1. Can I run a 1 ½” vanity drain horizontal along a 2x4 wall and around a 90deg corner? i.e. can I use a 90deg fitting to turn the corner?
You can have 135 degree of change of direction per cleanout.
2. After the shower’s p-trap, I am showing a 2” drain running to a ‘T’ and the top of the ‘T’ is a 1.5” vent. Can a 1.5” vanity drain ‘T’ to this 1.5” vent pipe for drainage purposes (creating about a 4’ section of vertical wet vent for the shower)? If I do this should the “wet vent” section be 2 inches?
A wet vent needs to be two pipe sizes larger and a minimum of 2". Since a lav can use 1-1/4", 2" would work, as long as you are on the same floor.
3. How far can a vanity 1 ½” sink drain run horizontal along a wall before it turns down to run to 3” sewer pipe?
As far as you like.
These are UPC trap arm lengths for the venting.
1.5" trap arm length, 42"
2.0" trap arm length, 60"
3.0" trap arm length, 72"
4.0" trap arm length, 10 feet unless it's for a toilet, then six feet.
4. I have a 3” toilet drain that runs 5’ horizontally to a “T”. Can the top of the “T” be 1.5” for the vent?
In many codes you would need at least a 2" vent.
5. What is the largest diameter pipe that can go through double 2x6” top plate on a structural wall?
You can cut the entire plate out, both top and bottom in most cases. It depends on the bearing of the structure. We would need structural plans and pictures to say more.
6. Am I correct that the minimum slope on a 2 inch or less pipe is ¼” per ft, 3” or larger is 1/8” per ft and on all pipe the maximum slope is ½” per ft ?
Minimum slope is 1/4" per foot unless you are running 4" and you have permission from the plumbing inspector.
8. If I have a 1 ½” kitchen sink vent pipe extending vertically to the 2nd floor and then running horizontal for 20’ before going vertical through the attic to the roof. During the 20’ horizontal length I will have the 1 ½” vents for a washer, 2 vanities, a shower and a tub connecting to it. Should this vent pipe be increased to 2” somewhere along the way or can 1 ½” run to the roof?
You need to count the units up as you go and work backwards.
If a vent's horizontal is more then 1/3 the distance of the vertical, then you will need to go up one size.
9. Am I correct that all drain pipes are a min. 1.5” except for a shower (2”) and a toilet (3”)?
No. Most sinks, including kitchen and laundry need 2". A lav can use 1-1/4".
Tubs require 2" as do showers.
If you have four toilets, then where the 4th toilet enters, you need to bump it to 4".
Outside the foundation, the sewer will be run in 4" regardless.
Your code book will list all of the minimum sizes for each fixture.
10. What’s the minimum size joist one can use below a shower if the p-trap is as tight to the drain as it can be? I.e. will a 2” shower p-trap fit within the 7” of a 2x8 joist bay below it (old house joists)?
It depends on how high the pipe is located. If you have to drill through the joist, then a 2x8 won't fit it. If you are running with the joist, then it should fit. The pipe would need to be run high.
12. Is there a min and maximum distance from a Tub or shower drain to the p-trap below it? I have 2x12 floor I-joists and then some space below those where I would like to locate the p-trap before the drains run horizontal.
We install the p-traps directly below the waste and overflow drains.
15. The existing house had a kitchen, 1 full bath and laundry – all draining down to a 3” PVC pipe running to the septic. Now we are adding 2 full bathrooms. Will the 3” PVC pipe be adequate?
3" will handle up to three toilets. When a fourth bathroom is installed, that's when you would need 4".