NOM-031-STPS-2011 classifies a site by built area or height: small with less than 350 m² or under 10.5 m; medium from 350 to 10,000 m² or 10.5 to 16.5 m; large with more than 10,000 m² or over 16.5 m. If the two criteria give different sizes, the larger one applies. Medium and large sites need a risk analysis and a safety and health system in place before work starts.
1How the site is classified
If area and height give different sizes, the larger one is used (secs. 7.1 and 7.2).
2What each size requires
| Obligation | Small | Medium | Large |
|---|---|---|---|
| List of activities, risks and measures | Yes | — | — |
| Signed risk analysis | — | Yes | Yes, by stage |
| Safety and health system in place before work starts | — | Yes | Yes |
| Machinery maintenance program and safety procedures | — | Yes | Yes |
Training: on large sites, programs by stage (secs. 20.1 and 20.2); on medium sites, information before hazardous work and safety talks (secs. 20.3 and 20.4). Emergency plan: every site needs one, according to its size (sec. 5.16); on large sites, with the drills set by the plan itself (sec. 19.3).
On all sites: basic and specific personal protective equipment, hard hats for visitors, extinguishers, signage, a safety and hygiene commission, authorization of hazardous work, facilities for staff (drinking water, toilets, eating area) and an accident log (ch. 5).
3The safety and health system
On medium and large sites, the system is put in place before starting (sec. 9.1) and includes:
- Safety and health policy and program for the site.
- Training, PPE, maintenance and supervision programs.
- First aid manual.
- Written authorizations for hazardous work: worker, type and location, date and time, measures and signature (sec. 9.4).
4Technical measures reviewed most often
| Topic | Requirement | Section |
|---|---|---|
| Excavations | Inspection at the start and end of the workday, and after heavy rain or heavy machinery traffic | 11.2 a) |
| Trenches | Stabilize walls deeper than 1.50 m; ladder from 1.20 m, no more than 7 m apart; material at least 2 m from the edge | 11.2 h), j), r) |
| Power lines | Minimum distance of 3.10 m (up to 50,000 V) to 8.60 m (up to 600,000 V) | 11.2 k) |
| Work at height | From 1.80 m above the reference level | 4.25 |
5What the inspector reviews
On the walk-through, the inspector checks PPE in use, extinguishers, machine guards, signage and staff facilities. In the documents, the site classification, the risk analysis, the safety and hygiene commission and contractor permits. The inspector also asks for vehicle logs, logs for work-at-height equipment and confined space entry logs (sec. 23.3).
Evidence must be kept for at least one year (sec. 23.5). A ruling from a verification unit is optional and valid for two years or the duration of the project (sec. 22.5).
- NOM-031-STPS-2011, Construction. Occupational safety and health conditions (DOF May-04-2011).
Information current as of October 15, 2026. For filings with the authorities, always confirm the current official text.