Can you understand this rendering?

Question

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 · Accepted Answer

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 · Answer

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.

Can you understand this rendering?

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