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I live in a rental house and I see this growing on roof. What is it: is it moss mold or some fungus/algae? Should i be worried for my health? enter image description here

The clear line where it grows is likely because it has shade from roof above.

This is North CA (Bay area), no rains in summer but they're growing since last 2 years.

p.s. the wet area at the bottom of photo is from me spraying some bleach.

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    It's moss. Not an immediate health risk, but can lead to mold growth because it retains water. I'd be more concerned about the integrity of the roofing materials in the short term. Sep 8 at 20:30
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    Also a heavy dose of lichen sprinkled in there.
    – Gunner
    Sep 8 at 20:35
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    Is this area of the roof permanently shaded ? There's a clear demarcation line between the two areas.
    – Criggie
    Sep 8 at 20:39
  • yes that area is under shade of another roof on top of it. (its 2 floors)
    – Taranfx
    Sep 8 at 20:40
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This is a combination of Moss (the thick rounded lumps) and lichen (the very thin plants) It is no worse for your health than having the same plants growing on your footpath or driveway or rocks.

However, while not a risk to you, you don't want this stuff growing on your roof, holding water, and rooting into your roof via the cracks. If the shingles were installed properly, there will be a sealing layer underneath which will stop anything coming through.

Since its a rental, its your landlord's problem, not yours.


If it were mine, I'd wait for a warm dry period of the year, and on a low/no wind day I'd get up and try one or more of:

  1. Scratch out the moss using an old screwdriver
  2. Broom/sweep off the roof in several directions
  3. waterblast (though the high pressures can damage roofing materials, use with caution)
  4. Spray with a suitable moss/lichen weedkiller

Wait a week for the spray to work, then on the next weekend:

  1. Sweep again to remove dead plant material
  2. Apply a sealant of some sort - either some roof paint suitable for your tiles, or a thicker mastic-type paint to seal up the gaps but still be flexible for temperature changes.

The idea is to stop the plants from rooting into the roofing material.

If the tiles/shingles were cracked and damaged, it would be better to just replace them, but these look to be in good condition.

Working on top of a roof generally means working at height, so do be careful.

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I agree; it definitely is moss and yes , it can lead to mold growth (as previously stated). Watch out though, as it accumulates quickly and in no time, it can cause your roof to collapse. And no, I wouldn't be worried about any immediate health risks.

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