Seahawk Highlights Video (206) 949-5683, Top Rated Plumber, Seattle

1. ## Plumbing Vent Size

I know that by code "The drainage system of the building must be vented by one or more pipes whose combined cross sectional area is at least equal to that of the largest required building drain line.

So..... I've got a 4" building drain and two 3" vents through the roof which means I'm good to go. But does that mean I need to vent one of my fixtures like a shower or toilet with a 3" vent all the way to the roof? Or can I just vent it with 2" and use a pipe increaser at the roof to connect to the 3"?

2. A pipe increaser at the top makes no difference in the size of the vent. Alternatively, you could put a single 4" stack through the roof - like the older plumbing codes required.

Also, 4 2" vents would do the job, so you could have 2" on one bath, 2" on another bath, 2" on the kitchen, 2" on the laundry.

3. Each fixture in a house that goes into a drainage system is given a unit of measurement. Drainage Fixture Unit, in the code books it is shortened in the charts to dfu. You have to look at the chart and give each fixture the number from the chart. Then there is a certain size of drain and vent pipe required for the total number of dfu's that you figure. You are correct, the smallest a vent can be is the size of pipe that is required by the main drain. It depends on the number of fixtrues on one of the 3" vents you have, calculated by their total dfu's, whether or not you can add additional fixtures into the vent. I know this is a bad explaination.

In some areas it is required that a certain size pipe go through the roof, because of icing. It is one of the only times you ever up size a pipe upstream in a drainage system without more pipes going into it. If needed you could change the 3" vent in the attic to a 4" and tie the 2" into it. Calculate your dfu's and use the correct chart for your local code.

As far as I know the chart information from the IRC code and others is copyright info so you will have to buy a book or find a website that paid for the information to get it. I won't list it.

I know this explaination is as clear as mud. Proper drain and vent size requires a certain amount of understanding of what a dfu is.

4. Hey Herk, I should have explained my situation a little more.
In my haste to get my metal roof on and the house dried in (long story), I put two 3" vents through the roof BEFORE I ran any of my other plumbing pipes.
I knew I was going to run a 4" building drain so I figured I couldn't go wrong with two 3" vents. But now that I'm running the pipes and connecting to these 3" vents, I'm wondering if I need to make one fixture have a 3" vent and then connect it to the "existing" 3" vent through the roof? Or (as stupid as it sounds) run 2" and use a pipe increaser to connect it to the 3" at the roof line?

5. According to the IPC, if an inspector wanted to, he could say that the waste stack must remain the same size until it goes through the roof. If a 4" main stack is required then it could have to stay 4" until it exits the roof. You should ask, just to be sure, if it is OK to split the stack into two vents like that. 910.3 of 2006 IPC. The vent stack can be half the size of the waste stack. I get confused by code books even with the difinitions in front of me. Needs more pictures. Stack vent and vent stack two different things.

6. Here's what confuses me, when does a main drain turn into a waste stack, in a single family dwelling, can a main drain turn into more than one stack IPC? Does the waste stack have to go through the roof without being divided, at the same size or is combined pipes OK? Given the idea of this post I figured someone could clear up a tiny bit of my confusion. I'm still on the search for knowledge.

7. This is getting a little more confusing than I thought.
My new house is a one story "L" shape with 2 Baths on one leg of the "L" and a Kitchen + half bath on the other leg of the "L".

As I mentioned earlier I have a 3" vent penetrating the roof at each leg of the "L". I really wasn't planning on having any type of soil stack. What I wanted to do was run one long 4" drain (see attached) and vent each fixture with the appropriate sized vent. Then connect them all in the attic and finally attach them to the 3" pipe I already have going through the roof.

I've added up each of the Fixture Units and if I'm reading my tables correctly I think I would need to convert to a 3" vent pipe after I've picked up more than 8 fixture units since my vent pipe in the attic would run horizontally. Right???

8. Fixture units 1 lav,2 k/s,3 w/c,1.5 tub,2 shower,l/t 2,f/d 3.

9. Do you know what plumbing code your area uses?

10. This was from old Ohio code book.We use IPC now.Been a long time for me.

11. They go by the International plumbing code.

12. Originally Posted by mlawrence
I knew I was going to run a 4" building drain so I figured I couldn't go wrong with two 3" vents. But now that I'm running the pipes and connecting to these 3" vents, I'm wondering if I need to make one fixture have a 3" vent and then connect it to the "existing" 3" vent through the roof? Or (as stupid as it sounds) run 2" and use a pipe increaser to connect it to the 3" at the roof line?

My code (UPC) says that a vent pipe can only be run 1/3 of it's total permitted length in the horizontal. But, if the vent is up sized by one pipe size, it's entire length, that no longer applies.

So, if the minimum vent required was a 2". the maximun length would be 120' and only 40' of it could be in the horizontal. If more than 40' is needed horizontally, it would need to be upsized to 2.5" or 3" for it's entire length, horizontally and vertically.

13. Originally Posted by mlawrence
I know that by code "The drainage system of the building must be vented by one or more pipes whose combined cross sectional area is at least equal to that of the largest required building drain line.
Where did you get "one or more pipes whose combined" in the IPC?

14. I would reduce each vent before it goes through the roof with a 3x2 san tee and a 2" bushing. I would then tie in one bathroom to one of the 2" connections, Then combine the kitchen and laundry and tie it into the other 2" connection. I would do basically the same thing on the other side, run one 2" vent up from one bath and tie it into a 3x2 before it vents thru the roof. Then run another 2" vent from the other bath into the 3x2 san tee. According to the UPC you can put 24 fixture units on a 2" vent and run it horizontally a maximum of 40 ft. The 8fu limit is on 2" horizontal drainage piping, not vent piping. SO you only have to run 3" in the attick if your horizontal piping goes more than 40 ft.

15. In the IPC 910.3 it says a stack vent shall be provided for the waste stack. The size of the stack vent shall be not less than the size of the waste stack.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•