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Pre-calculate DWV Pipe Lengths Between Fittings?

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Old 01-20-2009, 12:55 PM   #1
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Pre-calculate DWV Pipe Lengths Between Fittings?


I've always beena measure and cut guy for abs dwv piping. But I was wondering, is it possible to determine the exact length of pipe that's required between two fittings to get a perfectly-seated install if you know the exact distance between the vertical axes of the fittings?

Example: Installing a toilet drain. Between the center of the main stack and the center of the toitlet flange is x-inches. Immediately below the flange will be a 3" 90-degree elbow. The pipe will conform to the 1/4"-per-foot slope already built into the fittings. It'll join a santee at the stack.

I could put the santee in place, hold the elbow in place, take a measurement between the fittings and add two times teh pipe insertion depth.

OR, is there a way to calculate the pipe length from the x-inches, pre-assemble the riser, elbow, horizontal pipe and santee, so that it slips perfectly into the centres of the passages after assembly?

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Old 01-20-2009, 01:08 PM   #2
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Re: Pre-calculate DWV Pipe Lengths Between Fittings?

Got it - you gotta have the tech specs on the exact parts you're using. The catalog of my fittings' manufacurer lists centerline-to-seat dimensions. Measure centerline-to-centerline, subtract the centerline-to-seat dimension for the fitting at each end, should give a perfect fit IF you seat the pipe perfectly.

For example, while the centerline-to-seat dimension for a 3" elbow is 3", and you can see the logic behind that, the centerline-to-seat dimension for a sanitee is 3 1/16". So my pipe length will be x - 6 1/16" for a perfect fit.

Of course I suppose that only works on short runs where the triangle made by the 1/4"-per-foot slope is insignificant. On a 20-foot run, let's see, that'd be 240-inch horizontal, 5"-vertical, makes the hypoteneuse = sqrt(240^2 + 5^2) = 240.05", a difference of 0.05", or somewhere between 1/16" and 1/32" - ok, so never mind, we're good for pretty much any run.

Very nice.


Last edited by bob_cntrctr; 01-20-2009 at 01:17 PM.
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