by Paul Bianchina

While obviously unsightly, the presence of mildew on interior or exterior surfaces is more than just a cosmetic problem. It's almost always an indicator of a deeper and potentially much more destructive moisture condition. Painting over it won't cure the condition - the mildew will return sooner or later, and in the mean time the hidden moisture is working away at insulation, wood framing and other parts of your house.

Mildew is most often seen as a black, somewhat powdery stain on siding, drywall, roofing and other areas. In order to survive and grow, the mildew needs three things - heat, moisture and a food source. The food source is easy for the mildew to find, and, since it's virtually any organic material ranging from gypsum to wood to wallpaper paste, it's essentially impossible for you to eliminate. Since we heat the inside our homes and the outside is warmed by sunlight, eliminating the heat that the mildew needs is essentially impossible as well.

This leaves moisture as being the only one of the three necessary elements for mildew growth that you can have an impact on, and how you deal with moisture problems depends upon where in the house the moisture is and how it's getting there.


Inside, one of the most commonly seen areas of mildew growth is in the bathroom, where warm, wet air is at its most concentrated. Typically, the only thing needed to combat moisture here is the installation of ventilation fan that is ducted to the outside of the house (NOT into the attic!). As long as the fan is utilized during and immediately after using the bath or shower - and, if desired, you can ensure this by having the fan wired to come on every time the bathroom's overhead light is switched on - you shouldn't have any trouble containing the moisture before mildew can start.

If a ventilation fan isn't enough, then you have more moisture being generated in the bathroom then just that from the shower. Common hidden moisture sources include a leak in the tub or shower valves or supply pipes; loose and leaking drain lines; a bad wax ring seal below the toilet which is allowing seepage; or moisture buildup on the floor around the tub or shower from bad caulk joints or excessive splashing from tub users.

Detecting this moisture can be difficult, since the source is usually concealed and you typically won't even know you have a problem until it becomes bad enough to show visible signs - a buckled floor, crumbly drywall, etc. Here's one place where the presence of mildew is a blessing in disguise, since it tells you there's moisture present before it causes real damage. Once you begin to see the mildew, your best bet is to contact a contractor who specializes in water damage restoration - most have sophisticated moisture meters available that can help you track down the problem.


On the outside of your house, the two areas you're most likely to see mildew is on the siding and on the roof shingles, and here again it's the early warning sign of a moisture problem.

If the problem is localized to one or two patches of siding or roofing, then the moisture source is typically easy to track down. Some possibilities for localized mildew on siding include sprinkler heads that are misaligned, leaking or improperly adjusted; an underground leak in a water or sewer line; moisture accumulation from a dryer vent or exhaust fan vent; trees, shrubbery or other landscaping that's overgrown; and other similar "spot" sources. Do a little detective work in the areas of the mildew, and the problem is often easy to find and correct.

On the roof, mildew often forms in areas where trees overhang the roof, or where leaves or pine needles accumulate. Clearing debris off the roof and trimming overhanging branches will often be enough to solve the problem. You may also need to open up the areas around your house by removing or trimming closely packed trees and allowing sunlight and prevailing winds to reach mildewed areas and allow them dry out.

Wide-spread areas of mildew on walls and roof indicate a larger moisture problem, one that can usually be traced back to a lack of ventilation. If you have a high level of humidity inside the house that is not being adequately dealt with, normal pressure and convection will move the moisture through into wall cavities and attic spaces. Once it's there, a lack of ventilation will contribute to the moisture forming mildew. To combat this, first you need to deal with moisture inside the house through the use of ventilation fans in the kitchen, bathroom and laundry; mitigation of high-moisture sources such as indoor spas and hot tubs; reducing the number of house plants; or perhaps installing a dehumidifier. In the attic, be sure you have an adequate number of roof vents to allow moisture to dissipate naturally to the outside.


Once you've eliminated the source of the moisture, dealing with the resulting mildew is easy. Most paint stores sell mildew-killing mixtures, or you can mix your own from one quart of household bleach added to three quarts of warm water. You can also add about 2/3 of a cup of trisodium phosphate (TSP, available from paint stores and home centers) and 1/3 cup of powdered laundry detergent to this mixture for scrubbing down tough areas.

Wear goggles and rubber gloves, and apply the mixture using a sponge or a soft- to medium-bristle scrub brush. You can also spray the mixture on using a pressure washer, available from any rental yard. Allow the areas to dry, and check that the mildew has disappeared. You can then paint the areas with any common interior or exterior paint. If desired, you can add a mildewcide to the paint - also available from your paint store - to help prevent a recurrence of the mildew.

Paul Bianchina, 1999 All Rights Reserved

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