Ponderosa pine needles drop sap that bonds to asphalt shingle surfaces.
Over a few seasons, the sap collects oxidized minerals + creates dark
vertical streaks below tree-overhung areas. The streaks aren't
structural damage but they:
- Reduce solar reflectance (your AC works harder)
- Look like algae to a buyer's inspector
- Accelerate granule loss in the streaked area
Why pressure washing is wrong
A 2,000 PSI pressure wash will strip granules from asphalt shingles.
Most manufacturer warranties explicitly void coverage on
pressure-washed roofs. Save the pressure washer for concrete tile
only — and even then, low-pressure detergent application is safer.
The gentle-wash protocol
Materials (~$25 from a hardware store):
- 1 gallon sodium percarbonate cleaner (also sold as "OxiClean
pure" or "Sun Joe roof cleaner")
- Pump sprayer (1-2 gallon)
- Garden hose with standard nozzle
- Soft-bristle brush on extension pole (only for spot work)
Process:
- Wet the streaked area with the garden hose first.
- Mix 50/50 sodium percarbonate solution; spray on streaks.
- Wait 20 minutes (don't let it dry — re-wet if needed).
- Rinse thoroughly with the garden hose.
- Repeat once if streaks remain.
Avoid bleach + chlorine cleaners — they corrode the metal flashing
+ run-off kills your landscaping.
Prevention: trim + zinc strip
Two cheap upgrades stop future streaking:
- Trim branches within 6 ft of the roofline. Pre-monsoon
timing (May) is best.
- Install a zinc strip at the ridge. ~$40 in materials, $120-$200
installed. Dissolves a tiny amount of zinc on every rain — kills
sap-fed micro-organisms that darken the streaks.
When to call a pro
If you have:
- Tile roof (cracking risk on foot)
- Steep grade > 6/12
- More than 10% of the roof affected
- Any signs of granule loss already
Hire a roof-cleaning service. Average Prescott cost: $350-$650 for
a 2,200 sq ft house.