Plumber, HVAC, and a 2x10 Floor System
The floor system on a two-story house uses 2x10 (9-1/4 in.) joists at 16 in. o.c. spanning 14 ft between a center beam and the foundation wall. The plumber needs a 4 in. drain line through the joists near midspan, positioned about 1 in. below the top edge. He also wants a 3 in. deep notch cut in the top edge of one joist at the foundation bearing point to clear a vent. An HVAC contractor wants to cut a whole joist out to run a trunk duct and "box it later." Framing inspection is Wednesday and the drywall crew starts Monday.
1. Evaluate the 4 in. drain hole and give the options.
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Evaluate against the limits for sawn joists:
- Maximum hole diameter is one-third of the joist depth: 9-1/4 in. / 3 = 3-1/16 in. A 4 in. hole exceeds that.
- Holes must stay at least 2 in. clear of the top and bottom edges. A hole 1 in. below the top edge violates that too.
- Midspan is also the worst place for a hole - that is where bending stress is highest and shear is low, but the loss of section at the extreme fiber is what matters here.
So the requested hole fails on two counts and cannot be cut as drawn.
Options, in order of preference:
- Reroute the drain so it runs below the joists in a soffit or through a wall cavity - usually the cheapest and requires no engineering.
- Keep the drain in the joist bay but drop it below the bottom edge and box the soffit.
- As a last resort, use a designed detail: a double joist or a header with trimmers around the penetration, sized by a designer or engineer, with the hole kept within limits.
Any field-cut hole beyond the code limits needs a design before it is cut, and the inspector will look for it.
2. Evaluate the 3 in. top-edge notch at the bearing end.
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Evaluate: the notch limit for a sawn joist is one-quarter of the depth - 9-1/4 in. / 4 = 2-5/16 in. A 3 in. notch is over that, and it is cut in the top edge, which is the compression side on a simple span. Notches also are not permitted in the middle third of the span in any event, so location does not rescue this one.
Why it matters: a top notch at bearing removes the section that carries the compression and also interrupts the bearing area over the plate. It is a common source of cracked or deflecting joists and squeaky floors.
Correction:
- Move the vent so it does not require a notch; route it through the wall or in the stud bay above.
- If a penetration at bearing is unavoidable, cap or reinforce with a designed detail (a doubler or a header), not a field notch.
- Reject the notch on the spot and have the plumber re-route before drywall - after drywall, this becomes a demolition job.
3. The HVAC contractor wants to remove one joist entirely and box the duct later. What is the correct answer and process?
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Cutting a joist is sometimes acceptable, but only under a defined procedure - "box it later" is not a detail.
Acceptable approach:
- Generally no more than one joist may be cut in a floor system, and never two adjacent joists.
- Provide a double header across the opening, sized for the span and load it now carries, with trimmers (hangers) at each end and joist hangers at all connections.
- Support the cut joist's loads: the header takes the tributary load from the cut joist plus the floor above.
- Keep the subfloor supported at the opening - blocking and proper edge nailing.
- Get the detail from the designer or engineer of record, and show it to the inspector before Monday.
Reject: a cut joist with no header, two joists cut side by side, or a header sized by eye. The floor above a cut joist is where the tile cracks and the doors stop closing. Also, sequence matters: this work is inside the wall before drywall - after Monday, the same job costs ten times as much.