Manifold all vent pipes in attic to have one pipe into roof

Question

I'm building a new house, 2 1/2 baths, laundry room & kitchen. What size vent pipe would be needed to manifold all vents into one?

Answer 1

The required vent size is half the building drain size or 2" if your building drain is 4".

Then where you penetrate the roof you need 3" for the last couple feet, if you are in an area prone to frost.

You can certainly connect all the vents together into one vent penetrating the roof. That is a common practice. Every building requires a minimum of one roof penetration and you can have more if you wish to save pipe. Although every roof penetration is a point of possible failure in the future.

Happy Saturday!

Answer 2

Depends on where you live. In my area, AFAIK you can vent as many things as you want through a 4" pipe, and it must be 4" or greater in diameter. Even if your codes allow 3" stacks, I suggest using 4".

Answer 3

Do you have an itemized list of fixtures and the sizes of their pipes? I think it's going to be on a case by case basis but we should be able to position ourselves to make some determination on this specific question.

Here's some relevant materials to begin to dig into this problem. Ths is a hit for the Electronic Code of Federal Regulations (e-CFR) Title 24 - Housing and Urban Development deep down in

• Subtitle B - Regulations Relating to Housing and Urban Development
  • CHAPTER XX - OFFICE OF ASSISTANT SECRETARY FOR HOUSING - FEDERAL HOUSING COMMISSIONER, DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT
    • PART 3280 - MANUFACTURED HOME CONSTRUCTION AND SAFETY STANDARDS
      • Subpart G - Plumbing Systems
        • § 3280.611 Vents and venting.

24 CFR § 3280.611 - Vents and venting.

(c) Size of vent piping -

(1) Main vent. The drain piping for each toilet shall be vented by a 1 1/2 inch minimum diameter vent or rectangular vent of venting cross section equivalent to or greater than the venting cross section of a 1 1/2 inch diameter vent, connected to the toilet drain by one of the following methods:

(i) A 1 1/2 inch diameter (min.) individual vent pipe or equivalent directly connected to the toilet drain within the distance allowed in § 3280.611(c)(5), for 3-inch trap arms undiminished in size through the roof;

(ii) A 1 1/2-inch diameter (min.) continuous vent or equivalent, indirectly connected to the toilet drain piping within the distance allowed in paragraph (c)(5) of this section for 3 inch trap arms through a 2-inch wet vented drain that carries the waste of not more than one fixture. Sections of the wet vented drain that are 3 inches in diameter are permitted to carry the waste of an unlimited number of fixtures; or

(iii) Two or more vented drains when at least one is wet-vented, or 2-inch diameter (minimum), and each drain is separately connected to the toilet drain. At least one of the drains shall connect within the distance allowed in § 3280.611(c)(5) for 3-inch trap arms.

(2) Vent pipe areas. Each individually vented fixture with a 1 1/2 inch or smaller trap shall be provided with a vent pipe equivalent in area to a 1 1/4 inch nominal pipe size. The main vent, toilet vent and relief vent, and the continuous vent of wet-vented systems shall have an area equivalent to 1 1/2 inch nominal pipe size.

(3) Common vent. When two fixture traps located within the distance allowed from their vent have their trap arms connected separately at the same level into an approved double fitting, an individual vent pipe may serve as a common vent without any increase in size.

(4) Intersecting vents. Where two or more vent pipes are joined together, no increase in size shall be required; however, the largest vent pipe shall extend full size through the roof.

(5) Distance of fixture trap from vent shall not exceed the values given in the following table:

Maximum Distance of Fixtures From Vent Trap

Size of fixture drain (inches)	Distance trap to vent
1 1/4	4 ft. 6 in.
1 1/2	4 ft 6 in.
2	5 ft.
3	6 ft.

https://www.law.cornell.edu/cfr/text/24/3280.611#c

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There's a state level website that ties into similar themes, you may be able to look up your corresponding state here. Here's California details...

California Plumbing Code 2016

• Chapter 9 Vents
  • 904.0 Size of Vents

904.0 Size of Vents

904.1 Size

The size of vent piping shall be determined from its length and the total number of fixture units connected thereto, in accordance with Table 703.2. The diameter of an individual vent shall be not less than 1 1/4 inches (32 mm) nor less than one-half the diameter of the drain to which it is connected. In addition, the drainage piping of each building and each connection to a public sewer or a private sewage disposal system shall be vented by means of one or more vent pipes, the aggregate cross-sectional area of which shall be not less than that of the largest required building sewer, as determined from Table 703.2. Vent pipes from fixtures located upstream from pumps, ejectors, backwater valves, or other devices that obstruct the free flow of air and other gases between the building sewer and the outside atmosphere shall not be used for meeting the cross-sectional area venting requirements of this section.

Exception: Where connected to a common building sewer, the drainage piping of two or more buildings located on the same lot and under one ownership shall be permitted to be vented by means of piping sized in accordance with Table 703.2, provided the aggregate cross-sectional area of vents is not less than that of the largest required common building sewer.

904.2 Length

Not more than one-third of the total permitted length, in accordance with Table 703.2, of a minimum-sized vent shall be installed in a horizontal position.

Exception: Where a minimum-sized vent is increased one pipe size for its entire length, the maximum length limitation shall not apply.

https://up.codes/viewer/california/ca-plumbing-code-2016/chapter/9/vents#904.0

And here's a cross reference to that table (this website is pretty spectacular how it allows you to jump between cross references and glossary items, and footnotes and all that.)

703.0 Size of Drainage Piping

703.1 Minimum Size

The minimum sizes of vertical, horizontal, or both drainage piping shall be determined from the total of fixture units connected thereto, and additionally, in the case of vertical drainage pipes, in accordance with their length.

703.2 Maximum Number of Fixture Units

Table 703.2 shows the maximum number of fixture units allowed on a vertical or horizontal drainage pipe, building drain, or building sewer of a given size; the maximum number of fixture units allowed on a branch interval of a given size; and the maximum length (in feet and meters) of a vertical drainage pipe of a given size.

TABLE 703.2

MAXIUM UNIT LOADING AND MAXIMUM LENGTH OF DRAINAGE AND VENT PIPING

SIZE OF PIPE (inches)	1 1/4	1 1/2	2	3	4	5	6	8
Vent Piping Horizontal and Vertical Maximum Units	1	8	24	84	256	600	1380	3600
Vent Piping Horizontal and Vertical Maximum Lengths, (feet)	45	60	120	212	300	390	510	750

703.3 Sizing Per Appendix C

For alternate method of sizing drainage piping, see Appendix C.

https://up.codes/viewer/california/ca-plumbing-code-2016/chapter/7/sanitary-drainage#703.2