Mold/Moss on outside of house? How to clean?

June 25, 2021 1:56 PM

These dark moss(?) spots appeared on the outside of my house one day. I want to clean them, but I don't know what they actually are! And I don't want to make it worse somehow. Here's a picture. What are they and how do I clean it?
posted by cmcmcm to Home & Garden (8 answers total)

I can't quite tell from the picture if it looks like mold or moss or what. Any chance of a close-up, maybe an angle shot too? Possibly different lighting if it's not actually as black as it appears?

What does it feel like (gloved is fine)? Moss if it's desiccated will be greenish-black and sort of stiff and scratchy. Mold will be soft if it has any thickness. Or if it's actually thin like black paint that's useful to know too. I will say I ain't ever seen a clump of moss like that on painted siding myself.
posted by away for regrooving at 3:38 PM on June 25, 2021

I notice in the board above a sunken or crumpled look under the paint film? Unless the board started out weirdly grooved, I have suspicions of rot. :( Look at the "skirting board" photo here.

Poke that upper board and see if it's soft in those spots.

Look above, any drip source like a gutter or any vertical alignment to how these are showing up?
posted by away for regrooving at 3:43 PM on June 25, 2021

Has anything changed the windflow around your house? Like new adjacent buildings? Or have there been changes inside the house to insulation or vapor barriers, also air-con?
posted by unearthed at 4:22 PM on June 25, 2021

I also have suspicions of rot and an ongoing leak somewhere above that spot.
posted by aramaic at 5:13 PM on June 25, 2021

Response by poster: Here is another picture.

The clumps are 3-dimensional. It is not rot. (The crumpled look is because my house was repainted without completely removing an under layer of paint.)

It seems to be an animal or vegetable. Thanks for your help so far. Any idea what sort of expert to ask for advice?
posted by cmcmcm at 5:59 PM on June 25, 2021

Best answer: Chocolate Tube Slime Mold appears darker on siding.
posted by Iris Gambol at 6:26 PM on June 25, 2021 [5 favorites]

Missed: don protective gear, and scrub (not powerwash) with Oxiclean solution? Other DIY links, but you could consult siding experts.
posted by Iris Gambol at 6:32 PM on June 25, 2021 [1 favorite]

Glad to hear, "weird specimen id" is much more fun than "oh no house is rotting". Thanks for the pic, the stuff really sticks out! It sure has been dropping the spores or something.

I think I would try a mycologist first, if you have a local university perhaps. I'd ask if they think it might be a slime mold, always a sensible question when something appears fairly suddenly as you describe this. But it's not one I recognize. Doesn't look like "black hair" Nostoc either. Did you see them growing, or can you put a bracket on how fast they showed up between your seeing this siding?
posted by away for regrooving at 1:24 AM on June 26, 2021 [1 favorite]