How to Drill Through Floor JoistsUpdated: Jun. 30, 2017
How to position holes correctly so they don't weaken the joist
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Where to safely drill joists
Drill holes with a diameter of no more than one-third the depth of the joist, staying 2 in. away from the top and bottom.
Where and how to drill joists for electrical cables or plumbing runs depends on what type of floor framing you have. Keep the hole at least 2 in. from the top and the bottom if it’s a dimensional lumber joist. The hole cannot be larger than one-third the depth of the joist, so the maximum hole size for a 2×12 joist (actual size 1-1/2 x 11-1/4 in.) is 3-3/4 in. diameter—plenty big enough for running cable! You can drill the holes anywhere along the length of the joist (first photo).
If you have manufactured I-joists, you can drill holes up to 1-1/2 in. diameter almost anywhere in the web area (the area between the flanges). Just stay 6 in. away from any end or load-bearing wall (second photo). Holes up to 4 in. can be drilled in the middle of the I-joist, away from the ends and load-bearing wall. Keep the distance between adjacent holes at least twice the diameter of the largest hole. For holes larger than 4 in., consult the lumber supplier.