1/8th per inch
1/4 " per ft is average
Vertically it is unrestricted
It really depends on your local building code, but 2% should be the minimum so that your pipe will be self-cleaning.
.25 per inch per foot
1/8" per foot
1/8" per foot.
I would use 3 inch for that.
Yes. A reduction fitting can be used.
A 3 or 4 inch pipe sticking straight up out of the ground? This is the clean out for the sewer line. If sewage is coming out of it, this means the sewer line is clogged or collapsed from there to the main city sewer. If you have a septic tank, the tank is full and needs to be pumped out.
For proper fall. Pipe should slope 1" for every 8'.
In Tulsa Oklahoma you can, but i would still use 4 in.
The maximum flow rate through a 1.5 inch pipe is typically around 9 gallons per minute.