Mold in hard plaster and cement board

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Joined: Tue Jan 14, 2020 8:11 pm

Mold in hard plaster and cement board

Post by Noakes » Tue Jan 14, 2020 8:26 pm

In a house built in 1960s in which cement board was used (expanded metal mesh with concrete over it) To finish the walls and ceilings a hard plaster coat was applied.
We have a mold issue in the bathroom especially above the shower. I'll have to pull the surround off of the tub to view those walls but the walls above the surround and the ceiling area show signs of mold.
I also have a few spots that appear to be mold outside of the tub/shower area on the ceiling. I do not know exactly if the material covering the concrete is 'hard plaster' or other.
What I would like to know is: is there any kind of plaster or other material I could cover the concrete board with that will inhibit mold, and can the small spots of mold be dealt with without replacing the panels?
I believe I will need to replace the panels behind the surround but not sure what to replace them with. also have to replace the ceiling above the tub/shower.
I am also concerned about possible water damage to the structure behind the tub/surround. What is the criteria for replacing it? The wood is darkened but not pithy.
Drywall in other areas I am used to, damp/wet areas I am not used to. I want whatever fix I do to last a long time, more than 10 years.
Also, presently the exhaust fan is not above the tub, I plan to move it there (wiring ok, GFCI circuit). This may help in the future. This may have prevented this issue and may prevent a recurrence.
Any thoughts or suggestions appreciated.

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Joined: Wed Jan 15, 2020 1:22 am
Location: Western Canada

Re: Mold in hard plaster and cement board

Post by DNDPARA2 » Wed Jan 15, 2020 1:39 am


Whats best way to get rid of mold in baseboards & drywall, will it require just ripping it out back to studs & then build fresh or is there an easier way??

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Location: Canada

Re: Mold in hard plaster and cement board

Post by Shannon » Wed Jan 15, 2020 9:44 am

So what you have is basically a plaster finish not actually cement board as we know it. Personally I would tear it all out replace any actual rotted wood and drywall the bathroom . If the lumber is damp but not actually soft it will likely be fine once it dries out but if its really black I would remove it and replace. Also be sure your fan is the right size for the space (bigger is better IMO) and that it properly vents outside the house. A good rule of thumb for fan sizing is figure out the sq. footage of the bathroom and get a fan equal or greater to that number in CFM. If you have ceilings over 8' high then I would suggest using 1.5x the square footage and if you are over 100 sq.feet then a 200 CFM is for you. Also look at the "Sones" the fan produces, this is basically how loud it is the lower the sones the quieter the fan anything under 1 sone is really good.
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