plaster skim coat: effect of temperature of hairline crack formation

[noos999]

Hi all

Over the years, in a previous house, I had several rooms skimmed with a thin coat of plaster as the original plaster (eighty years old) was damaged in places. I noticed that in some rooms hairline cracks appeared in the skim coat over time. It seemed to be correlated with the weather conditions on the day each room was reskimmed. The room that had the most hairline cracks was the one that reskimmed during a heat-wave. Two rooms had no hairline cracks, and had been reskimmed in during very cold weather in the middle of winter. Another couple of rooms were skimmed during typical summer weather (20 deg Celsius) and acquired one or two hairline cracks. Has anyone else noticed this sort of correlation, if there is indeed a correlation?

thanks

J

[Andrew Gabriel]

Cracks which form while the plaster is setting and walls drying out could be due to conditions when applied.

Cracks which form weeks or months later are due to building movement. If your property is 80 years old, it is likely to be built using lime mortar. Lime mortar is flexible which means the whole house will routinely move around slightly. This is caused by things like ground moisture levels causing ground movements, timbers swelling and shrinking as humidity changes, etc. Original lime plaster is also flexible and will thus handle the movement without showing it. However, gypsom plaster is not flexible, and will instead form hairline cracks. If you reskim a lime mortar house with gypsom plaster, this is just something you will have to put up with. I've done this too and I have got some hairline cracks, but paint has for the most part filled them (I didn't get round to painting until about 12 months after doing the plastering).

[marvelus]

On the hot days maybe the PVA dried out before the plaster was applied.