How to Clean and Prevent a Mossy Roof | Ask This Old House

10February 2020


Push broom []
Hammer []
Caulking gun []
Garden hose []
Rubber gloves []

Shopping List:
Zinc strip []
Roofing nails []
Roofing cement []
Pump sprayer []
Bleach []

1. Start by securing the harness system to the roof, following the manufacturer’s instructions to ensure they’re installed properly.
2. Using the push broom, lightly sweep the roof to get off as much of the loose moss as possible.
3. Install a zinc strip near the ridge of the roof. When it rains, water will pick up the particulates in the zinc and deposit them along the roof, preventing more moss from growing.
a. Roll out the zinc strip underneath a course of shingles, allowing roughly half of it to be exposed and the other half underneath the shingles.
b. Nail roofing nails about halfway through the zinc strip, just to get it held in place.
c. Apply a bead of roofing cement around each nail before nailing them all the way through. This will help seal up the hole made by driving the nail.
d. Once the nail is all the way through, add another bead of roofing cement on top of the nail.
e. Repeat this process for all the nails.
4. Below the roof, spray any plants or grass with water to protect them from any runoff bleach that may fall off the roof during the next steps.
5. In the pump sprayer, pour a mixture of roughly 1/3 a gallon of bleach to 1½ gallons of water.
6. Spray the bleach mixture on the roof. The bleach will kill off any leftover moss over the next couple of days.
7. Remove the harness system from the roof.
8. Spray any plants and grass underneath the roof with water again once the process is completed.

When working on a roof, safety is very important. Tom added a roof anchor [] to the rafters, wore a harness, and used a safety line []. These can be purchased at home centers or hardware stores.

Tom used a push broom to gently remove the loose pieces of moss.

He then installed a roll of zinc strip moss preventer, which can be purchased online or at specialty retailers.

Finally, he sprayed down the roof using a solution of water and bleach. He started at the bottom of the roof and worked up so that he wouldn’t step on wet areas and slip.

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