Cracking plaster around fireplace

[stuartjbecker]

Our fireplace was modified two years ago for a wood stove by installing a stainless opening up the existing fire box, installing a lintel, adding brick and cement board w/portland cement, installing a chimney liner, and then plastering over it all with 0:1:3 (cement: hydrated lime: sand).

By the middle of that first winter, we saw cracks start to develop around the sides and top of the fire box where the cement board is. Now, two winters in, we are in dire need of plaster repair (see attached images).

Could anyone offer advice on why this cracking occurred, and how we can prevent it when we redo the job? We followed the installation techniques of this website, where cement board is recommended and they seem to not have any problems with it: http://www.stovefittersmanual.co.uk

I wonder if air may have been trapped behind the cement board when we installed it?

 

[begreen]

I suspect it is due to dissimilar materials used having different expansion rates. The stainless will heat up faster and move more than cement board or plaster.