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Ask the Carpenter: Why is this plaster ceiling failing?

Rob Robillard discusses surface preparation. Plus, the mystery behind the mildew. Get more home improvement advice at

Is this reader’s ceiling plaster cracking because of the cold? Handout

Q. A painter recently repaired our unheated vestibule ceiling by applying a thin coat of plaster over the entire surface. He gave the same treatment to the walls. A big piece of the plaster has pulled away from the ceiling, however, and the obvious conclusion seems to be that the recent frigid temperatures caused it to fail. He has agreed to fix it when things warm up. Why did this happen, and what should be done so that it doesn’t occur again? Are the walls at risk of failure, too?


A. The extreme cold probably caused this damage, but many times the failure can be attributed to improper preparation.


If the crack separated above the “old’’ layer, it’s probably cold-related. If only the new plaster failed, it’s probably due to poor surface preparation.

Let me explain:

Proper preparation for plaster work includes cleaning all surfaces thoroughly. Glossy (painted) areas must be sanded to etch the surface, and porous ones must be primed. My plasterer always uses an adhesive bonding product called Weldbond. Weldbond gives extremely high adhesive strength to all types of plaster— cement or finishing plaster— as long as the surfaces are clean and structurally sound.

To make a sturdy plaster bonder: Use Weldbond straight from the container or diluted (1 part Weldbond to 3 parts water) and apply with a pump-type sprayer, roller, sponge, or brush. If it is a painted surface, be sure to plaster while the adhesive is still tacky. Porous surfaces, however, must be primed with a solution of 1 part Weldbond to 5 parts water and be allowed to dry.


Q. I live on Cape Cod, and have a long-term problem with mildew along the junction of my bathroom wall and ceiling. I have cleaned it with various off-the-shelf products and repainted the ceiling with high-quality paint with a mildewcide additive. The window is open all year long, and we run the fan when we shower. After we have finish showering, we leave the fan on for at least 10 minutes. What can we do to combat the mildew?



A. The first places I’d look are the bath fan and fan duct. Is your ceiling fan rated for the proper cubic feet-per-minute airflow and exchanges for your size bathroom? Remove the fan cover for the sizing information, and then do the research online.

You can also do a quick suction test: Hold a piece of printer paper up to, but not touching, the bath fan. Turn the fan on. If the paper is sucked up to the fan, your fan is probably working OK. That doesn’t mean, however, that it is exchanging the air enough times for the room’s size.

If the suction is poor, make sure the exhaust vent hood is not clogged and the damper is working properly. Also check the exhaust pipe for damage, disconnects, and kinks. I’m not a fan of flexible hoses because they tend to sag. Ridged pipe is better, and if it is running through an unheated space, make sure you put an insulated sleeve over it.

If you can access it, look under the fan duct for water stains, indicating a leak in the pipe. This could be the source of the mold. This is also a good time to make sure that the pipe is pitched away from the fan and toward the exit point.


Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to [email protected] or tweet them to @robertrobillard. Subscribe to our newsletter at


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