I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?
Was the cement too old (about 3 months)?
Did I over float the top coat to get it level?
I thought I'd done a very nice job and wish I'd stuck at one coat of
render!

Many thanks Alan of Exeter.

alanjones wrote:
I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?

Is there a DPC in the wall? Did the render get frosted?

Chris Bacon wrote:
alanjones wrote:
I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?


I think it was because you used waterproofer and did the second coat a
week later. After a week the render is probably fully cured and ther
will be no suction available for the second coat because the
waterproofer wil not allow it. I'm no expert so its just a guess.

"alanjones" wrote in message
oups.com...
I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?
Was the cement too old (about 3 months)?
Did I over float the top coat to get it level?
I thought I'd done a very nice job and wish I'd stuck at one coat of
render!

Many thanks Alan of Exeter.


Sorry to hear about your wasted work. 3 months is certainly getting on a
bit for cement. The mix was maybe a bit on the strong side too, but I don't
expect either of these is the main reason for the failure.

You should ideally apply the second coat the next day, or sometimes the same
day in warm weather if the first coat has set and stopped shrinking.
Leaving it for a week means the first coat probably became too dry. Did you
wet the surface really thoroughly? It has to be wet because otherwise the
dry surface sucks the moisture out of the new render and kills the setting
process, and then you end up with a thin powdery layer between the two
coats.

Pete near High Wycombe )

Peter Taylor wrote:
"alanjones" wrote in message
oups.com...
I built a concrete block wall this summer and rendered it about a month
ago. I applied 2 layers of render the first was well scratched before
the second was applied a week later. I used sand:cement 4:1 with
waterproofer 1:25.
About a week ago the top later detatched itself from the base layer and
fell off. The other walls I did are also losing thier top coat (they
sound hollow).
What did I do wrong?
Was the cement too old (about 3 months)?
Did I over float the top coat to get it level?
I thought I'd done a very nice job and wish I'd stuck at one coat of
render!

Many thanks Alan of Exeter.


Had exactly the same problem about 2 years ago. Bricked up a window and
had to rended the surface to match the house. Problem was the render
had to be about 1 inch thick ( given my inset bricks) so I put it on in
two layers..First layer went on fine...Left it for about 4 weeks during
which time it also got a bit dusty, then got around to doing the second
before chucking lots of wee pebbles at it to match the house. 2 weeks
later I thought it looked blown and tapped it..HOLLOW sound. prodded
away and the whole 2 feet by 4 feet lump slid off! Kind of realised
what I had done wrong..Got out watered down PVA solution and soaked the
wall in it..Really soaked the wall until it stopped drying out quickly
then re-did the render.. Suffice to say its still up there!

Ged

In article .com,
writes:

Had exactly the same problem about 2 years ago. Bricked up a window and
had to rended the surface to match the house. Problem was the render
had to be about 1 inch thick ( given my inset bricks) so I put it on in
two layers..First layer went on fine...Left it for about 4 weeks during
which time it also got a bit dusty, then got around to doing the second
before chucking lots of wee pebbles at it to match the house. 2 weeks
later I thought it looked blown and tapped it..HOLLOW sound. prodded
away and the whole 2 feet by 4 feet lump slid off! Kind of realised
what I had done wrong..Got out watered down PVA solution and soaked the
wall in it..Really soaked the wall until it stopped drying out quickly
then re-did the render.. Suffice to say its still up there!

Sand and cement should really be recoated in 24-48 hours (might
stretch a bit longer if it's cold). Any longer, and it needs to
be handled not as a recoating, but as a scratch (base) coat
application, as you did.

--
Andrew Gabriel

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick. There was no DPC and no frost at the time.

Many thanks for the prompt advice I will take it back to the concrete
blocks next year and apply a single layer of rendering to be on the
safe side. Fresh cement and a weaker mix with waterproofer should do
the trick.