At no time did you imagine you’d be called upon to channel your inner biologist as a part of home ownership. Yet, here you are. Green things are growing on your roof and you’re not sure what to do about it. Roof fungus, algae, lichens, moss, and molds are the common culprits of roof growth that can create a green (or multi-colored) roof, especially if you live in areas where there is a lot of moisture or humidity.

Add a little sun to the equation and you have a virtual biology experiment setting up on your roof. If you are seeing stains or streaks of various colors appearing on your roof, it’s a sign that you should take a closer look.

Here are a few dos and don’ts of roof maintenance.

Colorful, but Potentially Damaging Roof Growth

As much as you might enjoy a little touch of color on your roof, the growth that is happening isn’t something to ignore. Roof growth caused by moisture-loving algae, moss, or lichens can lead to mold and real damage to your roof’s integrity if left untreated. Treating this growth at its first sign should be a priority.

Safety First

If you can get a ladder and climb up to take a closer look at the growth appearing on your roof, you may be able to easily identify what is going on and put a plan in motion to treat it. For those uncomfortable or unable to climb a ladder, this inspection and identification process is best left to an experienced roofing professional. Here’s what you need to know.

Identifying Types of Roof Growth

  • Algae. This is a fungus, often black or green, that thrives in moist, wet environments. It finds its way to your roof via airborne spores that quickly set up housekeeping on wet spots where they spread and multiply. Algae will quickly cause damage to your shingles and need to be addressed without delay.
  • Mildew. Roof growth of the mildew variety is a lot like the black gunk you might find in a shower or bathtub inside your home. It can also be light gray or pink, but regardless of its appearance, it is not only potentially damaging to your roof but also hazardous to your health. Since it is closely related to mold, this mildew may be toxic to those in your home.
  • Mold. Mold is also a fungus but of the most dangerous variety. It often has a distinct smell, is slimy, and presents as black, brown, or dark green. You’ll likely find it growing on surfaces that are easily permeated by water. Missing shingles or deteriorating decking on your roof are perfect locations for mold to begin growing, and it is as bad for your health as it is for your roof.
  • Moss. You may have seen moss growing on the side of a tree where moisture, shade, and debris create the perfect home for it to grow. Moss can also grow on your roof because of these same conditions. As it spreads, moss can trap moisture that then seeps into the underlying structures, leading to extensive roof damage.

Dealing with Roof Growth

If you have identified any of the common forms of roof growth at your home, the next step is to get rid of it. As long as you can do this safely, and the roof’s structural integrity hasn’t already been damaged, you can address this growing biology experiment head-on. Always call a roofing professional if you can’t or don’t wish to take these steps on your own.


  1. Check and eliminate moisture causing issues first. While you’re on the roof, it is also a good time to remove any overhanging branches or debris from roof or gutters that may be allowing moisture to access your roof and contribute to the problem.
  2. Prepare a bleach rinse. Fungus hates bleach, so it isn’t surprising that it is a common remedy for the things that may be growing on your roof. It is important to use gloves and eye protection as you mix a solution of one-half bleach to one-half water. This is easily applied using a pump-up sprayer. There are also ready-to-use formulas available at your local home improvement store, or roofing contractors are always happy to provide this service for you. It is often combined with a soft wash of the rest of your home’s exterior, just to make sure all fungus and spores are covered by the treatment.
  3. Set the timer. After spraying the bleach mixture, let it set for at least 15 minutes to ensure it kills the spores. If you have a lot of growth, it is best to apply it liberally and leave it on for up to one hour.
  4. Rinse. The next step is to take a garden hose using low to medium water pressure to thoroughly rinse off the mixture. Start at the pitch or top of the roof and wash the formula down toward the gutters. Continue to rinse until you’ve removed all of the bleach/water solution. If you feel any residue still on the roof, repeat the garden hose rinse again to make sure you’ve removed all of the bleach formula. The roof shouldn’t feel slimy if you’ve completely rinsed it off.
  5. Inspect and repeat if necessary. Once complete, you’ll want to make a point to check the roof and repeat this process as often as necessary to keep roof growth at bay. The timeframe for having to do this will depend on your specific situation and the amount of moisture in your area.

Helpful Hints to Keep in Mind

Be aware that this process can be potentially damaging to any plants or trees that may receive runoff from your bleach roof treatment. Be sure to rinse surrounding vegetation that you want to preserve, or you can cover them with tarp or protective plastic prior to applying the treatment. If you don’t wish to make a fashion statement later, you’ll also want to wear old clothing to do this work, since bleach will leave stains on fabric that are permanent.

This can be a complicated and time-consuming process. From first time assessments and treatment of roof growth to ongoing maintenance, residents of Warren County and the surrounding area can rely on our roofing teams to not only manage your growing problem, but to also keep it at bay for good.

Be sure to ask about Roof Maxx® waterproofing to ensure long-lasting protection from moisture and roof growth. Find out more by contacting Warren Thompson & Son Roofing & Siding today or call us at (908) 774-9229.

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