Can you understand this rendering?

[Davy]

A year ago we bought a house that was rendered two years ago. Last Spring, about 3 sq metres of the rendering on the main south-facing wall began to peel off. Investigating it today I find that there appears to be three layers of rendering:

- bottom layer of unknown thickness which has been scored

- middle layer which is about 10mm thick and is very powdery and is the bit that is failing

- top layer which is hard and only about 2mm thick and comes away in sheets

- smooth paint on top of this

Does this sound right?

Since the middle layer is failing in patches is it reasonable to assume that I can hack back to the bottom layer in the failing patches and then just apply a patch render over this?

many thanks in advance, Davy

[Andy]

IANAE but rendering is done in two stages, bottom coat and top coat. The bottom coat is a stronger mix than the top coat I think. 10mm sounds about right, I would think 2mm was a very thin topcoat, difficult to apply unless it was very smooth in composition. The second coat is presumably failing due to either a poor mix/materials or it was applied in a way that didn't allow it to adhere ( poorly applied or perhaps too dry ). I expect more expert opinions will follow. I think your plan sounds OK, though who knows whether more will fail. Careful pulling the blown stuff off, it can weaken the stuff around it. Chopping at it with a bolster chisel might be better. Check for soundness by tapping at the render with the end of a trowel or even your knuckle. Remove anything that's blown. Use a topcoat render mix.

Andy.

[Stuart Noble]

Could be one of those "never paint your house again" treatments gone wrong. The middle layer is usually some kind of powder which appears to do nothing other than provide a good environment for ants.