If you’re designing, installing, or inspecting plumbing systems in South Carolina, you’ve likely encountered the term “ICC Plumbing Code South Carolina 100 Year 1-Hour Rain Fall.” But what does it really mean—and why should you care? Heavy rainfall events are becoming more frequent, and outdated drainage designs can lead to flooding, property damage, and code violations. This guide breaks down exactly how this critical standard affects your projects, with actionable insights grounded in current codes and real-world data.
What Is the “100-Year, 1-Hour Rainfall” Standard?
The phrase “100-year, 1-hour rainfall” refers to a statistical rainfall event that has a 1% chance of occurring in any given year—not something that happens once every century. In practical terms, it’s an extreme but plausible storm used by engineers and code officials to size stormwater and plumbing drainage systems.
In South Carolina, this value is codified under the International Plumbing Code (IPC), adopted statewide with local amendments. The IPC references rainfall intensity data from sources like NOAA and ASCE to determine required pipe sizing, roof drain capacity, and sump pump specifications.
💡 Key Insight: A “100-year storm” doesn’t mean it’s rare—it means there’s a 1-in-100 annual probability. Over a 30-year mortgage, that risk jumps to nearly 26% (NOAA).
How Does South Carolina Apply the ICC Plumbing Code to Rainfall Design?
South Carolina adopts the International Plumbing Code (IPC) as part of its statewide building regulations, typically aligned with the most recent edition (as of 2026, many jurisdictions use the 2021 IPC). Section 1106.2 of the IPC mandates that storm drainage systems must be designed based on the 100-year, 1-hour rainfall event for the project’s specific location.
But here’s the catch: rainfall intensity varies across South Carolina. For example:
Ignoring the ICC Plumbing Code South Carolina 100 Year 1-Hour Rain Fall standard can have serious consequences:
Failed inspections during final walkthroughs
Flooding due to undersized roof drains or gutters
Voided insurance claims if systems don’t meet code during storm damage
Costly retrofits after construction is complete
Consider this real case: In 2021, a new residential development in North Charleston experienced basement flooding during a moderate storm. Investigation revealed the storm drain system was sized for a 10-year event, not the required 100-year standard. The builder faced over $220,000 in remediation costs.
Step-by-Step: Designing a Compliant Storm Drainage System
Follow these steps to ensure your plumbing design meets South Carolina’s ICC-based rainfall requirements:
Determine Your Location’s Rainfall Intensity Use NOAA Atlas 14 to find the exact inches per hour for your site’s 100-year, 1-hour storm.
Calculate Total Roof Catchment Area Measure the horizontal projection of all roof surfaces draining to a single point (e.g., 2,400 sq ft).
Use IPC Table 1106.2 to Size Roof Drains Example: For Charleston (6.5″ rainfall), a 2,400 sq ft roof requires at least two 4-inch roof drains or one 6-inch drain.
Design Horizontal Storm Sewers Pipes must maintain a minimum slope of 1/8 inch per foot and be sized using Manning’s equation or approved hydraulic tables.
Install Backflow Prevention If the storm system connects to a combined sewer or is below flood level, include a code-compliant backwater valve (IPC Section 1109).
Document & Submit for Permit Review Include rainfall data, calculations, and equipment specs in your plumbing permit application.
⚠️ Warning: Do not rely on older editions of the code or neighboring states’ standards—South Carolina’s coastal climate demands higher rainfall allowances than, say, Tennessee or Georgia.
Common Misconceptions About the 100-Year Storm Standard
Let’s clear up some confusion:
❌ Myth: “It’s just a theoretical number—I’ll never see that much rain.” ✅ Fact: Charleston recorded 7.2 inches of rain in 2 hours during Hurricane Matthew (2016)—exceeding its 100-year rate.
❌ Myth: “Residential buildings are exempt.” ✅ Fact: The IPC applies to all structures, including single-family homes (Section 101.2).
❌ Myth: “Gutters and downspouts don’t count as plumbing.” ✅ Fact: Under IPC Section 1101.2, roof drainage is part of the plumbing system and must comply.
Q1: Is the ICC Plumbing Code legally enforceable in South Carolina?
Yes. South Carolina adopts the IPC through the South Carolina Building Codes Council. Local jurisdictions (like Charleston County or Richland County) may add amendments, but the 100-year rainfall requirement is statewide and mandatory.
Q2: Where can I find the official rainfall data for my project site?
Use the NOAA Atlas 14 Precipitation Frequency Data Server (https://hdsc.nws.noaa.gov/). Select “South Carolina,” enter coordinates or address, and download the 1-hour, 100-year depth.
Q3: Do I need an engineer to design the storm system?
For commercial buildings, multi-family units, or complex roof geometries, yes—a licensed professional engineer (PE) must stamp the drainage plans. Single-family homes may be designed by licensed plumbers or contractors using prescriptive IPC tables.
Q4: What happens if my system fails during a heavy rain?
If the system was not designed to code, you could face liability for water damage, failed inspections, or even stop-work orders. Insurance may deny claims if non-compliance is proven.
Q5: Are there exceptions for green roofs or permeable paving?
Partial credits may apply under green building programs (like LEED), but the IPC still requires the primary drainage path to handle the full 100-year event. Secondary systems (e.g., overflow scuppers) are often needed.
Q6: How often is the rainfall data updated?
NOAA updates Atlas 14 periodically using newer climate records. South Carolina began transitioning to Atlas 14 Version 3 in 2023, which increased rainfall intensities by 5–12% in coastal zones due to observed climate trends.
Conclusion: Build Smarter, Not Just Faster
Understanding theICC Plumbing Code South Carolina 100 Year 1-Hour Rain Fall isn’t just about passing inspections—it’s about protecting people, property, and your professional reputation. With rising rainfall extremes across the Southeast, cutting corners on drainage design is a risk no contractor or homeowner can afford.
✅ Key Takeaways:
Always use location-specific rainfall data from NOAA.
Size every component—roof drains, pipes, pumps—to the 100-year standard.
When in doubt, consult a licensed plumber or engineer familiar with SC codes.
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