OSHA Trench Slope Chart

The maximum allowable slopes from OSHA 1926 Subpart P App B (Table B-1) — federal law, and the most consequential numbers in the trade. They apply to excavations under 20 ft; the competent person classifies the soil and has final authority on site, state OSHA plans can be stricter, and signs of distress flatten the cut immediately.

Maximum allowable slopes (excavations < 20 ft)

1926 Subpart P App B
Steepest permitted excavation face by soil classification, as horizontal:vertical ratio and degrees. Soil type comes from the Appendix A classification — unclassified soil takes the Type C slope.
Soil / rock typeMax allowable slope (H:V)Angle from horizontal
Stable rockVertical90°
Type A¾H : 1V53°
Type B1H : 1V45°
Type C1½H : 1V34°
Short-term exception: Type A open ≤24 hours and ≤12 ft deep may cut ½H : 1V (63°). Signs of distress: cut back at least ½h : 1v less steep than maximum. Surcharge loads near the edge reduce the allowable slope — the competent person sets the reduction.

The configurations beyond a simple slope

Appendix B also blesses combination cuts. Type A allows an unsupported vertically sided lower portion of 3.5 ft — with ¾:1 above at total depths to 8 ft, flattening to 1:1 above between 8 and 12 ft. Every soil type allows a vertical lower portion when it is shielded or supported, with the box or shoring extending at least 18 inches above the top of the vertical side (the rule people mis-file under the trench-box section — see the trench box rules). Benching has its own table — the benching chart — and everything starts from the soil classification.

Common questions

What slope does OSHA require for a trench?

By soil type, for excavations under 20 ft: stable rock may be cut vertical, Type A slopes at ¾:1 (53°), Type B at 1:1 (45°), and Type C at 1½:1 (34°). If nobody classifies the soil, the whole cut defaults to the Type C 34° slope. A trench 10 ft deep in Type C soil therefore needs the top of the cut 15 ft back from the toe on each side — sloping eats real estate fast, which is why boxes and shoring exist.

What is the short-term exception for Type A soil?

An excavation in Type A soil that stays open 24 hours or less and is 12 ft or shallower may be cut at ½H : 1V (63°). Past 12 ft the standard ¾:1 applies regardless of duration — and remember how hard true Type A is to come by (see the classification chart's disqualifier list).

How steep can you slope past 20 feet deep?

That is no longer a table question: sloping or benching for excavations deeper than 20 ft must be designed by a registered professional engineer (Appendix B, Table B-1 note). The published slopes — and the no-classification 34° default — all stop at the 20-ft line.