SWRainAlertPermit thresholds
The 0.25″ Federal Rain Trigger Is Probably Not Your Number
Most rainfall triggers quoted in this industry are quoting somebody else's permit. Here is how to find yours, and why the number changes when the permit is reissued.
Ask five people on a construction site what rainfall triggers a stormwater inspection and you will get 0.25″, 0.5″, and “an inch, I think.” All three answers are correct somewhere in the United States. Only one of them is correct at your site.
Where the 0.25″ figure comes from
The EPA’s Construction General Permit (CGP) covers construction stormwater discharges in the handful of states, territories and tribal lands where EPA remains the permitting authority. Its inspection trigger is a storm event of 0.25 inches or greater. That is the number that gets quoted in national training material, and it is the number most software defaults to.
But the Clean Water Act delegates NPDES permitting to states that qualify for it, and the overwhelming majority have. Those states write their own construction general permits. They are not obliged to copy EPA’s trigger, and most do not.
What the states actually set
Here are the thresholds we ship as defaults, each checked against the primary permit text rather than a summary of it:
| State | Trigger | Window | Respond within | Permit |
|---|---|---|---|---|
| Georgia | 0.50″ | 24 h | 24 h | GAR100001 |
| Texas | 0.50″ | 24 h | 24 h | TXR150000 |
| Florida | 0.50″ | 24 h | 24 h | FDEP Generic Permit |
| North Carolina | 1.00″ | 24 h | 24 h, business hours | NCG010000 |
| Tennessee | 0.50″ (>50-acre sites) | 24 h | 24 h | TNR100000 |
| South Carolina | 0.50″ (rain log) | 24 h | Weekly report | SCR100000 |
| Federal baseline | 0.25″ | 24 h | 24 h | EPA CGP |
Rainfall inspection triggers by state, with the permit each comes from
The spread matters more than it looks. A contractor who moves a crew from Charlotte to Atlanta has just halved the amount of rain it takes to start an inspection clock — 1.00″ in North Carolina, 0.50″ in Georgia. Storms that were routine on the last job are now reportable events on this one.
The permit-era trap
Construction general permits are issued for a fixed term, usually five years, and then reissued — often with changes. Georgia’s current permit, GAR100001, took effect on 1 August 2023 and runs to 31 July 2028. Tennessee’s TNR100000 expires on 30 September 2026, which means anyone relying on its figures should be watching for the renewal now.
This is where most published threshold information goes wrong. A blog post written against a permit that has since been replaced does not announce that it is stale; it just keeps ranking. Worse, reading two permit eras as though they were concurrent is how a “dual trigger” that never existed ends up hard-coded into a compliance product.
Check the effective and expiry dates, not just the number. Any source that gives you a threshold without telling you which permit version it read, and when that version took effect, has given you a number you cannot defend in an audit.
Three things that override the state default
- Your individual permit or NOI conditions. Coverage under a general permit can carry site-specific conditions that are stricter than the general text.
- Sensitive or impaired waters. Many states impose tighter inspection frequencies, or additional monitoring, on sites discharging to designated sensitive waters.
- Local programs. Some jurisdictions run their own delegated stormwater programs on top of the state permit. North Carolina alone has dozens.
This is why every threshold in SWRainAlert is editable per site. A shipped default is a starting point, not an answer — and we would rather you override us than trust a number that does not match your paperwork.
How to find your actual number in ten minutes
- Find your permit’s identifier — it is on your Notice of Intent and usually looks like
GAR100001orTXR150000. - Pull the current permit text from the state agency’s site, not a third-party summary.
- Search it for “rain,” “storm event,” and “inspection.” The trigger, the measurement window and the response window are usually in the same paragraph.
- Note the effective and expiry dates. Put the expiry in a calendar.
- Check whether your site is in a sensitive-waters or local-program area that adds conditions.
Common questions
What rainfall amount triggers a SWPPP inspection?
It depends on the permit that covers your site. The EPA Construction General Permit uses 0.25 inches, but most states issue their own permit: Georgia, Texas, Florida and Tennessee use 0.5 inches, and North Carolina uses 1.0 inch. Your individual permit or a sensitive-waters designation can be stricter.
Is the rainfall trigger measured over a calendar day or 24 hours?
Permits generally specify a 24-hour period, which is a rolling window rather than a calendar day. A storm that starts at 22:00 and runs to 02:00 is one event measured across midnight, not two separate daily totals.
How do I know if my state's permit has been reissued?
Every construction general permit carries an effective date and an expiry date in its own text. Check both. If the expiry date has passed, or is close, the agency has either reissued the permit or administratively continued it, and the figures you are relying on may have changed.
A note on what this is. General guidance, not legal advice. Your permit, your SWPPP and your state’s requirements govern, and they differ. SWRainAlert is a monitoring and recordkeeping aid, not a compliance guarantee — where a permit names an on-site rain gauge as the system of record, the gauge is the record.