Why Is The Clog Forming In The Top?

PipeGuy

I cut open a bath tub drain today and noted that the top 2/3 of the pipe is developing a a grey colored build-up (presumably its soap scum, body oils, etc.). It's thickest right at the top of the pipe, about 1/4 - 3/8 of an inch thick.
Why does it form at the top?

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Ron The Plumber

On the vertical or the horizontal run of pipe?

Teetorbilt

I'm assuming that it is the water level of the trap.

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Squench

I cut open many drains and the top always has the build-up
because the water flow is 'cleaning' the bottom portion. All the floaties
are sticking to each other. If you take a 'mini Mag' and illuminate a plastic continuous waste from behind, you will see the build-up at the top.