When does a trench need shoring?

OSHA requires a protective system at 5 ft deep unless it's in stable rock; over 20 ft it must be engineered.

Type A ¾:1, Type B 1:1, Type C 1½:1 (horizontal:vertical).

What shoring options exist?

Hydraulic shores, trench boxes and timber shoring.

1 ft horizontal for every foot of depth (45°).

Can I combine sloping and shoring?

Yes — benching or sloping on top with shoring below is common.

Who chooses the protective system?

A competent person under OSHA; over 20 ft it must be designed by a registered engineer.

Trench shoring calculator

Trench size and soil type: compare the shoring area you need with the extra volume of sloping the walls instead.

Volume with vertical walls

41.854 yd³

Shoring area (both walls)861.113 ft²
Volume with shoring (vertical walls)41.854 yd³
Top width if sloped15.748 ft
Volume if sloped146.49 yd³

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How it's calculated (visible formula)

vertical = L × W × D; top width = W + 2 × k × D; sloped = L × D × (W + top width) ÷ 2

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure trench length, bottom width and depth.
Classify the soil (stable rock, Type A, B or C) per a competent person.
Work out shoring area for both walls if you dig vertical.
Work out top width and volume if you slope instead.
Compare shoring cost with extra excavation and backfill, and available space.

Worked example: 66 ft × 2.6 ft × 6.5 ft trench in Type B soil (1:1)

Shoring: 2 × 66 × 6.5 ≈ 860 sq ft (80 m²)
Vertical: 42 yd³
Sloped: top width 15.7 ft → 146 yd³

→ Shoring saves about 104 yd³ of digging.

Common mistakes

Entering a trench 5 ft or deeper without protection.
Piling spoil within 2 ft of the edge.
Assuming clay holds the same after rain.

Pro tips

Keep spoil and equipment at least 2 ft from the edge.
Provide a ladder within 25 ft of travel in trenches 4 ft or deeper.
Inspect the trench after rain, freeze or vibration before entry.
Trench boxes are dragged along as the pipe advances.

From calculation to quote

Add labor, overhead and your margin, and generate the professional PDF.

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FAQs

When does a trench need shoring?: OSHA requires a protective system at 5 ft deep unless it's in stable rock; over 20 ft it must be engineered.
What slope is safe?: Type A ¾:1, Type B 1:1, Type C 1½:1 (horizontal:vertical).
What shoring options exist?: Hydraulic shores, trench boxes and timber shoring.
What is a 1:1 slope?: 1 ft horizontal for every foot of depth (45°).
Can I combine sloping and shoring?: Yes — benching or sloping on top with shoring below is common.
Who chooses the protective system?: A competent person under OSHA; over 20 ft it must be designed by a registered engineer.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.