What is the 33% rule in solar panels?
The 33% rule is a fire-code limit on rooftop solar. Under International Residential Code section R324.6.2, panels covering no more than 33% of a roof's plan-view area need an 18-inch clear setback on each side of the ridge, and anything above 33% needs a 36-inch setback. In homes with fire sprinklers, that limit rises to 66%.
Why the rule exists
The rule sits in the code's roof-access section. The setback keeps a clear strip along the ridge so firefighters can work on the roof without walking on live panels.
What it means for your system size
Here's a worked example. Take a roof with 2,000 sq ft of plan-view area (the roof's footprint seen from above). 33% of that is 660 sq ft. A typical 400-watt panel takes up about 21.1 sq ft, so:
- About 31 panels fit under the 33% line.
- That's roughly a 12.4 kW system, which covers the average home's use almost anywhere in the U.S. (see is 10 kW enough?).
Go above the 33% line and you can still install more panels. You just keep 36 inches clear at the ridge. On a small or cut-up roof, that can cost you a row of panels. It's one reason two homes of the same size can get very different quotes. Our 2026 solar price guide covers the other reasons.
Your town's rule may differ
Each jurisdiction adopts its own version of the code. Clark County, Nevada, for example, applies the 18-inch setback up to 66% coverage. Some installers also use "33% rule" for something else: sizing panels at up to 133% of the inverter's rating. That's a design guideline (and an Australian standard), not a U.S. code limit.
Facts verified
- IRC R324.6.2 thresholds (33%/18 in, over 33%/36 in): Charleston County, SC, Residential Solar Panel Information Form.
- 66% sprinkler alternative: Leon County, FL, Residential Roof Access Requirements.
- Local variation: Clark County, NV, policy BD-PP-206.
- 133% inverter-sizing usage: SolarQuotes (Australia), on Clean Energy Council oversizing guidelines.
- Panel size: 400 W at 20.4% efficiency, the representative residential module in the NLR Q1 2025 PV System Cost Benchmark. The panel count is our own calculation and ignores setbacks at roof edges, vents and valleys, so real layouts fit fewer panels.
More solar cost questions
- How much do solar panels cost for a 2,000 sq ft house?
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- Is 10kW enough to run a house?
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- How much do solar panels cost per month?