newbie trap question

I’m a tad confused about the washer stand pipe standard:

18"-42" above the crown weir, but 30" above the trap weir?

I don’t get the implied difference. What am I missing here?


I get 34 inches.

Washer drain needs to go up so there is no siphon effect.

Hmm. Not sure how to interpret that.
Where does 34" come from?


The 18" to 42" rule is from IPC.

The 30" rule is in the IRC.

Somehow, in whatever you referenced, they appear to have been combined.

Thanks, Jim. Now it makes sense.