Venting a New Bathroom Over a Garage Through the Attic
A plumber is adding a bathroom over a garage in a two-story house in Traverse City. The lavatory has a 1-1/4-inch trap and a 5-foot trap arm; the shower has a 2-inch trap and a 9-foot trap arm. Because the nearest wall is far away, the installer plans to run a single 1-1/2-inch vent 38 feet horizontally through the attic and connect it into the existing 3-inch vent stack 2 feet above the new lavatory's flood-level rim. He intends to pitch this horizontal vent slightly downhill toward the stack 'so it will drain.' The existing vent terminates through the roof as a 1-1/2-inch pipe in an area of the state that sees heavy snow and deep frost. He asks whether the arrangement will pass inspection.
1. Check the two trap arms. Are they within the allowed length, and what would you do if one is not?
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The governing rule: a trap arm is limited so that the total fall on the arm does not exceed the diameter of the pipe, and the length limit follows from that at 1/4 inch per foot:
| Trap arm size | Maximum length (at 1/4 in/ft) | Maximum fall |
|---|---|---|
| 1-1/4 in | 5 ft | 1-1/4 in |
| 1-1/2 in | 6 ft | 1-1/2 in |
| 2 in | 8 ft | 2 in |
| 3 in | 12 ft | 3 in |
| 4 in | 16 ft | 4 in |
Lavatory: 1-1/4-inch arm, 5 feet - exactly at the limit, acceptable, but there is no room for error. Measure the developed length, not the straight-line distance, and correct it back to 1-1/2-inch if the run grew during framing.
Shower: 2-inch arm, 9 feet - not acceptable (8-foot maximum). Options:
- Increase the trap arm to 3 inch, which allows 12 feet - but that requires a 3-inch trap and a larger floor cut, and is usually impractical under a shower.
- Preferred: relocate the vent takeoff so the shower arm is 8 feet or less, or individually vent the shower and tie that vent into the 38-foot vent run.
Do not solve this by pitching the arm flatter than 1/4 inch per foot; the arm still needs enough slope to scour, and the limit is based on fall in the pipe, not slope alone.
2. Evaluate the 38-foot horizontal vent: its slope, and its point of connection to the existing 3-inch stack. What corrections are needed?
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Slope: reversed and must be fixed.
A horizontal vent must be graded back toward the drain it serves, at not less than 1/4 inch per foot, so that condensate and any water entering the vent drains to the drainage system rather than pooling in the attic or blocking the vent with ice. Pitching it 'downhill toward the stack' traps water in the low point of the run and can freeze shut in a Traverse City attic. Re-pitch it to fall back to the fixture drain.
Connection height: an individual vent must connect to a vent stack not less than 6 inches above the flood-level rim of the highest fixture served. Two feet above the new lavatory rim is acceptable for the new bathroom, but confirm what the existing 3-inch stack serves. If that stack picks up fixtures on the floor above the new bathroom, the connection must be above the flood-level rim of the highest of those fixtures, not just the lavatory - otherwise a stoppage in the stack can back up over the new vent connection.
Other conditions to verify:
- The 38-foot run counts in the developed length used to size the vent; if the developed length is long, the vent may need to be increased in size. Check the vent sizing table for the total developed length and the drainage fixture units served.
- Support the horizontal run and keep it out of the insulation envelope where it can freeze.
- 1-1/2 inch is generally adequate to vent a single bathroom group, so the size through the attic is not the problem - the slope and the point of connection are.
3. What must be done at the roof penetration in this climate, and what is the minimum height above the roof?
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Frost closure. Where frost closure is likely - Michigan qualifies - the vent must be increased at least one nominal size at the point it passes through the roof, and it need not be larger than 3 inches. A 1-1/2-inch vent becomes 2 inch through the roof. The reason is that moist air freezing at the terminal gradually closes a small pipe; the enlarged terminal resists icing over. Include an approved flashing and make sure the pipe is supported and the penetration is sealed.
Height above the roof: the vent terminal must extend not less than 6 inches above the roof surface. If the roof is used for anything other than weather protection (a deck, a place people walk), the terminal must extend at least 7 feet above the roof surface. Keep the terminal clear of windows, doors, air intakes, and roof overhangs so gas and odors are not drawn back into the building.
Inspection package for this job: corrected shower trap arm, vent re-graded back to the drain, connection elevation confirmed against the highest fixture on the existing stack, and a 2-inch roof terminal on a proper flashing.