Vent plumbing

Most modern homes have adequate vent plumbing because of local codes. Proper venting is actually fairly new. Many older homes have always had plumbing problems due to poor vent pipes.
We know how to run vent lines that keep the right atmospheric pressure and still maintain green building codes. It’s more mandatory these days to get the venting correct. That’s because of more environmental laws in place and also because homes are different and more complex structures than they used to be.

vent plumbing

When installing the septic lines, it’s easier to start from the end and work your way forward.

The best place to start is where your septic mainline enters the house. From that point, the line is probably a 4-inch pipe that needs a Y with a clean out. That’s where I will start the process and proceed by the numbers.

1. From mainline, install Y with cleanout and plug, go up with 3-inch PVC pipe.

2. Install 3-inch reducing Y into main waste stack for 1st floor wastewater fixtures.

3. Put in 2-inch PVC with 2-inch Y with cleanout. Add long radius elbow.

4. Install a 2-inch T for sink wastewater and vent can be 1 ½ inch to vent line.

5. Install 2-inch sink trap usually with slip fittings to hook up to sink drain.

6. Put 3-inch reducing Y into main stack with 2-inch reducer for sink or washer.

7. Install waste T with side inlet for closet bend and sink wastewater.

8. Install 2-inch trap for shower/bath or sink.

9. Put in 3-inch closet bend for the toilet install.

10. Install 3-inch reducing T into main waste stack for lavatory trap access.

11. Put in 2-inch lavatory trap.

12. Continue vent-plumbing stack to the 2nd floor where you will install 3-inch waste T with reducer.

13. Install PVC wastewater pipe and bath/shower trap on 2nd floor.

14. Put in closet bend for 2nd floor toilet.

15. Put 3-inch reducing T into main stack. Reducer will be for sink, bath, or washer.

16. Put in trap for sink, bath, or washer.

17. Put final 3-inch reducing T into main waste stack. Reducer can be 1 ½ for vent.

18. Main waste stack continues up through the roof for proper vent-plumbing.

19. Install the 2-inch Y with cleanout back into the stack for sink wastewater.

20. Put 22-½ degree elbow for wastewater to flow smoothly into Y.

21. Install 2-inch waste T for access to sink and vent.

22. Install 2-inch sink trap.

23. Install vent T. Can be either 2-inch or 1 ½ inches for venting.

24. Put in vent elbow and run line back down to 2-inch T wastewater fitting.

You can assemble the parts anyway you want. It’s not difficult. There are a few rules about vent plumbing however. Many locations don’t allow, “wet venting”. This is where a vent pipe running horizontally carries water for a distance greater than normal. The drawing below has an example of wet venting so you can better understand what this means.


The bath/shower in number 8, and the lavatory in number 11 are mild cases of wet venting because the wastewater and the venting airflow are sharing the same pipe for about 5 or 6 feet. This could cause a problem with draining, so it should be remedied. The shaded pipes shows how to solve the problem. It’s a simple, but correct example of venting.

A few decades ago, venting wasn’t a major consideration and there was a lot of leniency in the area of wet venting. Trial and error has taken us down learning lane once again. If a vent is full of water or has restricted airflow, an imbalance in atmospheric pressure can suck the water out of traps and also prevent wastewater from draining. Take the time to make sure everything is properly vented.

The illustration below shows a typical bathroom plumbed with galvanized pipe. Copper is exactly the same layout. This shows how things look after rough in. It helps to see the plumbing in respect to the walls and floor joists so we know how it should look. I did fail to mention the use of air chambers in metal plumbing to prevent noise.

bathroom plumbing diagram

The air chambers are pipes that extend about ten inches above the lavatory water line outlets. They create an air buffer to absorb rapid water movement that sometimes causes pipes to vibrate and jump. They could be a good idea. I don’t really know much about galvanized pipe. Air chambers are a code requirement in a few areas only, but by and large, it is left up to the owner to decide.

There are only a few things left to cover about what to expect from vent plumbing. I didn’t mention much about the slope ratios for waste lines. A person unfamiliar with the aspects of plumbing might think that down is down no matter the angle, but in truth, if the slope is too much on the waste lines, the water will run down and the solids will stay behind. Bad formula!

The idea is for the angle to be just right so that the wastewater carries the solids away and out of our lives forever. That’s why the codes will state something like having a ¼ inch slope every 12 inches of run. The inspectors are very particular about things like that.

Also, if you live in a cold climate zone, you may want to get frost bib spigots. They have a longer internal attachment that allows the spigot bearing to be closer to the warmer part of the house and not the exterior foundation. That can reduce the likelihood of frozen pipes through the spigots. You should have at least one spigot on each side of the house.

As for now, that should give us a general idea of what we need to do.

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