How-to Vent Your Island
One of the thorniest problems encountered with islands that incorporate sinks or cooktops is how to vent them. Each case is different. Plumbing fixtures like sinks have to be vented through the roof to keep harmful gasses from building up. Cooktops are vented to draw off moisture, grease and odor-laden air when cooking. Both typically make use of walls or upper cabinets to conceal piping. Since that's not an option with islands, this venting begins its journey headed south.

Running venting below the floor is easy in homes with a basement or crawl space—everything is run between the floor joists (it's particularly easy if the joists run "the right direction" ). But it gets trickier with a concrete slab: A trench must be jackhammered to run the new pipes.

Whichever floor you have, here are some of the details on both types of island venting:

Sink smarts. Although island plumbing eventually connects to a vent stack that penetrates the roof, most building codes require a high venting loop within the base cabinet. This drawing is based on requirements in the National Plumbing Codes Handbook.

•Use 1 1/2-in. PVC pipe in the vent loop as well as to the main drain line.

•Plan drawers and cabinet space around the plumbing, and make sure there's room to get at the pipes for repairs.

Cooktop concerns. There are two choices for venting an island cooktop: overhead vents, which are very expensive, and downdraft vents, which are featured in many cooktops designed for islands.

•Downdraft ventilators should be routed to the nearest exterior wall. Keep the duct run as short and straight as possible. And follow the manufacturer's restrictions on installation, or you might void the cooktop warranty.

•Ductwork for a downdraft system has longer runs and is forced to make more turns than the ductwork for a hood. Stay away from flexible ducting because its spiral ribs grab grease and can inhibit proper air movement on long runs. Use straight-wall galvanized-steel or stainless-steel ducting, except in concrete, where PVC is the best choice.

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