I'm remodeling and adding a second floor bathroom. King County (Seattle) plumbing code says that a 3" stack can accommodate 48 DFUs, which is more than enough for me. However, there is a footnote that states:

"Only four water closets or six-unit traps allowed on any vertical pipe or stack..."

I only have 2 toilets connected to the stack. But I'm confused by "six-unit traps". At first I was reading this as limiting you to "six trap arms" connected to the stack (which would be more than I have planned to connect to the stack -- if you count the toilets). But, now I'm thinking that a "six-unit trap" is something else altogether (like a 4" trap?) and hopefully I don't have to worry about it. Google wasn't much help so I thought I'd ask here.


Here's the reference (see footnote "d"):