How do you clean algae out of tile roof valleys?
Short answer. Clear leaves and needles out of the valley first, then apply a roof-safe algae cleaner or a diluted bleach solution at low pressure, working from a ladder or with an extension pole, and rinse gently with a garden hose, never a pressure washer. Valleys are where water concentrates on a tile roof, so the priority is keeping the channel open without walking on or cracking tiles.
Why valleys grow algae first
Valleys collect the most water and debris on any roof. On tile, leaves and pine needles lodge in the valley metal and against tile edges, stay wet in Georgia humidity, and feed algae, moss and lichen. Worse, debris can dam the valley and push water sideways under the tiles onto the underlayment, which is the real waterproofing layer on a tile roof.
Cleaning it safely
- Stay off the tile if you can. Clay and concrete tiles crack under foot traffic. Work from a ladder at the eave, use an extension pole, or hire someone who knows where to step.
- Clear the debris from the valley channel by hand or with a soft brush, working downhill.
- Protect plants. Wet and cover nearby landscaping, then apply the cleaner following its label for tile roofs.
- Let it dwell as the label directs, then rinse with a garden hose at normal pressure.
- Never pressure wash. High pressure erodes tile surfaces, strips coatings, and drives water under the tiles and past the valley metal.
Slowing regrowth
- Trim overhanging branches so the valley gets more sun and fewer leaves.
- Clear the valley a couple of times a year, especially after fall leaf drop.
- Zinc or copper strips near the ridge release ions that slow algae on the slopes below, though their effect in valleys is less reliable.
Check what is under the algae
Look for cracked or slipped tiles, rusted valley metal and stains in the attic under the valley. Tiles often outlast the underlayment beneath them, so an older tile roof with valley leaks may need the underlayment and valley metal replaced. See moss and lichen on roofs and black streaks and algae.