Island venting...

Originally Posted By: bnelson
This post was automatically imported from our archived forum.

Here’s the scenario: Island kitchen sink, three basins, (insikerator in the middle), with a high-loop vent that tops out BELOW the drain level. In the text I have, it reads that these vent configurations should top out “as high as possible” but doesn’t list how low is too low. Is the one I’m describing effective or is it too low to prevent cross connection? I’ll try to attach a jpeg, but haven’t figured that out yet. Thanks!

Originally Posted By: dhoffman
This post was automatically imported from our archived forum.

909.0 Special Venting for island Fixtures

Traps for island sinks and similar equipment shall be roughed in above the floor and may be vented by extending the vent as high as possible, but not less than the drainboard height and then returning it downward and connecting it to the horizontal sink drain immediately downstream from the vertical fixture drain. The return vent shall be connected to the horizontal drain through a wye-branch fitting and shall, in addition, be provided with a foot vent taken off the vertical fixture vent by means of a wye-branch immediately below the floor and extending to the nearest partition and then through the roof to the open air or may be connected to other vents at a point not less than six (6) inches (152 mm) above the flood level rim of the fixtures served. Drainage fittings shall be used on all parts of the vent below the floor level and a minimum slope of one-quarter (1/4) inch per foot (20.9 mm/m) back to the drain shall be maintained. The return bend used under the drainboard shall be a one (1) piece fitting or an assembly of a forty-five (45) degree (0.79 rad), a ninety (90) degree (1.6 rad) and a forty-five (45) degree (0.79 rad) elbow in the order named. Pipe sizing shall be as elsewhere required in this Code. The island sink drain, upstream of the returned vent, shall serve no other fixtures. An accessible cleanout shall be installed in the vertical portion of the foot vent.

910.0 Combination Waste and Vent Systems

910.1 Combination waste and vent systems shall be permitted only where structural conditions preclude the installation of conventional systems as otherwise prescribed by this Code.

910.2 Plans and specifications for each combination waste and vent system shall first be approved by the Administrative Authority before any portion of any such system is installed.

910.3 Each combination waste and vent system, as defined in Chapter 2, shall be provided with a vent or vents adequate to assure free circulation of air. Any branch more than fifteen (15) feet (4572 mm) in length shall be separately vented in an approved manner. The minimum area of any vent installed in a combination waste and vent system shall be at least one-half (1/2) the inside cross-sectional area of the drain pipe served. The vent connection shall be downstream of the uppermost fixture.

910.4 Each waste pipe and each trap in any such system shall be at least two (2) pipe sizes larger than the sizes required by Chapter 7 of this Code, and at least two (2) pipe sizes larger than any fixture tailpiece or connection.

910.5 Unless specifically required or permitted by the Administrative Authority, no vertical waste pipe shall be used in any such system, except the tailpiece or connection between the outlet of a plumbing fixture and the trap therefore. Such tailpieces or connections shall be as short as possible, and in no case shall exceed two (2) feet (610 mm).

Exception: Branch lines may have forty-five (45) degree (0.79 rad) vertical offsets.

IRC 2000
P3112.2 Vent connection. The island fixture vent shall
connect to the fixture drain as required for an individual or common vent. The vent shall rise vertically to above the drainage outlet of the fixture being vented before offsetting horizontally or vertically downward. The vent or branch vent for multiple island fixture vents shall extend to a minimum of 6 inches (152 mm) above the highest island fixture being vented before connecting to the outside vent terminal.

P3112.3 Vent installation below the fixture flood level rim.
The vent located below the flood level rim of the fixture being vented shall be installed as required for drainage piping in accordance with Chapter 30, except for sizing. The vent shall be sized in accordance with Section P3113.1. The lowest point of the island fixture vent shall connect full size to the drainage system. The connection shall be to a vertical drain pipe or to the top half of a horizontal drain pipe. Cleanouts shall be provided in the island fixture vent to permit rodding of all vent piping located below the flood level rim of the fixtures. Rodding in both directions shall be permitted through a cleanout.


Dennis D. Hoffman