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.