2.2 Structural Safety: Boring, Notching & Cutting of Structural Members

2.2 Structural Safety: Boring, Notching & Cutting of Structural Members

Exam Focus: Violations of structural safety codes represent one of the most dangerous and heavily tested subjects on the Michigan Journeyman Plumber exam. Plumbers must memorize the Rule of Thirds for joists, exact dimensional formulas (D/6, D/4, D/3, 2-inch edge clearance), wall stud percentages (40% vs. 60% bored, 25% vs. 40% notched), and the uncompromising rules regarding engineered wood I-joists and trusses.

---

1. Code Authority & Structural Mechanics (MPC 307)

Plumbing systems must be installed without weakening the structural safety of the building. Michigan Plumbing Code Section 307 incorporates by reference the structural requirements of the Michigan Residential Code (MRC R502.8, R602.6) and Michigan Building Code (MBC 2308).

The Physics of Structural Members

When a floor joist or beam supports gravity loads:

• Upper Fibers: Subjected to horizontal compression (pushing inward).
• Lower Fibers: Subjected to horizontal tension (pulling outward).
• Neutral Axis: The geometric horizontal center line of the member where internal longitudinal bending stress equals zero.
• Span Stress Distribution: The maximum bending moment occurs at the exact center of the span (the middle one-third), while maximum vertical shear stress occurs at the bearing ends.

  LOAD-BEARING FLOOR JOIST STRESS ZONES
  +-------------------------------------------------------------+
  | Compressive Stress Zone  (Top Fibers)                       |
  | - - - - - - - - - NEUTRAL AXIS - - - - - - - - - - - - - -  |
  | Tensile Stress Zone      (Bottom Fibers)                    |
  +-------------------------------------------------------------+
  |<- Outer 1/3 Span ->|<- Middle 1/3 Span ->|<- Outer 1/3 Span ->|
  (Max Shear at ends)   (Max Bending Moment)  (Max Shear at ends)
                        *NO NOTCHING ALLOWED*

---

2. Solid-Sawn Floor Joists: Notching Rules (MRC R502.8.1)

To preserve the load-carrying capacity of solid-sawn lumber joists, cutting and notching must comply with strict mathematical limits:

1. Middle One-Third Prohibition: Notches in the top or bottom of joists are strictly prohibited in the middle one-third of the joist span. Because tensile forces on the bottom edge reach maximum intensity at mid-span, any notch cut in this zone creates a stress riser that triggers catastrophic joist cracking.
2. Outer One-Third Notches: Notches located in the outer one-third of the span (measured from the face of each bearing support) are permitted, but:
   • Maximum Depth: Cannot exceed one-sixth of the joist depth (D/6).
   • Maximum Length: Cannot exceed one-third of the joist depth (D/3).
3. End Notches (Over Bearings): Notches cut at the extreme ends of the joist directly over the bearing plate or beam foundation are permitted to facilitate framing alignment:
   • Maximum Depth: Cannot exceed one-fourth of the joist depth (D/4).
   • Location: Must not extend into the clear span beyond the inner face of the bearing support.

  JOIST NOTCHING & BORING RULES
  +---------+
  |  D/4    |                          Outer 1/3: Max D/6 Depth
  |  Max    |                          Length <= D/3
  +--+      +------------------+     +--------------------------+
  |  |                         |     |                          |
  |  |                         +-----+                          |
  |  |               (O) Hole <= D/3                            |
  |  |               Min. 2" from edges                         |
  +--+----------------------------------------------------------+
  |<- End ->|<-------- Outer 1/3 -------->|<---- Middle 1/3 ---->
  Notch D/4                                *NO NOTCHING PERMITTED*

---

3. Solid-Sawn Floor Joists: Boring Rules (MRC R502.8.1)

Boring holes for water supply pipes, hydronic loops, or drainage lines is generally preferable to notching because holes can be centered along the neutral axis where bending stresses are minimal.

Dimensional & Location Mandates for Bored Holes

• Maximum Hole Diameter: The diameter of any bored hole cannot exceed one-third of the joist depth (D/3).
• Minimum Edge Distance: The edge of the hole must be at least 2 inches (51 mm) from both the top edge and the bottom edge of the joist.
• Spacing Between Holes & Notches: A minimum of 2 inches (51 mm) of solid, uncut wood must separate any hole from another hole or notch.
• Holes in Middle Third: Unlike notches, properly sized holes maintaining the mandatory 2-inch edge clearance are permitted in the middle one-third of the span, though best trade practice centers them on the neutral axis.

Quick-Reference Calculation Guide for Sawn Joists

Note: Nominal lumber dimensions differ from actual dressed dimensions (e.g., 2x8 is 7.25", 2x10 is 9.25", 2x12 is 11.25"). Code limits must be calculated using actual dressed lumber dimensions.

Crucial Exam Takeaway: A 3-inch DWV pipe has an outside diameter of 3.50 inches. Therefore, a 3-inch DWV line cannot be bored through any 2x6, 2x8, or 2x10 joist under standard code rules (as 3.50" exceeds the 3.08" maximum hole size for a 2x10). It may only pass through a 2x12 (maximum 3.75" hole) provided the hole is bored dead-center to maintain the required 2-inch top and bottom edge clearances.

---

4. Wall Stud Boring and Notching Limits (MRC R602.6)

Plumbing stacks and horizontal supply branches frequently penetrate load-bearing and non-load-bearing wall assemblies. Plumbers must strictly distinguish between wall types.

  STUD BORING & NOTCHING LIMITS

  LOAD-BEARING STUD                 NON-BEARING PARTITION STUD
  +-------------+                   +-------------+
  | Notch <=25% |                   | Notch <=40% |
  | +--+        |                   | +----+      |
  | |  |        |                   | |    |      |
  | +--+        |                   | +----+      |
  |             |                   |             |
  |    ( O )    | Hole <= 40%       |   (   O   ) | Hole <= 60%
  |   (60% if   |                   |             |
  |   doubled)  |                   |             |
  |             |                   |             |
  | Min. 5/8"   | Edge Dist.        | Min. 5/8"   | Edge Dist.
  +-------------+                   +-------------+

Load-Bearing Stud Walls

• Bored Holes: Maximum diameter is 40% of the stud width.
  • Nominal 2x4 (3.5" depth): 40% = 1.40 inches.
  • Nominal 2x6 (5.5" depth): 40% = 2.20 inches.
  • Exception for Doubled Studs: A hole up to 60% of the stud width is permitted in a load-bearing wall stud if the stud is doubled (two studs nailed together side-by-side), provided that not more than two consecutive doubled studs are bored in this manner.
• Notches: Maximum notch depth is 25% of the stud width.
  • Nominal 2x4 (3.5" depth): 25% = 0.875 inch (7/8").
  • Nominal 2x6 (5.5" depth): 25% = 1.375 inches (1-3/8").

Non-Load-Bearing (Partition) Stud Walls

• Bored Holes: Maximum diameter is 60% of the stud width.
  • Nominal 2x4 (3.5" depth): 60% = 2.10 inches.
  • Nominal 2x6 (5.5" depth): 60% = 3.30 inches.
• Notches: Maximum notch depth is 40% of the stud width.
  • Nominal 2x4 (3.5" depth): 40% = 1.40 inches.
  • Nominal 2x6 (5.5" depth): 40% = 2.20 inches.

Edge Clearances for Studs

Under all circumstances, the edge of any hole bored into a stud must remain at least 5/8 inch (15.9 mm) from the edge of the stud. If the hole is located closer than 1-1/4 inches to the face, steel shield plates (No. 16 gage) must be installed.

---

5. Stud Shoes & Framing Reinforcement

When a large pipe penetration (such as a 1-1/2" or 2" horizontal waste branch or vent) exceeds allowable stud boring or notching limits, the structural strength of the stud must be restored using engineered stud shoes.

• Approved Stud Shoes: Heavy-gage stamped steel brackets (e.g., Simpson Strong-Tie SS or HSS series) wrapped around the stud perimeter and secured with structural screws or 10d/16d common nails.
• Application: Stud shoes are typically code-listed to reinforce studs notched up to 50% in bearing walls or bored up to 2-3/8 inches in 2x4 framing.
• Plumbing Chase Walls: In standard modern practice, running 3-inch or 4-inch vertical soil and waste stacks through a standard 2x4 wall is prohibited because the pipe diameter exceeds the total stud depth. Plumbers must coordinate with general contractors to frame a plumbing chase wall (2x6 framing or a double 2x4 wall with a center air gap) to house large plumbing risers without compromising structural safety.

---

6. Engineered Wood Products, I-Joists & Roof Trusses

Modern residential and light commercial construction relies heavily on engineered structural components. The rules for solid-sawn lumber DO NOT APPLY to engineered wood.

Wood I-Joists (TJIs)

1. Flanges (Chords): The top and bottom horizontal flanges are composed of laminated veneer lumber (LVL) or solid structural timber. These flanges carry 100% of the tensile and compressive bending forces. Flanges must NEVER be cut, notched, drilled, or beveled under any circumstances. Severing a flange causes immediate structural failure.
2. Webbing: The vertical panel connecting the flanges consists of oriented strand board (OSB). Web penetrations are permitted only in strict compliance with the manufacturer's web hole chart.
   • Round knockouts provided by the manufacturer may be tapped out with a mallet.
   • Holes must be centered in the web height and must maintain minimum distances from bearing supports based on joist depth and span load (typically, larger holes must be located closer to mid-span where shear forces are lowest).

Roof & Floor Trusses

Under MPC Section 307.4 (Alterations to Trusses), roof trusses, open-web floor trusses, and glue-laminated beams are custom-engineered assemblies. No truss chord, web diagonal, or metal connector plate may be cut, notched, drilled, spliced, or altered unless approved by a written engineering specification from a Michigan registered design professional (Professional Engineer or Registered Architect).