Blueprint Elevation Query 2

[WaterLou]

If the invert elevation of a pipe is 93.25', isn't this the distance down from grade? If so, when calculating the upstream invert elevation 85' away at 2% grade, why is the answer 95.02'? Isn't this a deeper point? What am I missing here?

 

[MikeHalloran]

Some arbitrary, likely immovable, point on or near the site has been chosen as 100' elevation. Everything is then relative to the theoretical 0 associated with that. Higher numbers are closer to the sky.



Mike Halloran
Pembroke Pines, FL, USA

 

[PEDARRIN2]

A lot of times the architect picks the first floor slab elevation and calls it 100.00'. Inside elevations are taken from that point of reference.

With plumbing piping, the inside invert elevation is also taken in reference to that elevation. Therefore underslab piping will be less than 100.00. With required negative slope, the invert of a plumbing pipe will get less. That is why the numbers decrease in the direction of flow.

What gets sticky is that the plumbing piping ties into site piping and the civil engineer who designs the site piping uses inverts which are typically from a site survey with inverts based upon that with numbers in the hundreds or thousands.

It is the plumbing designer's responsiblity to keep them straight.