Blown render!! Please Help!

[dougalhouse]

Is there anyone who can give me some advice on external rendering?

I've recently rendered (overall thickness about 15 mm) 3 external walls (new build concrete block) one of which has blown twice.

The only thing i can think of that caused it to blow the first time was that the unibond dried out before i applied the scratch and subsequently it blew as it stuck to the dried unibond not the wall!!

So i hacked it off (it came away in bigs chunks) wire brushed the wall, soaked it and reapplyed scratch and top coats minus the unibond this time!

I did a 4 : 1 mix with a waterproofing plasticiser (Rendamix) as directed on the instructions. BOTH COATS

However the render has blown again!

What am i doing wrong? I've rendered several times before and this has never happened and besides the other 2 walls are ok!

I used washed sand and maybe added too much rendamix! could either of these things be the cause?

 

[oilman]

OK legs, sort this one for the poor guy.

 

[Freddie]

Why did you put unibond on the outside of a building????????

 

[jbonding]

raking out the joints would of helped to key the scratch coat, the mix should be different your scratch coat should be stronger than your top coat, a waterproofer in your scratch coat will even out the suction on your top coat. Concrete blocks are high suction so the problem lies in your first coat, control the suction with unibond but provide a key ie raking out the joints.

 

[Freddie]

pva is glue

 

[jbonding]

mix up the unibond with some cement and a handfull of sand and apply with a roller and let it dry, make sure you rake the joints this is the key.

 

[dougalhouse]

Thanks for your replies guy's seems like i'm not the only one who gets confused with all these differnent method's I just copied what i learned from my time as a labourer without the experience tho!!!

This is the last rendering job i do. From now on i'm gonna stick to inside jobs!

Where's 'Legs' when you need him? I laboured for a gang of irish plasterers for a couple of years and they were quality spreads. Wish i'd stayed in touch!!

 

[Freddie]

no pva

 

[canusmellgas]

rake back all affected render...romving all lose materials....form slurry coat using SPR ..from your local builders merchants and mix this with portland cement and course grit or building sand............leave too dry..dont unibond .....form new scratch and leave to cure and then top of with finnishing coat................hey presto.dont paint to soon

 

[Freddie]

no pva

 

[legs-akimbo]

Hey Freddie it may be for six year olds to stick fuzzy felt to bright pink cardboard but as is commonly perceived it is a cure for all ills (see my screw fix post reva)....or so many would have you believe by the amount of posts pertaining to unibond it unibond it unibond it.........its feckin school glue not the elixir of life for gords sake. Many may have read a very informative post by a guy called mudster and the properties of using pva in ceramic tiling as a sealant to the walls ....it doesent...it acts as a barrier between wall and tile, in the same fashion pva ing a concrete block wall has had the same effect. Your scratch coat has adhered (or not as the case may be) to the unibond but not the wall itself.
My remedy would be to clean upoff the original and sparrow peck the wall which is to take say a scutch hammer and whack the feck out of the wall, actyually just chipping it all over (sparrow pecked) give the wall a soak then half an hour later a 1:1 slurry mix spatter dash /scud coat. leave for at least 24 hrs then a hose down and give it a good scratch coat. The dampened walls with the scud coat will cure slowly and not suck out the moisture from the mix and its strenghth also. Scratch with or without waterproofer floating coat WITH waterproofer regardless of what others have stated on here and a shovel of hydrated lime per single barrow mix.

 

[legs-akimbo]

Freddie are you saying I am an old Culchie??

 

[legs-akimbo]

If someone would care to tell me how to post some pics I will put up some of my work here ...Some rendering jobs with details like raised 4" window bands, formed quoin stones, Napp, dash and tyrolean if anyone is intrested.

 

[jbonding]

your still not convinced about the water proofer in the top coat are you legs. unibond is a brand name , its pva not wood glue, even though its one of its purposes. poly-vinyle-accitate or spelt something like that. its used for a number of things (do a search on google i cant be arsed).ive heard about it failing on tiled walls but ask yourself should the water have got that far. Ive re-skimmed hundreds if not thousands of walls using pva and not once have i ever seen plaster fail due to it getting wet and the pva reversing and turning ****. like i said before waterproofer is used in the top coat on ashlar, theirs a reason if you know about ashlar. its a finish that is supposed to look like big stone bricks or what ever style you want ie key stones and quoins. i will agree maybe if you had a house that faced the sea you would go with the waterproofer in the top coat. The top will absorb water but will not readilly pass it on. the problem with having a waterproofer in the top coat, any water that gets through cannot get out and may freeze and then you know what?

 

[legs-akimbo]

I know all there is to know about ashlar work and forming quoins and raised band work J bonding and if you consider that 70,000 houses are built anually in Ireland and all would have the majority fully rendered. There is very little brick facing ...only in the city and none whatsoever rurally. I dont have time right now to be specific but who is right you or 70,000 new builds per year? Remember I do this on every house I work on, how often do you get to do external rendering in England, and I know I am Engluish!
As for pva I know exactly what it does and does not do .....observe the obvious facetiousnes within my previous posts ......basically its IS FEECKIN SCHOOL GLUE its not a cure for all, it has its uses, but they are limited. End of