New Rendering Falling Off

[naomi24]

What would cause the top layer of new render on a brick wall to fall off? It’s bubbling up and cracking in places and then dropping off. To allow the wall to ‘breathe’ it was newly lined with an expensive metal mesh material before the render was applied, and it is just the top layer.

The builders are coming back to re-do it, but I just wondered what might have caused it.

[dannyk13]

Probably a combination of rain and frost.

[naomi24]

It started happening before the bad weather arrived, danny, and has gradually become worse. We thought at first the new paint had been out on too thickly and was kind of splitting, but now we know it's not that.

[dannyk13]

Maybe the builders can explain it when they return.

[naomi24]

They've had a look and are perplexed. They're having to wait for better weather before they sort it out. I'm wondering if the top layer was put on when the base coats were wet after heavy rain - not that I know if that would cause this.

[KARL]

Without knowing a collection of details it is impossible to be definitive. If the "render" is a sand-cement mixture then the following possibilities come into the picture.

The substrate may have been, in one way or another, unsuitable (type of material, dusty/friable, very dry, etc.).

The failing render may have been in some way inadequate (mix proportions, type, etc.).

The failing render has dried out too rapidly - water curing is required if weather conditions cause rapid drying out before the cementitious material has developed its strength and bond.

A damp (but not wet) substrate is good for cementitious materials and humid/damp conditions help them to cure (develop maximum ultimate strength). It is a common myth, even among tradesmen, that drying out is good for concrete, mortar, etc. - it is for plaster (a totally different material) but not for anything containing portland cement which takes 28 days and more to reach its maximum potential strength. Portland cement's chemical processes require moisture to proceed, curtail them before they are complete and you permanently stunt the development of strength. Deliberate curing with water (or the water in the mix is encapsulated with membranes to stop it from escaping into the atmosphere) is typically carried out over a minimum of 7 days where proper supervision is maintained, for important structural members it may be 14 days and occasionally longer.

[naomi24]

Thank you, Karl. Much appreciated.

[The Builder]

Good post from Karl, as always. I guess the expensive mesh is stainless steel expanded metal lath. Certainly a good product.

It does sound as though the finish coat of render has lost its "key" to the base coat. I've known this to happen when render lath has not been used, but never in this situation. I can only imagine that the job was done in too low a temperature... hindering the chemical process which "sets" the render.

Possibly, it was painted too soon after finishing. Some modern masonry paints are not "breathable". Not a problem in most cases, but may be a factor here.

[naomi24]

Builder, It was painted shortly after it was finished so that could be it, although the other walls are fine. Thanks so much for answering. There are some ideas there from you and Karl that I can pass on to my perplexed builders. A really great bunch of guys who've done so much in my house and have been utterly reliable and trustworthy. (I've gone away several times and left them my keys so that they could carry on with the work both inside and out). I didn't like calling them back but I had no choice. Thank you again.