If you’re renovating your bathroom, installing a new sink, or rerouting pipes through your floor, you’ve probably asked yourself: “How many holes can I safely cut in one joist for plumbing?” You’re not alone. Many DIYers and even some contractors struggle with balancing plumbing needs and structural safety. Cutting too many holes—or cutting them in the wrong places—can seriously compromise your home’s floor integrity. In this guide, we’ll break down exactly how many holes are allowed in one joist for plumbing, what building codes say, and how to do it right the first time.
Why Joist Integrity Matters for Plumbing Installations
Floor joists aren’t just wooden beams under your floor—they’re critical load-bearing components. When you drill holes for pipes, you reduce their ability to carry weight. According to the International Residential Code (IRC), there are strict rules about where and how large these holes can be.
The American Wood Council (AWC) emphasizes that joists should never be weakened beyond 1/3 of their depth in the center third of the span. Violating these guidelines can lead to sagging floors, cracks in drywall, or even structural failure over time.
⚠️ Real-world example: A 2022 study by the National Association of Home Builders (NAHB) found that 23% of structural floor complaints in remodeled homes were traced back to improper joist notching or excessive drilling.
How Many Holes Are Allowed in One Joist for Plumbing?
The short answer: It depends—but usually no more than 1–3 holes per joist, and only if they meet specific size and spacing rules.
Let’s break it down:
✅ Per IRC Section R502.8 (2021 & 2024 editions):
Hole diameter must not exceed ⅓ of the joist depth.
Example: For a standard 2×10 joist (actual depth = 9.25″), max hole = ~3″.
Holes must be at least 2 inches from the top or bottom of the joist.
Holes must be at least 2 inches apart from each other (edge-to-edge).
No holes allowed in the middle third of the joist span for notches—but holes are permitted in the center if properly sized and spaced.
So technically, you could drill multiple holes in one joist—say, one for a 2″ drain pipe and another for a 1″ supply line—as long as:
Both holes are ≤ ⅓ joist depth
Spaced ≥2″ apart
Not too close to ends (minimum 2″ from top/bottom)
Located outside high-stress zones (near supports)
💡 Pro Tip: Always use the smallest hole necessary. A 1.5″ hole for a 1.25″ pipe is safer than a 3″ hole “just in case.”
Where to Cut Holes in a Joist: The Safe Zones
Not all parts of a joist are equal. Stress is highest near the ends (supports) and lowest in the center—but the center third is actually the only place where holes are fully permitted.
Joist Section
Hole Allowed?
Notching Allowed?
End 1/3 (near supports)
❌ No
❌ No (or very limited)
Center 1/3
✅ Yes (with size limits)
❌ No
Top/Bottom 2″
❌ No
❌ No
Visual guide: Imagine your joist as a ruler from 0″ to 144″ (12 ft span).
Safe drilling zone: Between 48″ and 96″ (the center 48″).
Avoid: Within 2″ of top/bottom edges, and within 24″ of either end.
Step-by-Step: How to Safely Cut a Joist for Plumbing
Follow these steps to stay code-compliant and safe:
Identify the joist type and size
Measure actual depth (e.g., 2×8 = 7.25″, 2×10 = 9.25″).
Confirm it’s solid-sawn lumber (not engineered I-joists—those have stricter rules).
Locate the center third of the span
Use a tape measure from support to support.
Mark the middle ⅓ section.
Plan pipe routing
Use smallest pipe diameter needed (e.g., 1.5″ for bathroom sink drain).
Avoid running multiple large pipes through the same joist if possible.
Drill with precision
Use a hole saw or spade bit.
Keep holes ≥2″ from top/bottom edges.
Ensure ≥2″ spacing between holes.
Reinforce if needed
For multiple holes or large diameters, add sister joists or steel plates.
Consult a structural engineer if modifying load-bearing floors.
🛠️ Case Study: A contractor in Portland installed a laundry room by drilling two 2.5″ holes in a 2×10 joist (depth = 9.25″). Holes were 3″ apart, centered in the span, and 3″ from top/bottom. The floor passed inspection and showed zero deflection after 18 months.
Common Mistakes to Avoid
Drilling near the ends: High stress = risk of cracking.
Oversizing holes: A “just-in-case” 4″ hole in a 2×10 removes nearly half its strength.
Ignoring spacing: Holes too close act like a single large hole.
Cutting I-joists without approval: Engineered joists often require manufacturer specs—never assume.
FAQ Section
Q1: Can I drill three holes in one floor joist for plumbing? A: Yes—if each hole is ≤⅓ joist depth, spaced ≥2″ apart, and located in the center third of the span. For a 2×10, that means max ~3″ holes, properly placed.
Q2: What’s the maximum hole size allowed in a 2×8 joist? A: A 2×8 has an actual depth of 7.25″, so the max hole diameter is 2.4″ (⅓ of 7.25″). Round down to 2″ for safety and pipe fit.
Q3: Can I notch a joist instead of drilling? A: Notching is far more dangerous. The IRC allows notches only at the ends (max ¼ depth) and never in the center third. Drilling is preferred for plumbing.
Q4: Do I need a permit to drill holes in joists? A: Minor plumbing retrofits often don’t require permits, but structural modifications might. Check with your local building department—when in doubt, get approval.
Q5: What if I already drilled too many holes? A: Don’t panic. You can sister the joist (attach a new joist alongside) or install metal joist hangers/braces. A structural engineer can assess and recommend fixes.
Q6: Are rules different for I-joists or TJI joists? A: Yes! Engineered joists have pre-punched knockouts or require manufacturer-specific guidelines. Never drill I-joists without consulting the spec sheet.
Conclusion
Knowing how many holes you can cut in one joist for plumbing isn’t just about convenience—it’s about safety, code compliance, and protecting your home’s value. Stick to the ⅓ depth rule, keep holes in the center third, and never overcrowd a single joist. When done right, your plumbing will flow smoothly—and your floors will stay strong for decades.
If this guide saved you from a costly mistake, share it with a fellow DIYer or contractor on Facebook, Pinterest, or Reddit! And remember: when structural integrity is in question, always consult a licensed professional.
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