There are dozens of signs for the English concept of "layer."
The right sign for "layer" is going to depend on what you mean.
If you mean "layer" as in a "level" you can use the "flat" sign.
FLAT / level / floor / carpet / shelf:
(Also see: FLOOR)
Suppose you want to say, "A layer of snow."
What you would do is first show (one of the many versions of snow)
and then show the thickness of the snow layer.
Below I'm showing snow coming down by fluttering my fingers as I
moved my hands downward. Then I slide a "C" hand along the top of my
non-dominant forearm to show a layer of snow about 4 inches (8
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American Sign Language (ASL) online at American Sign Language University ™
ASL resources by Lifeprint.com © Dr. William Vicars