A reader would like to remove the growth from this siding or hire someone to do it.

Q: I have lived in my house for 25 years. Just in the past couple of years, some kind of mold or mildew has formed on the vinyl siding on the north side of the house. Why is this happening, and what do I need to do to address it? If it is physically challenging or involves chemicals, can I hire someone to remove it?

Rockville

A: Whatever the type of siding, algae, mold and mildew are most likely to form on the north side of a house because it stays shaded and damp. Several factors might explain why that didn't happen before. Perhaps trees weren't as tall? Or it could be that the vinyl was slicker when it was new and that bits of grime that became attached over the years now give rain a better grip.

Regardless of the explanation, the growth should come off fairly easily if you wash the wall with a little soap and water, using a soft cloth or a long-handled, soft-bristle brush, like what you'd use to wash a car. Hardware stores and home centers carry specialty cleaners that are gentle and work well.

The Vinyl Siding Institute, a trade group (vinylsiding.org), recommends starting at the bottom of the wall and working up, taking care to rinse thoroughly as you finish each area. Although it might seem to make more sense to start at the top, working up from the bottom minimizes the chance of having dirt-laden, soapy water dry on the siding and stick.

You can probably use a power washer, although the Vinyl Siding Institute recommends checking with the manufacturer first. But when siding is 25 years old or older, as in your case, the likelihood that the company is still in business, even if you know the name, is slim.

If you want to hire someone, the crew will almost certainly want to use a power washer. As a rough guide for what it might cost, Kevco Building Services (800-391-5291; kevco1.com), a company that cleans siding throughout the Washington area, charges a minimum of $189 to clean one or two sides of a house, a minimum of $229 to clean three sides, and a minimum of $269 for four sides. The price could rise if the house is large or especially tall. Kevco looks online to see if there is a picture of your house; if there is, the company can give you a more precise estimate over the phone. When necessary, it sends a crew to take a look and then gives an estimate.