How to Clean Roof Stains
Updated: May 03, 2024
Is your roof marred by streaks and stains? Here's how to remove that ugly discoloration and keep it from coming back.
Introduction
Roof stains are unattractive discolorations that can ruin a home's curb appeal. Luckily, they're often not too difficult to deal with.
I reached out to roofing and siding professional Niki O'Brien to learn what's behind these mysterious stains and what can be done about them.
"Roof stains can be caused by many different things," she told me. "In more humid climates, roof stains are often the result of some sort of organic growth — such as mold, moss, or even algae."
But while organic stains are a common cause of roof stains, they're far from the only culprit. "In Colorado, the stains we see on roofs are typically water stains, rust stains, stains from overhanging trees or bushes, or some sort of environmental factor," O'Brien says.
Here I've provided a step-by-step guide for dealing with one of the most common types of roof stains: moss or algae growing on asphalt shingles. If your home has a different type of stain, you may need to use a different kind of cleaner, but most of the steps will be the same.
Of course, if you don't have any interest in getting on top of your roof, it's best to call in a pro. But if your roof slope isn't too steep and you're comfortable working at heights, you can typically clean it yourself in a single afternoon.
Tools Required
- Full-body safety harness
- Garden hose and spray nozzle
- ladder
- Specialty rinsing tool (optional)
- Spray pump
Materials Required
- Nontoxic or noncorrosive roof cleaner
Project step-by-step (7)
Choosing the Right Tools
As with most jobs, the right tools can save you a great deal of aggravation in the long run. You’ll need a full-body harness, a spray pump, and a nontoxic, noncorrosive roof-cleaning chemical. You may opt for a special roof rinsing tool, which can make the process easier, but aren’t absolutely required.
Look for a roof-cleaning product that’s as noncorrosive and plant-safe as possible. Chlorine bleach is a corrosive agent that can damage metal flashing and gutters, and the runoff harms plants. Those perfect peonies you planted? You definitely don’t want to dump gallons of bleach on them.
O’Brien suggests starting with the lowest water pressure, gentlest cleaning method, and mildest cleaning solution. “We’ve seen instances where a homeowner got carried away with the pressure washer and created a lot of damage,” she says.
Remember, you can always add a more aggressive cleaner or use more pressure, but once you damage the roof, you’ve created a bigger repair to deal with.
Proper Preparation
Aim to do the work on an overcast day with little or no wind. Those conditions allow the cleaning solution to soak deep into the algae colonies without evaporating too quickly. (And it will limit the risk of cleaning solution blowing back into your face.)
Repair any loose shingles or flashing before starting the cleaning process. Clean the gutters and downspouts to ensure they can drain freely.
Prepare the area by moving lawn furniture and covering vegetation. Even if you’ve selected a relatively plant-friendly cleaner, it still pays to take precautions. You don’t need to prepare the entire home perimeter, just the areas that will have runoff from the cleaning.
Pre-Wet the Roof
Before applying the cleaner, spray the roof with water. This will prevent the cleaner from drying out too quickly.
If your roof is low enough, and you have a good angle, you may be able to do this from the ground by using a garden hose and a spray nozzle. Spray up in the air and let the water fall like rain. (Never spray directly up into the roof, as that will push water underneath the shingles and can lead to roof leaks.)
Apply the Cleaner
Fill a spray pump with the cleaning solution, following the manufacturer’s instructions.
At this point, it’s time to break out that full-body harness. You will be walking on a wet roof and applying additional liquid to (already slick) mildew or algae spots. Even experienced roofers can slip in situations like that, and the inconvenience of working in a harness pales compared to a trip to the emergency room.
Once you’ve secured the harness, it’s time to get on the roof. Use the spray pump to generously apply cleaner onto the shingles. Follow the manufacturer’s instructions on how long to let it soak, before rinsing.
Saturate as much area as possible without letting the solution dry out. Depending on the weather, you may need to work in smaller sections.
Start at the bottom and work up to the peak so that water isn’t running underfoot. Spray until you see runoff. Respray any areas that dry out.
Rinse Away the Stain
Once the algae has been treated, you can remove it. If the staining is fairly light, you can use a regular garden hose and nozzle to rinse it off. Go slowly and use even strokes, overlapping slightly to avoid a zebra stripe effect.
For severely stained roofs, a garden nozzle won’t exert enough pressure to dislodge the stains. In that case, you’ll want to invest in a specialized rinsing tool. Drag the rinsing tool in a forward-and-back motion as if you’re vacuuming. That places the water jets at the correct angle to blast off the dead algae colonies.
Prevent the Stain from Reappearing
Simply removing the stain isn’t a permanent fix. You need to identify and address the source. Otherwise, you’ll be in a perpetual cycle of stain remediation.
The steps to take will depend on the cause of your specific stains. Rust stains might require flashing to be replaced or painted, while stains from tree debris may simply need you to cut back a limb or two.
In our example of algae stains, consider applying a stain-blocking solution. When applied shortly after you’ve cleaned the roof, it can significantly delay the return of the stain-causing algae.
The best preventative measure, says O’Brien, is to perform a regular (she recommends bi-annually) inspection to address staining before it happens.
FAQ’s
How often should I clean my roof to prevent stains from recurring?
It depends on the type of stain and your individual roof. But if you can’t rectify the stain’s source, expect to clean it every 3 – 5 years.
Can cleaning roof stains improve the longevity of my roof?
“Metal roofs and flat membrane roofs can withstand and actually require more frequent cleanings,” says O’Brien. “However, frequent or harsh cleaning on an asphalt shingle roof could result in granule loss and a shorter lifespan for your roof. Maintenance, more than cleaning, will most definitely improve the lifespan of your roof.”
About the Expert
Niki O’Brien is the operations manager at Custom Exteriors, an exterior remodeling company based in Berthoud, Colorado. She has been in the roof, siding, and window replacement for over a decade, performing roofing installations, replacements, and repairs all across Colorado.
