Guide
Grease depth thresholds explained (secondary-sourced; verify NFPA)
Inspection cadence and cleaning trigger are different jobs. Secondary NFPA 96 explainers say Table 12.4 sets how often someone measures grease; Section 12.6.1.1 sets how much grease forces a clean. Operators often hear “one-eighth inch” as a universal rule — secondary sources say that number is the fan-housing threshold, not the whole system. Numbers below are secondary summaries. Buy or cite the edition your AHJ adopted; this is not the official text and not legal advice.
Short answer
Secondary 2024-edition explainers describe three measured limits checked with a grease depth-gauge comb: about 0.002 in (50 μm) on hoods / grease-removal devices / fans / ducts (often framed as the tight limit or clean-to target), about 0.078 in (2000 μm) on other surfaces as a cleaning-required depth, and about 0.125 in (3175 μm) inside fan housings. Crossing the applicable limit triggers cleaning under the secondary §12.6 framing. Your AHJ’s adopted edition and local amendments win — verify before you argue with a marshal.
Secondary threshold table
| Where (secondary framing) | Depth (secondary) | Rough feel | Cited section (secondary) |
|---|---|---|---|
| Hoods, grease-removal devices (filters), fans, ducts | 0.002 in / 50 μm | ~ copy-paper thickness | §12.6.1.1.1 |
| Other surfaces (non–fan-housing) | 0.078 in / 2000 μm | ~ US nickel thickness (secondary analogy) | §12.6.1.1.3 |
| Fan housings | 0.125 in / 3175 μm | 1/8 in (~ two dimes stacked, secondary analogy) | §12.6.1.1.4 |
Micron note: secondary sources use 3175 μm for 0.125 in — not the informal “3250” sometimes seen in shorthand. Always prefer the inch figure plus your edition’s text.
How the measurement works (secondary)
- Secondary §12.6.1.1.2: place a grease depth-gauge comb on the surface — opinion is not the method
- Inspection happens on the Table 12.4 cadence (solid monthly / high quarterly / moderate semiannual / low annual — secondary summaries)
- Cleaning is required when the applicable depth is observed — not automatically on every inspection in every kitchen
- Secondary §12.6.2 also frames a “clean before heavily contaminated” intent alongside the measured triggers
- Who measures: properly trained, qualified, and certified person acceptable to the AHJ (secondary §12.4 / §12.6.1)
Field note — framing variance to watch
- Some secondary explainers treat 50 μm / 0.002 in as the cleaning-required limit on hoods/filters/fans/ducts
- Others treat 50 μm as the clean-to / acceptable target after cleaning, and emphasize 2000 μm / 0.078 in as the “cleaning required” notch on surfaces, with 3175 μm / 0.125 in reserved for fan housings
- Both camps agree the popular “1/8 inch everywhere” shorthand is wrong for ducts and hoods
- Do not litigate millimeters from a blog — open the edition your AHJ named on the notice
Operator use that holds up
- Ask the inspector where they measured and which threshold they applied
- Require the written report to list inaccessible areas instead of silent skips
- Budget cleaning at every inspection for solid-fuel / high-volume (secondary operational reality)
- Keep the depth reading (or photo of the comb) with the visit file when provided
Common misunderstandings
- “We’re under 1/8 inch on the canopy — we’re fine” (wrong surface / wrong number)
- Eyeballing grease instead of a gauge
- Confusing inspection sticker cadence with automatic cleaning
- Treating a secondary blog table as the legal text
How this ties to scheduling
| Cooking tier (secondary Table 12.4) | Inspect | Practical cleaning plan (secondary) |
|---|---|---|
| Solid fuel | Monthly | Plan a clean nearly every cycle — creosote + grease load fast |
| High volume (24-hour, charbroil, wok) | Quarterly | Usually clean at the same visit — 0.002 in is easy to cross |
| Moderate volume | Semiannual | Inspect on schedule; clean when depths trip |
| Low volume (church, seasonal, senior) | Annual | May pass without a clean some years — still need the written report |
Who it’s for / not for
- For: managers who heard “1/8 inch” and want the three-number split; anyone reading a deficiency that cites depth; facilities leads training staff what “clean when measured” means
- Not for: substituting this page for the official NFPA text, DIY AHJ arguments, or inventing local amendments
Sources
- Facilitec Southwest — how often to clean (2026 NFPA 96 guide) (secondary; three-threshold table, paper/nickel/dime analogies, inspection vs cleaning split)
- Facilitec Southwest — NFPA 96 codes reference (secondary; §12.6.1.1.1 / .1.1.3 / .1.1.4 framing; “1/8 in is fan-housing-only” note)
- Cleaning Pros Plus — NFPA 96 certificate / grease levels (secondary; comb method; 50 / 2000 / 3175 μm ladder; clean-to vs cleaning-required framing)
- NFPA first-revision / committee PDF excerpt (public docinfo) (committee materials referencing 0.078 in / 2000 μm and 0.125 in / 3175 μm fan-housing language — not a substitute for your adopted standard)
- TFP1 — NFPA 96 cleaning frequency (secondary; inspection cadence overview used elsewhere on this site)
Next step
At the next inspection, ask the tech to show the comb reading and name which surface threshold they used. Put that line on the written report. If a deficiency cites “1/8 inch” on a hood or duct, ask which section of your adopted edition they mean — then verify against the official text, not this page.