A slippery slope

Asked February 20, 2019, 1:32 PM EST

In the most appropriate measurement (e.g. micrometer), how thick is the algae/slime/scum that grows on decks, walkways or virtually any surface left outside and unattended during the PNW rainy season. I understand it obviously varies depending on the type of growth, the medium on which it’s growing, the environment it’s in, the length of time it’s had to grow and so forth. I’m just interested in getting an average estimate on the thickness of the type of green growth that might cause an unwary person to slip. It’s a question that’s been rattling in my mind for a bit. Thanks!

Clackamas County Oregon

2 Responses

Algae thickness and related slipperiness are not items I’ve seen research about in the Extension Service publications. I’ll refer your question along to other experts here. Perhaps someone will have an answer.


I've also searched for resources about algae thickness and correlation to slipperiness, but I haven't found any such resources. In sum, all resources agree that presence of algae on walkways is enough to make them slippery and potentially hazardous. Having just slipped this very morning on an algae and moss-covered sidewalk, I have to agree.

I would hazard a guess that it isn't the thickness of the algae, per se, but the amount of area it covers. Algae is slippery because holds water, keeping pathways slippery after everything else has dried. If there are only trace amounts of algae or moss on the walkway, it is likely less of a problem. However, when you have several square inches covered, it becomes problematic.

I'm sorry we cannot answer your question.