This is not as simple a question as it might seem if you are talking about doing the bare minimum to follow code. There are a couple of major plumbing codes in use in America and they don’t all agree as this conversation from the Ask Me Help Desk forum makes clear.
From this Forum Post….
Plumbing codes in the USA mandate that pipes 3″ and smaller get 1/4″ pitch per foot minimum.
Pipes 4″ and larger can have a minimum 1/8″ pitch per foot.
If not on commercial job using a transit level to keep track of slope I take a 4 foot level and I tape a 1/2″ block of wood at the very end of the level which is equal to 1/8″ pitch per foot so that when I place the level on the 4″ or 6″ underground pipe, for example, and get a level reading from the level I know I have a properly pitched pipe for sure!
From another post on the same forum…
I got this from Google,
Minimum 1/4″ of fall per foot on 2″ pipe or smaller.
Minimum 1/8″ of fall per foot on 3″ or larger.
1/2″ per foot is Maximum.
Which garnered this response…..
MUST be IPC code…all others that I know of require as I posted…1/4″ per foot 3″ and smaller, 1/8″ pitch per foot 4″ and larger.
Always seems to be some exception to every rule!! I swear!
I know I wouldn’t go with only 1/8″ pitch per foot on a 3″ toilet line…just seems to be asking for trouble.
The IPC coder referred too is the International Plumbing Code. It is one of the plumbing codes that are used in America.
I agree with the last commentator that 1/8″ pitch per foot is risking it for a 3″ toilet line. In general, I am a steeper is better kind of guy.
But some people argue you should never pitch any waste drain for more then 1/2″ drop per foot. If you over do your pitch, the water is going to out run your waste. This means that the waste could build up and block your drain. Other people argue that this is just an old wives tales.
I think that there is usually some truth to those old wives tales. I would not doubt that you can overdo pitch if you try hard enough. However, I have never seen good sold scientific support for the “no greater than 1/2″ drop per foot rule.”
I wonder why this is. One would think that someone would have settled the issue in a scientific matter by now. But if they have, I have never come a crossed it.