Optimal slope for trap arms (according to physics, not particular code)

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tarlax

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The reasons for not exceeding the maximum slope is easy to understand: (1) the risk of siphoning water from the p-trap, and (2) water failing to transport solids.

So is the reason for ensuring a minimal slope: stagnation of solids and risk of negative slope should the house structure move slightly.

For toilets, it appears (according to most codes if I'm not mistaken) the optimal slope is anywhere between 1/8 and 1/4 per feet.

In the case of bathtubs, showers and sinks where there are no solids, I suspect that the optimal slope is somewhere close to the pipe diameter for the complete length of the trap arm.

I say "close", because the drop can't be equal the pipe diameter for the anti siphon to work, minus another "safety margin", to compensate for the level reading error (because plumbing levels are not precision instrument), and also to compensate a slight movement of the house structure, which could slightly change the horizontal plane.
 

Reach4

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For toilets, it appears (according to most codes if I'm not mistaken) the optimal slope is anywhere between 1/8 and 1/4 per feet.
I don't think plumbing codes specify an optimal slope for toilet waste, but they do specify a minimum slope.

Trap is built into a toilet, and the trap is supposed to siphon in normal operation. I think the vent for a toilet is to get air out of the way, rather than the purpose it also plays in other traps (prevent siphoning).
 
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