Trench collapses kill workers every year, and a cubic yard of soil can weigh as much as a car. US federal rules (29 CFR 1926 Subpart P) require a protective system in any excavation 5 ft deep or more, unless it is entirely in stable rock, and one of the simplest systems is sloping: cutting the sides back at an angle the soil can hold. This page gives OSHA’s maximum allowable slopes for each soil type, how wide that makes the top of a trench, and a calculator for top width and excavation volume.
Maximum allowable slopes (29 CFR 1926 Subpart P, Appendix B)
| Soil or rock | Maximum slope (H:V) | Angle from horizontal |
|---|---|---|
| Stable rock | Vertical | 90° |
| Type A | 3/4:1 | 53° |
| Type B | 1:1 | 45° |
| Type C | 1-1/2:1 | 34° |
Trench sloping calculator
Trench top width by depth and soil type
| Depth | Type A top width | Type B top width | Type C top width | Type C excavation (cu yd per ft) |
|---|---|---|---|---|
| 4 ft | 8.0 ft | 10.0 ft | 14.0 ft | 1.19 |
| 5 ft | 9.5 ft | 12.0 ft | 17.0 ft | 1.76 |
| 6 ft | 11.0 ft | 14.0 ft | 20.0 ft | 2.44 |
| 8 ft | 14.0 ft | 18.0 ft | 26.0 ft | 4.15 |
| 10 ft | 17.0 ft | 22.0 ft | 32.0 ft | 6.30 |
| 12 ft | 20.0 ft | 26.0 ft | 38.0 ft | 8.89 |
| 15 ft | 24.5 ft | 32.0 ft | 47.0 ft | 13.61 |
| 20 ft | 32.0 ft | 42.0 ft | 62.0 ft | 23.70 |
The numbers show why sloping deep trenches in poor soil is rarely practical: a 10 ft Type C trench with a 2 ft bottom is 32 ft wide at the top and removes more than 6 cubic yards of soil per foot of length. In those cases contractors use trench boxes (shields) or shoring, designed from the maker’s tabulated data or by an engineer.
Soil types in plain words
| Class | What it is (Appendix A) | Max slope |
|---|---|---|
| Stable rock | Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed | Vertical |
| Type A | Cohesive soil (clay, silty clay, sandy clay, clay loam; also caliche and hardpan) with unconfined compressive strength of 1.5 tons per sq ft or more, and not fissured, not subject to vibration, not previously disturbed, not layered over weaker soil and not with seeping water | 3/4:1 |
| Type B | Cohesive soil of 0.5 to 1.5 tsf; granular cohesionless soils such as angular gravel; previously disturbed soil; Type A soil that is fissured or subject to vibration; dry unstable rock | 1:1 |
| Type C | Cohesive soil of 0.5 tsf or less; granular soils such as gravel, sand and loamy sand; submerged soil or soil with freely seeping water; submerged rock that is not stable | 1-1/2:1 |
Other trench rules that apply at the same depths
- Ways out: a ladder, stairway or ramp in any trench 4 ft or deeper, placed so no one has to travel more than 25 ft sideways to reach it (1926.651(c)(2)).
- Spoil and equipment: at least 2 ft from the edge, or held back by a retaining device (1926.651(j)(2)).
- Air: test the atmosphere before entry in excavations deeper than 4 ft where low oxygen or hazardous gas could be present, such as near landfills or fuel tanks (1926.651(g)).
- Deeper than 20 ft: sloping or benching must be designed by a registered professional engineer (Appendix B limits stop at 20 ft).
- Inspections: a competent person inspects the excavation daily, before each shift and after rain or anything else that could change conditions (1926.651(k)).
- Utilities: locate underground lines before digging; in the US call 811 a few days ahead.
Homeowners and small jobs
The OSHA rules apply to employers, but soil does not care who is digging. A trench for a footing, drain or water line that is deeper than about your waist can trap you if a wall slumps. Keep people out of narrow vertical-sided trenches deeper than 5 ft, slope or step the sides back, keep the spoil pile well back from the edge, and never work alone in a trench. For the depth of footings and post holes, see the fence post depth guide and the footing calculator.
Frequently asked questions
At what depth does OSHA require trench protection?
5 ft. Excavations 5 ft deep or more need a protective system (sloping, benching, shoring or a shield) unless they are entirely in stable rock; shallower ones need one too if a competent person sees signs of a possible cave-in (29 CFR 1926.652(a)).
What is the OSHA slope for Type C soil?
1-1/2 horizontal to 1 vertical, about 34 degrees from horizontal. It is also the slope to use when the soil has not been classified.
What is the slope for Type A and Type B soil?
3/4:1 (53°) for Type A and 1:1 (45°) for Type B, for excavations up to 20 ft deep.
How wide is a sloped trench at the top?
Bottom width plus twice the depth times the slope. A 6 ft trench in Type B soil with a 2 ft bottom is 2 + 2 × 6 × 1 = 14 ft wide at the top.
When does a trench need a ladder?
At 4 ft deep or more, with no more than 25 ft of sideways travel to reach it (1926.651(c)(2)).
Sources
- 29 CFR 1926.650-652 and Subpart P Appendices A (soil classification) and B (sloping and benching), Occupational Safety and Health Administration. US federal regulations, read at law.cornell.edu (text cached 24 October 2026). Table B-1 slopes as printed in Appendix B.
Last reviewed: 24 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.