Venting a Kitchen Island Sink

Properly Venting a Kitchen Island Sink is Tricky

By Mark J. Donovan

Question: I am planning a kitchen remodeling project where I am installing a kitchen island with a sink. I have been told by my building contractor that installing a kitchen island sink is tricky relative to the pluming. In particular, he says the venting of a kitchen island sink is difficult to do. I spoke with a plumber about this situation and he indicated that I need to tie the kitchen island sink drain pipe and vent into the main stack. To do this he wants to run 2″ PVC pipes through the floor joists.

A total of eight floor joists will have to be drilled. My kitchen sits over a basement with limited headroom, so he can not drop the pipes below the floor joists.

I also am planning kitchen cabinets with granite countertops for the kitchen island, and am concerned that if they drill the floor joists that the floor will be severely weakened. My question to you is it okay for the plumber to drill the floor joists?

Also, I already have ceramic floor tile installed. What tool would you recommend to drill through the sink base cabinet and the ceramic floor tile?

Answer: Venting a kitchen island sink is tricky. Normally a kitchen sink vent is tied into the main vent stack which is hidden in a wall. It is very important that a kitchen sink vent be installed such that sufficient air is supplied to the plumbing drain pipes to allow the waste water to flow properly through the plumbing drain system.

To do this correctly, the kitchen sink vent needs to rise approximately 6 inches above the level of the sink before the vent pipe turns horizontally. This is impossible to do with a kitchen island sink, so a more clever way to implement the kitchen island sink vent is required. I leave that discussion for another time.

Venting a Kitchen Island Sink

Regarding Drilling Floor Joists and Cutting Ceramic Floor Tile

Drilling holes through floor joists for plumbing pipes is common, however there are specific codes the plumber needs to follow in terms of the maximum hole size, relative to the width of the floor joist.

For example, I think the maximum hole size is 2″ in diameter for 8″ wide floor joists. You could ask the contractor to show you the actual code to confirm that he will not exceed the maximum hole size for the particular floor joists that you have. Also, these requirements are based on drilling the hole in the CENTER of the floor joist, not near the edges. You never want to have the floor joists notched or drilled at the top or bottom as this will severely weaken the joist.

You could also have the contractor scab in some additional lumber just above and below the hole if you still feel concerned. Scabbing is the process of nailing to the cut or drilled floor joist a couple of pieces of 2″ thick lumber, which can be up to a couple of feet in length, above and below the hole. The scabs act as reinforcement to the cut / drilled floor joist.

Kitchen Remodeling Bid Sheet
Regarding cutting your sink base and ceramic tile, you can use a screw gun or drill, with a carbide tip, 2″ in diameter, hole saw, such as the Milwaukee 49-56-2123 Hole Saw Carbide Tipped 2-1/8-Inch. If you need to cut out a larger section of ceramic tile, then I would recommend the use of a circular hand-held wet saw. A wet saw will help keep the dust down and make short work out of your tile cutting. You could also use a coping saw with a carbide blade; however it is slow and takes a lot of elbow grease.


For help on your kitchen remodeling project, see my Kitchen Remodeling Bid Sheet. The Kitchen Remodeling Bid Sheet provides a request for quote checklist section that you can provide to prospective kitchen remodeling contractors. It also includes a comprehensive kitchen remodeling cost breakdown table, in Microsoft Excel format, that allows the contractor to include his projected remodeling costs for every phase of the project.

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