some bricks had lost their face due to frost damage. The builder mixed PVA with the render mix before applying to the wall. It's as solid today as when it was applied. As far as I can recall no treatment was given to the existing brickwork.
It also depends on the make and type of brick, Tweed. NHBC didn't allow render on LBC (London brick) unless it was "wired" ............. expanded metal mesh.)
The way D97 describes is one of the best methods.......... also "scatting" the brickwork first (roughcast) to give a "key".
Wiring is probably best, but more expensive.
To join to existing render ......... don't try to "feather out" the new render over the old. Always cut old render to give a precise "step", butt new render to it, and roughcast over................. or flexible filler if a smooth finish is wanted.
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Many thanks for your answers folks. I'm pretty sure the bricks are LBC so will probably scatt the brickwork first. The builder. Do you mean I should cut a straight vertical line rather than have a patchy seam?
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I'd appreciate your opinions on my intentions to repair this. The walls are finished in roughcast and painted. I plan to:

- Remove all old plaster (with straight seams) to expose red brick
- coat with 1:3 PVA/water mix and let dry
- coat with 1:4 PVA/water mix and while just starting to set, fling a shallow layer of a wet 3:1 sand/cement to gain a fixing for next coat of render. Allow to dry.
- Apply a normal render 6:1 sand/cement and while it's just starting to set, throw a slurry of 4:1 sand/cement with added chips on to the wall.

What do you think?
I would add some waterproofer to the first 3:1 coat of render and lightly scratch it using a 8" piece of baton with some nails in with the heads removed.
The 6:1 coat I would add 1 part lime to the mix, but I'd wait to see what The Builder has to say on that one as he seems far more knowledgeable than I!
Ok Tweedy ........ no, I didn't mean the "join" had to be straight lines. I was referring to the edge of the existing render. That should be cut at right-angles to the wall to create a step that the new render could be butted up to (that will obviously at right-angles as well) I wish we could draw sketches on here lol

Your plan looks great, except that 3:1 was used a lot in the "old days", but its considered far to strong today. The stronger it is, the more likely it is to crack (taking the top coat with it.)
I would use 5:1 with Plasticiser.......... it's resin based, and gives strength to the render, making up for the "missing" cement. Then 6:1 (with plasticiser) for the second coat. The general rule is always for subsequent coats to be "weaker" than the previous.
A lot of plasterers will prefer lime instead of plasticiser. You can argue both ways.
With lime, perhaps ......... 6:1:1, then maybe 7:1:1.
Lay on the first coat on one day ........... then the 2nd coat on the next day while the 1st is still "green".
You'll find each plasterer has his own tried and tested method. Within certain parameters, there are several ways of getting there.
Oh Monsieur le Builder I love it when you talk technical
............... ahhhh .......... Madame O .............. plenty of potential smut in that lot eh?
Question Author
Again, many thanks D97*9 (?) and The Builder for replying. I get you, I think. You mean that the edge of the old render should be perpendicular to the brickwork, not feathered. And what you're saying regarding weaker coats etc, that's why I thought the 3:1 would be good for a strong initial scat coat to key the next. Your reasoning is appreciated and makes sense. Anyway, I intend to use plasticiser AND a waterproofer to the mixes. Is this necessary, or is it a case of one or the other?
Put the waterproofer in the base coat and the plasticiser in the top coat of render, as it essentially aerates the mix. One tip, make sure the waterproofer coat is mixed very well otherwise it will be quite difficult to use.
Good luck.
Yep, that's it .......... perpendicular.
Waterproofers usually have a plasticising capability as well......... but......... not as much as the real thing. Here, I would have to do things a little differently from what D97 has said. I would use a squirt of plasticiser in the base coat as well as the waterproofer. It would work without, but a 6:1 would be hell to lay on without plenty of aeration. As I said before, the resins give the coat strength too.

Be prepared for the top coat to "hang around" for a while (ie... take a while before it can be floated up). This is because the waterproofer in the "scratch" coat will mean that it has little suction (cannot absorb water from the final coat.
Question Author
Great answers folks. I think I now know all I need to, to do the job. Thanks again.

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