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Where to safely drill joists
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 2x12 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.