The North-Facing Streaks That Were Not Dirt
A New Castle homeowner called us last July because her neighbor told her the roof looked dirty. She was embarrassed, thinking the pressure washer in the garage would handle it on a Saturday morning. When our inspector climbed up, he found classic Gloeocapsa magma, the blue green algae that leaves those dark vertical streaks on north and west facing slopes. The shingles were 11 years old, still had good granule coverage, and had another solid decade in them if treated correctly.
We talked her out of the pressure washer. High pressure strips granules in seconds, and those granules are the UV shield keeping the asphalt from cooking. Instead, we did a soft wash with a 50/50 sodium hypochlorite and water mix, rinsed with low pressure, and installed zinc strips along the ridge. Total cost was roughly one twelfth of a replacement. Two years later the streaks have not returned.
What made that job easy was the timing. She called in July, which is prime soft wash season because the solution dwells on the shingle long enough to kill the algae at the root before a rain rinses it off. We tell New Castle homeowners that the window between late May and early October is ideal. Outside that range, cold nights dilute the mix and we end up needing a second application.
The Moss That Hid a Real Leak
Another call came in from a New Castle homeowner who noticed a ceiling stain in his dining room after a spring storm. He assumed storm damage and wanted an insurance inspection. We ran one, but the real culprit was not wind. It was a four inch moss colony on the north slope that had been holding moisture against the shingles for probably three or four years. The moss had lifted the shingle tabs just enough to let water wick sideways under the course above it.
That is the part people miss about moss. Algae is mostly cosmetic in its early stages. Moss is structural. It holds water like a sponge, and in central Indiana we get freeze thaw cycles 40 or 50 times a winter, which means that sponge expands and contracts against the shingle edges every single night for months. On that particular roof, we ended up doing targeted roof repair on two sections where the decking had started to soften, then cleaned the remaining moss and treated the whole north slope. If he had waited another season, we would have been writing a replacement quote instead.
When We Tell Homeowners to Wait
About one in four algae calls we run in New Castle ends with us recommending no cleaning at all that year. If the growth is light, the shingles are young, and the ridge already has zinc, spending money on a wash is not honest work. We will note it, photograph it, and tell the homeowner to call us in 18 months. That approach is why repeat customers and referrals drive most of our schedule.
The Landscaping Question Nobody Asks
One New Castle homeowner we worked with last spring had prize winning hostas and hydrangeas ringing the entire north side of her house. She was terrified of the sodium hypochlorite mix dripping off her gutters and killing everything she had nurtured for 15 years. That is a legitimate concern, and it is why New Castle Metal Roofing crews tarp foundation beds, pre soak plants with clean water before we start, and continuously rinse during the wash. We also keep a hose running in any downspout that drains near ornamentals. She checked her beds every day for a week after we finished. Not a single leaf yellowed. Good prep work is the difference between a clean roof and a lawsuit, and we treat it that way on every job.
Why New Castle Roofs Get Hit Hard
Central Indiana has three conditions that feed algae and moss aggressively. First, we get humid summers with dew points that regularly sit above 65 degrees for weeks. Second, we have mature tree canopies in older New Castle neighborhoods, which means shade and constant leaf litter on north slopes. Third, our winters keep shingles wet for months between snow melts. That combination is why we see algae on roofs as young as six or seven years old here, when the national average is closer to ten.
The fix is usually straightforward if caught early. Zinc or copper strips along the ridge release trace metal ions with every rain, and those ions keep algae from establishing. Trimming back overhanging branches by even three or four feet makes a measurable difference in how fast a slope dries after rain. An annual soft wash on the shaded slopes, priced modestly for most New Castle homes, prevents the moss stage entirely.
The Pressure Washing Disaster
Not every story has a happy middle. A New Castle homeowner hired a general pressure washing company, the kind that does driveways and fences, to clean his roof in 2022. They used a 3000 PSI tip at close range. When he called us that fall because shingles were curling and he was finding granule piles at every downspout, the damage was already done. The roof was eight years old and should have lasted another 15. Instead, we walked him through the signs a roof actually needs replacement and confirmed that blasting had stripped so much granule surface that the asphalt was already oxidizing. His insurance would not cover it because the damage was not storm related. That job became a replacement, and it did not need to.
If you remember nothing else from this post, remember this short list of what not to do on an algae or moss covered roof:
- No pressure washing above 1000 PSI, ever, and honestly not above 500 for shingles
- No wire brushes or stiff bristle scrubbing, which rip granules
- No bleach applied without rinsing plants and gutters first
- No walking on wet moss, which is slicker than ice
The Commercial Flat Roof in New Castle
Algae and moss are not just a shingle problem. A property manager called us about a low slope TPO roof where moss had colonized around the drains and HVAC curbs. Standing water plus organic debris equals a biofilm that eats through membrane seams faster than most people realize. We cleaned it, resealed the flashings, and got him on a twice yearly maintenance rotation. That story lives closer to our commercial roofing work, but the principle is identical. Organic growth plus trapped moisture equals accelerated failure.