Basement drain run
What's wrong here?
- The joists are notched for the drain in the middle third of their span, and the notches are deeper than 1/6 of the joist depth
- Holes for the copper water line may not be bored through floor joists
- Joists may be notched up to 1/4 of their depth anywhere, so a notch this size is fine
- Nothing. This installation is compliant
Show the answer and the code reference
The answer
Every joist has been notched on its bottom edge at midspan to seat the drain, and the notches are deeper than 1/6 of the joist depth; R502.8.1 does not allow notches in the middle third of the span at all.
Correct choice: 1. The joists are notched for the drain in the middle third of their span, and the notches are deeper than 1/6 of the joist depth
A simply supported floor joist carries its highest bending stress at midspan, and the bottom edge there is in tension, so a notch in that zone takes away the wood doing the most work and starts splits at the notch corners. R502.8.1 limits notches in sawn joists to 1/6 of the joist depth (about 1-1/2 in. on a 2x10), no longer than 1/3 of the depth, and keeps them out of the middle third of the span entirely; only end notches may go to 1/4 of the depth. These notches are roughly 3-1/2 in. deep in a 9-1/4 in. joist and sit right in the middle of the span, so they fail on location and on depth. The copper line in bored holes near mid-depth is the right way to cross joists.
How to fix it
Reroute the drain below the joists on hangers (or through properly sized and placed bored holes where the joist depth allows) and have the damaged joists repaired, typically by sistering full-length joists or by a repair designed by a registered design professional. Do not just fill the notches.
Code references
IRC 2018 R502.8.1 — Sawn lumber
Notches in solid lumber joists, rafters and beams may not exceed 1/6 of the member depth, may not be longer than 1/3 of the depth, and may not be located in the middle third of the span; notches at the ends may not exceed 1/4 of the depth. Bored holes are limited to 1/3 of the depth and must be at least 2 in. from the top and bottom edges, other holes and notches.
Drills citing R502.8.1IRC 2018 R502.8 — Cutting, drilling and notching
Structural floor members may not be cut, bored or notched beyond the limits in this section.
Drills citing R502.8
2024 IRC:same rule and number. Published 2024 IRC text found in search results keeps the same notch limits (1/6 depth, 1/3 depth long, not in the middle third, 1/4 depth at the ends) and hole limits under the same section number. Not checked line by line against the printed 2024 IRC. Confidence: medium
Minnesota
Minnesota adopts the 2018 IRC with Minnesota amendments as the Minnesota Residential Code (Minn. Rules ch. 1309). No Minnesota amendment to R502.8.1 is known. DLI is adopting the 2024 IRC as the next edition; see the 2024 IRC note.
About the Minnesota Residential CodeWhy the other choices are wrong
- Holes for the copper water line may not be bored through floor joists. Bored holes are allowed in sawn joists if they are no larger than 1/3 of the joist depth and at least 2 in. from the top and bottom edges and from other holes (R502.8.1); a 1 in. hole near mid-depth easily qualifies.
- Joists may be notched up to 1/4 of their depth anywhere, so a notch this size is fine. The 1/4-depth allowance is only for notches at the ends of a joist. Notches elsewhere are limited to 1/6 of the depth and are not allowed at all in the middle third of the span (R502.8.1).
- Nothing. This installation is compliant. Notching the bottom (tension) edge of a joist at midspan, where bending stress is highest, is exactly what R502.8.1 prohibits.