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Cutting Into Joists

Yes, you can reroute water supply lines through the floor

Richard Trehewey holds a toilet

I would like to reroute some water-supply lines and a drain line from the kitchen sink through some floor joists. Is it okay to drill holes in joists, and if so, how big can the holes be?
— Anthony Butcher, Troy, MI


Richard Trethewey replies: Now you know the kinds of obstacles plumbers face every day. Most wood framing is just not designed to make it easy to install pipes — or ducts for that matter.
Fortunately, it is possible to drill holes or cut notches in joists made out of sawn lumber without compromising their strength. You just have to make sure that the holes aren't too big or too close to the edge and the notches aren't too deep or too wide. The illustration shows you what the building codes allow.
Supply lines are fairly small, so you shouldn't have any problem working within these restrictions. A sink's drain line isn't that big either, but remember it has to be pitched at least an inch in every 4 feet, so the holes in the joists won't line up horizontally. And in a long run, the drain could end up being too low on the joist to create a penetration. At that point, it will have to be rerouted.
For engineered joists, such as laminated veneer lumber or I-joists, follow the manufacturer's hole-drilling specifications exactly. And never, ever notch the top or bottom of an I-joist, which looks a bit like an I-beam that's made entirely of wood.


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