External Plaster Problem

[jobber]

I am currently re-plastering a new build resulting from a very poor plastering job. I would appreciate comments on possible causes of the disaster.

Not long after completion cracks became noticeable particularly after a shower of rain. Most of the finished plaster had the appearance of a spiders web of lines spreading across large areas of theplaster. In other areas long straight lines, mostly vertical, could also be seen. On closer inspection a considerable area of theplaster was boast or hollow sounding when tapped & a milky white residue can be clearly seen along some cracks.

The render mix used consisted of:- sand, cement, lime and evo plast but I cannot say what the ratio was. The walls were scudded, one scratch coat applied & one finish coat applied.

Having stripped the walls it is now evident that the plaster in some areas has separated between the two coats and other considerably large areas have easily come away from the block with no traces of scudding left behind. It looks like the scratch coat was combed with a trowel which left 1/4" lines in it before the finish coat was applied.

 

[Simeon]

What strength was your scudding? What type of blocks? Why the Evoplast, as you were using lime?

 

[jobber]

1) Don't know
2) Standard 4" Solid Blocks
3) Don't know, can only assume the "expert" new what he was doing.

 

[Simeon]

Scudding will come away for a number of reasons. Wrong mix? It should be 2:1 washed sand:cement. No plasticisers, lime or brickie's sand. If the temp drops below freezing point before the set. A film of dirt/dust on the blocks. If the scud dries out too quickly. The list goes on ........ . Why not take a sample and get it analysed.

 

[Magunn]

The render could have been mixed with a dirty sand. This will cause the "crazing" (spiders web) effect. Also, what was the time delay between coats? If it was too long, the bond between coats can be reduced considerably (usual practice is scratch coat today, finish coat tomorrow) - something to do with suction/drying out.

 

[Simeon]

The recommended time between coats is five days. This allows he scratch coat to settle - so that any cracks that would appear in that would not travel through to the rub-up coat. Finishing the two coats in two consecutive days will promote cracking.