Black streaks on a roof bother a lot of homeowners in Lexington, Columbia and around Lake Murray, and a pressure washer looks like the obvious fix. It is also one of the riskiest do-it-yourself jobs around the house. Here is why, and what to do instead.
Why does my roof have black streaks?
Those dark streaks are usually algae. In humid places like the Midlands, it thrives on shingles, especially on shaded slopes, and spreads down the roof with rain. It is mostly a cosmetic problem at first, but it also holds moisture against the roof, and left alone it can spread. Moss can follow in damp, shady spots.
So the streaks are worth dealing with. The question is how.
Can you pressure wash a roof?
Technically, yes. Practically, it is a bad idea for most homes. There are three separate risks: to you, to your shingles and to your warranty.
The risk to you
The biggest danger is not to the roof but to you.
Roofs are steep, and shingles covered in algae are slippery, especially when wet. A pressure washer adds a second problem: the wand pushes back when you pull the trigger, which can throw you off balance. Ladders on soft ground add another hazard. Falls from roofs and ladders cause serious injuries every year.
Even if you stay on the ground and spray up, you are working overhead with a heavy wand, and it is easy to lose control.
If you remember one thing from this post, let it be this: no clean roof is worth a fall. Stay off the roof.
The risk to your shingles
Asphalt shingles are coated with small mineral granules. They shield the roof from sun and weather and are part of how long the shingle lasts. High-pressure water can knock granules loose.
When granules wash off, you may see them piling in your gutters or at the end of the downspout. The exposed surface ages faster, and the roof can wear out sooner than it should. That kind of damage cannot be undone.
Pressure can also:
- Lift or crack shingles and break their seal
- Force water underneath, where it can reach the underlayment and decking
- Damage flashing, vents and the sealant around them
The risk to your warranty
Many shingle manufacturers advise against pressure washing. Warranty terms vary by manufacturer and product, so check the paperwork for your roof or ask the manufacturer or your roofer. If pressure washing is excluded, damage it causes may not be covered.
Why the streaks come back
Blasting algae with water may make the roof look better for a while, but force alone does not treat the growth. In our humid climate, what is left behind can spread again. And if the pressure wears the surface, you may end up with a roof that streaks faster in the future.
DIY chemical mistakes
Some people skip the pressure washer and mix their own cleaning solution. That is not risk free either:
- Strong cleaners can harm landscaping, grass and plants when runoff hits them.
- Some cleaners can stain or damage metal flashing, gutters, paint or siding below.
- Mixing products can create dangerous fumes.
- Using too much, or leaving it on too long, can hurt the shingles.
If you use any product, follow the label, wear protection, and rinse plants first.
What a roof soft wash does instead
A professional roof soft wash uses low pressure and a cleaning solution made for the job. The solution treats the algae, and the roof is rinsed gently. The shingles stay in place and the granules stay put.
We also protect your landscaping and check the gutters and downspouts as part of the job, since runoff from the roof ends up there. Learn more in our comparison of soft washing versus pressure washing.
Safer things you can do yourself
You do not have to ignore your roof. A few things are safe and useful:
- Look from the ground. Binoculars or your phone’s zoom show a lot.
- Trim branches that hang over the roof, so there is less shade, fewer leaves and faster drying. Hire out the high or heavy work.
- Keep gutters clear so water leaves the roof edge. See our gutter clean-out service.
- Take photos once a year so you can see how it changes.
When to call a professional
Call someone if:
- Streaks are spreading or moss is growing
- Shingles look cracked, curled or missing (you may need a roofer, not a washer)
- The roof is steep, tall or hard to reach
- You plan to sell, refinance or file an insurance claim and want it looking its best
If we see damaged shingles when we look at your roof, we will tell you before we start, and we may suggest a roofer first.
Questions homeowners ask
Can I use a low setting on a pressure washer? A lower setting reduces some risk, but nozzle, distance and angle still matter, and the fall risk does not change. Most homeowners are better off leaving roofs to a professional.
What about a garden sprayer and a store-bought roof cleaner? It is gentler than a pressure washer, but you still have to climb onto a slippery roof, and the runoff can harm plants and finishes below.
Will cleaning the roof make it last longer? Cleaning is mostly about appearance and removing growth that holds moisture. It does not fix worn or damaged shingles. If the roof is near the end of its life, you may need a roofer first.
Is it safe to clean a roof when it is wet or cold? Wet and cold surfaces are slippery. Professionals pick conditions carefully, and homeowners should too, which usually means not doing it at all.
The bottom line
A pressure washer on a roof puts you at risk of falling and your roof at risk of premature wear. The better choice is a professional low-pressure soft wash.
Ready to get a clear roof without the risk? Request a free quote or call (803) 528-1996. We serve Lexington, Irmo, Chapin, Red Bank, Ballentine, Gilbert and Columbia, and we respond within 24 hours.