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Community and Q&A

Mold on a wood ceiling

6621023 | Posted in General Questions on

I just built a house in Mississippi and it has been a very wet winter here.  I have noticed something that looks like mold or  mildew on the wood  ceiling of the garage.  I decided to paint the ceiling  with an oil based paint without cleaning it first,  now I am worried.   If this is mold will it grow out the other side of the wood into my attic. What can I do to make sure this does not turn into a major problem?

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  1. Expert Member
    BILL WICHERS | | #1

    Mold won’t grow through the wood, mold doesn’t really penetrate things like that. It’s entirely possible mold is growing on the other side though if the conditions in the attic above (I’m assuming there is an attic above) are right. Mold needs moisture, and food. The wood is the food. Very high humidity and/or condensation can get the growth started. You haven’t provided enough info about the attic space to offer a solution to a moisture problem there. The usual advice is to make sure walls and ceilings/floors between the attic and the house are properly air sealed, and that the attic has sufficient ventilation if it’s a vented attic.

    For the mold on the garage ceiling, you can probably clean it. Bleach or hydrogen peroxide can be used to kill the mold, and there are also special purpose disinfectants intended for use with mold. Chances are you’ll have some discoloration of the wood after cleaning, but if this is a painted ceiling a fresh coat of paint will hide the damage.

    You probably can’t do much about the humidity levels in your garage. There are some paints out there that inhibit mold growth so that’s about all you could do to minimize future mold problems.


  2. GBA Editor
    Martin Holladay | | #2

    First of all, can you tell us your name?

    I wouldn't worry too much about a little mold on your garage ceiling. The only way to prevent the mold from coming back is by reducing the moisture level in your garage -- and that may be an expensive proposition that isn't worth the trouble. (I doubt that you have any interest in installing a dehumidifier or an air conditioner in your garage.)

    Some garages are damp due to a roof leak or poor grading (bad drainage) at the exterior. If you've got a roof leak, of course that should be fixed. If there is a grading issue, it's probably worth adjusting the grade it you can. Ideally, the exterior grade should slope away from the garage on all four sides.

    In damp climates, it's sometimes helpful to install gutters at the eaves of your roof, and to connect the gutters to a buried drainage pipe that directs the water to a distant dry well. Whether you want to go to all that trouble because of a spot of mold is entirely up to you.

  3. Expert Member
    Peter Yost | | #3

    The oil in the paint can also be the food that mold needs:

    But moisture is still the key; it's really the only way to manage mold, regardless of the food source.


  4. BazookaSkill | | #4

    Humidity problems occur when warm, moisture laden air interacts with a cool ceiling, causing condensation and subsequent mold growth. Liquid moisture problems are due to direct liquid water exposure, i.e. plumbing failures, roof leaks, etc.

  5. Expert Member
    Peter Engle | | #5

    Untreated wood has lasted centuries in your climate without treatment and without decay, if protected from bulk moisture (and termites). It may grow some surface mold in damp weather, but this is generally not significant enough to cause any damage. Good grading around the garage, roof overhangs and diverting roof water away from the building are good primary preventions.

    If you really need to inhibit mold growth in wood that will be damp in service but not exposed to liquid water, borate-based treatments can be effective. The two most common products are Boracare and Tim-Bor. Both are water-based formulations that will last the life of the structure as they are based on mineral salts that will not degrade over time. These products also provide prophylactic treatment for termites, carpenter ants and other wood-boring insects.

  6. Jon_R | | #6

    Monitor relative humidity at the ceiling and if it's higher than outside, increase ventilation and circulation.

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