Water Damage

[Craig]

While attempting to repair a spot of water damage in the ceiling of one of my spare bedrooms (yes, the roof has already been repaired) I began to chip away at what I thought was Gypsum Type Wallboard. I figured maybe I would simply have to replace a small portion, patch & paint. I eventually chipped off what appeared to be a large chunk of Plaster… to uncover cement backerboard… only unlike any I’ve ever seen. Rather than a mix of fiber & cement, like Blueboard, this is acutally Metal Lath covered in real cement! I have never seen this before. My question is… how do I patch up these huge chunks of… I guess.. plaster that I have removed. The Cement Board is still in good shape, though there are rust spots coming through.. I obviously don’t want the rust to bleed through the plaster. I was thinking about applying a skim coat of Thinset Mortar, followed by plastering. I was hoping to use Joint Compound to patch.. but I figure it won’t bond too well to the cement/mortar. Am I on the right track here??

[BobB]

Craig, I’m not a professional plaster, only a die hard doit yerselfer with some electrical training. However, over the years I have had occasion to try different techniques of accomplishing the patching you are talking about. One good thing is if you don’t like the looks of the method you tried, you can always cut it out and try something else. And yes I have encountered the construction you are talking about in a house built in the early 1950’s. My presently preferred method is to cut the hole all the way out, back to the center of the closest ceiling joist. Carefully measure the thickness of the plaster in several locations ( it will vary ) and come up with a combination of wood shims and drywall to get back as close as possible to the thickness you need. Then use drywall tape, joint compound, etc.. to finish the joints. Good Luck. Take your time, and have fun with the challenge. BobB

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