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alan12

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alright chaps, earlier this year i did a full house sand and cement on new block work, mixed at 5-1 with waterproofer, devil floated and skimmed...the job went fine no problems at all... untill now!!!
The customer rang up the other day saying the plaster is coming of the walls in one of the rooms in big sheets, ive spent hours searching the web for info and advise but cant find anything...
i do remember it being quite cold at the time we were doing it and the walls in this paticular room were thicker than the rest of the house. also he said there are a few white marks on the render under the plaster what could this be??
any help guys cheers al
 
Hi Alan,

1. What blocks did you work over,and what consideration did you give to their characteristics ie suction/key ect/ strength of float coat

2. Do you think you may have built up , too much in one hit ( thickness of floating coat)

3. white marks may be sulphates from the blocks ( did they get overly wet during the build ?

4. Was the failed work carried out below ground level ie basement.?

Mark
 
Im thinking that only the skim is coming off in sheets ? I had this once in a college in cambridge and it was probably below freezing at the time with windows open, so it can happen itf its too cold at the time of skimming.
 
the sand was of good quality not the best but definatly not the worst ive used..
1. the blocks were master lite or concrete solids definatly one of them.
2. the build up of to much gear has crossed my mind may be we skimmed it to quickly, not letting the sand and cement to dry properly.
3. not to sure about that ill be asking the customer that one
4. nar all the work was at ground level

thanks all
 
just the skimming mate, sand and cement was put on just ya average thickness, ya know one coat then another ruled off..apart from some areas it was really thick id say maybe 1.5 to 2 inch thick...thos areas have come off but it has happend else where aswell mate cheers
 
Doubt it if it's devil floated I've skimmed s/c next day in freezing conditions and never had probs don't get what the White s**t is though
 
How long did you leave the float coat before skimming it Alan? And be honest because I think I already know the answer.
 
nar the skimming took a while to set.. im thinking its got to be the cold weather or thickness or this salt type build up thats coming threw that i know fook all about
 
I've had it sheet off sand and cement if it hasn't dried enough. you need a couple of weeks for it to dry depending on how thick it is i think. Sand and cement shrinks alot, so if it's still drying and you skim it, it'll debond or shell off.
 
the walls in that room were left over the weekend i think they defo had a couple of days before skimming.. maybe not long enough
 
alan12 said:
the walls in that room were left over the weekend i think they defo had a couple of days before skimming.. maybe not long enough
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appen you over did the waterproofer?...
skim next day on render an the waterproofer hasnt had a chance to reach its full potential but leave it 3 days and its got where its gonna be... just a thought?
 
I've skimmed loads next day you shouldn't but I have and have never seen anything fall off
 
yeah true.. to be fair the blokes being sound about probably a little pissed off its happend but s**t happens..thinking of uni bonding it to be on the safe side..still this white s**t baffles me!!
 
It sounds like salts from the sand Alan was it a domestic job were you behind closed doors they didn't just bang the heating on the moment you finished?
 
Spunky's right.....................Only time it happened to me was when punter put heating on when specifically told not to. :RpS_thumbup:
 
The reason 99% of skimming shells from S&C is that it's skimmed before the S&C has had a chance to fully cure and shrink.
When people say they've skimmed the day after and there's never been a problem it's just they aren't around when the problem shows up or they've been very lucky with conditions. I've seen no end of walls where as a householder has started to strip the wallpaper the skim just comes away from the render as clean as a whistle. Now that hasn't just happened, it's been like that since the wall was first skimmed, well within a few days of it being skimmed.
Depending on the sand used you can get a 3mm shrinkage on a normal coat of internal render, and if the render is skimmed before this shrinkage has taken place it will just carry on shrinking back away from the finish and the two will separate. When this happens sometimes the skim with shell and bubble away from the render (when there's a lot of moisture trying to get out quickly) but more often the skim coat will just stand there looking fine for years often being decorated many times until wall paper is stripped once to many times or as is the case in recent years a steam stripper is used. If you find one shelled wall in a house you can be damned sure there will be others if not most of the floor or house.
 
alan12 said:
yeah true.. to be fair the blokes being sound about probably a little pissed off its happend but s**t happens..thinking of uni bonding it to be on the safe side..still this white s**t baffles me!!
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sounds like efflorescence you never mentioned why the job was done in the first place and at a guess i would say a damp problem and with another guess mis-diagnosed (defo wasnt rising damp) damp problem that hasnt been fixed.
 
essexandy said:
The reason 99% of skimming shells from S&C is that it's skimmed before the S&C has had a chance to fully cure and shrink.
When people say they've skimmed the day after and there's never been a problem it's just they aren't around when the problem shows up or they've been very lucky with conditions. I've seen no end of walls where as a householder has started to strip the wallpaper the skim just comes away from the render as clean as a whistle. Now that hasn't just happened, it's been like that since the wall was first skimmed, well within a few days of it being skimmed.
Depending on the sand used you can get a 3mm shrinkage on a normal coat of internal render, and if the render is skimmed before this shrinkage has taken place it will just carry on shrinking back away from the finish and the two will separate. When this happens sometimes the skim with shell and bubble away from the render (when there's a lot of moisture trying to get out quickly) but more often the skim coat will just stand there looking fine for years often being decorated many times until wall paper is stripped once to many times or as is the case in recent years a steam stripper is used. If you find one shelled wall in a house you can be damned sure there will be others if not most of the floor or house.
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Your spot on there Andy.

In general it is wise to leave SC backing coats for a minimum of 4 days before setting.This same basic rule also applying to traditional external renders.
From experience we have also observed poor mechanical key in under coats, attributing to faliure of skim coats.
 
flynnyman said:
Exactly........................like i said earlier not enough key.
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Sorry Flynny but I don't think the amount of key makes a jot of difference. I've seen walls shell where a really deep scratch has been used and yet I use a very light scratch with no issues. Multi is made to use on the relatively keyless surface of plasterboard and will stick to a rubbed up S&C render no problem.
 
It could be a number of things but i think without a good key it will fail and skimming on a smooth render i think your asking for trouble, also a sandy render you will have problems from the off and like you say shrinkage will cause problems if you get on it too early. Every time ive seen finish fail on render the key has been the problem coz it aint usually there or it aint deep enough.
 
mark ross said:
Your spot on there Andy.

In general it is wise to leave SC backing coats for a minimum of 4 days before setting.This same basic rule also applying to traditional external renders.
From experience we have also observed poor mechanical key in under coats, attributing to faliure of skim coats.
Click to expand...

Exactly Mark on a S&C external render even when using the correct drag key, if curing time isn't given then the two coats will still separate just as skim and render will.
On decent dry walls in normal temperatures I reckon you're spot on with 4 days. The way I work through an average house my float coat usually gets between 5 and 6 days. If doing smaller houses I just work 2 at a time, the job doesn't take any longer overall and there are no comebacks.
 
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