I have a kitchen drain that goes across my basement ceiling in a floor joist bay and then uses two 90s to exit the joist bay and take a 90 degree turn to run perpendicular to the joists. This reduces the headroom enough to be bothersome.

Can the two 90s be replaced with a 45 and a closet 45 to give me additional headroom?

Steve

If I understand correctly you would want a 90 in the joist Bay that's rolled so it's pointing at a 45° angle and then you put a 45° to bring you basically horizontal and perpendicular to the joists. Installing 2 45s are either going to drop it down and go straight or will give you a weird angle depending on the relationship of the two 45s.

you not only can use a 45 in place of a 90 but it will be a better job.

Is there a code issue? The plumber who installed the drain insisted it had to be done this way. I suspect he didn't have 45s in his truck.

from the internet...

"Where a horizontal drainage pipe, a building drain or a building sewer has a change of horizontal direction greater than 45 degrees (0.79 rad), a cleanout shall be installed at the change of direction."

the 2 90s can be replaced by a single 90 and a single 45

if done properly the 'perpendicular' pipe can begin its run strapped onto the bottom of a joist - remember it must pitch downwards slightly before it 90s downward to vertical

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638