Timber Shoring

Timber shoring uses wood posts and braces to hold up the sides of a trench so the walls can’t collapse on workers. It was the first trench protection and is still used for small jobs, odd-shaped excavations and places where trench boxes won’t fit. Timber shoring only works if it’s designed, built and inspected correctly.

• Construction
• Excavation
• Utilities

Why shoring is used

• OSHA requires a protective system for trenches 5 feet or deeper, unless they’re in stable rock, and also for shallower trenches when a competent person finds a risk of cave-in. Options are sloping or benching, shoring or shielding (29 CFR 1926.652). See Protective Systems and Trenching Basics.
• Shoring supports the trench walls. A trench box (shield) doesn’t hold the walls up. It only protects workers inside it. See Trench Boxes and Hydraulic Shoring.
• Timber shoring is useful when there is no room to slope the walls, near buildings, roads and utilities, and for short, narrow or irregular trenches.

Parts of a timber shoring system

Systems may be spot (intermittent) bracing, where uprights are spaced apart, or continuous sheeting, where boards form a solid wall. Looser or wetter soil needs tighter spacing.

OSHA’s timber shoring tables

• Appendix C of Subpart P (1926 Subpart P) gives timber shoring tables that list the sizes and spacing of uprights, wales and cross braces for trenches up to 20 feet deep, based on soil type (A, B or C) and trench width and depth. If the trench fits the conditions, an employer can use the table instead of hiring an engineer.
• The tables apply only when the conditions match. They assume certain wood qualities, soil conditions and loads, and they need a competent person to pick the right table. See Soil Classification and Soil Tests.
• If the trench is deeper than 20 feet, or the conditions don’t match the table, a registered professional engineer must design the system. See Deep Excavations (Over 20 Feet).
• Designs can also be based on manufacturer’s tabulated data or a professional engineer’s design. Keep a copy of the table or design on site while work is under way.
• Use the actual table in the standard. It’s based on exact measurements, and a shortcut can be fatal.

Materials

• Use sound, straight, solid timber of the species and grade the table or design requires. Don’t use rotten, split, twisted or knotty lumber, or material with large cracks.
• Use full-sized timbers. Nominal sizes (such as “2 by 6”) are different from actual sizes. The tables use actual dimensions, so check them.
• Make sure wood is dry enough and not weakened by prior use. Inspect reused timbers for damage.
• Use hardware, jacks and wedges made for the load.

Installing timber shoring

1. Plan first. A competent person classifies the soil, picks the system and checks for utilities, water, surcharge loads and nearby structures. See Surcharge Loads, Call 811 and Excavations Near Structures.
2. Dig carefully in stages. Install shoring as the trench is dug, not after it is already deep. Don’t let anyone enter a trench until protection is in.
3. Set uprights against the wall and drive them down as digging proceeds, keeping them at the spacing in the table.
4. Install wales across the uprights at the specified heights, and install cross braces tightly between wales, level and square to the trench.
5. Secure the braces so they can’t drop or kick out. Use wedges, cleats or jacks. Make sure the braces are tight against the wales.
6. Install from the top down. Install the first bracing at the top, and keep going down as the trench deepens.
7. Provide ladders or ramps within 25 feet of workers in trenches 4 feet or deeper. See Access, Egress & Spoils.
8. Keep spoil piles and equipment at least 2 feet back from the edge. See Barriers & Fall Protection at Excavation Edges.

Removing shoring

• Remove from the bottom up. Backfill in lifts as shoring is taken out, and release only the lowest braces as the trench is filled. See Backfilling & Compaction.
• Take out one set of braces at a time, and make sure the next set is still in place. Use cables, ropes or other methods so workers don’t have to stand in unsupported ground.
• Have a competent person supervise removal. Backfill as you go, so no one works in an unprotected trench.
• OSHA requires that shoring be removed progressively and safely, and that any cave-in risk be controlled during the process.

Inspection and daily duties

• A competent person must inspect the trench, the shoring and the adjacent area every day before work and after rain, thaw, vibration or other changes. See Competent Person Inspections.
• Check braces for movement, wedges for looseness, timbers for cracks, uprights for shifting and the trench walls for cracks, sloughing and water. See Cave-In Warning Signs.
• If any sign of danger shows up, get workers out until the problem is fixed.
• Don’t use shoring as a ladder, and don’t allow people to hang from braces.

Common mistakes

• Using undersized, weak or damaged lumber.
• Spacing uprights or wales farther apart than the table allows.
• Letting braces become loose, or forgetting to re-tighten after the ground moves.
• Working in the trench before shoring is installed, “just for a minute.” Many deaths happen this way.
• Removing braces from the top down, or removing them before backfill is in.
• Using the Appendix C table on a trench that doesn’t match its conditions.
• Digging beyond the depth of the shoring, or leaving a gap at the bottom.

Timber shoring checklist

• Competent person classified the soil and chose the table or the engineer’s design
• Sound timber of the right size and species; hardware and jacks rated for the load
• Uprights, wales and cross braces spaced as designed; braces wedged or secured
• Installed from the top down; ladders within 25 feet; spoil 2 feet or more from the edge
• Inspected daily and after any change; workers out if there are danger signs
• Removed from the bottom up as backfill goes in

Last updated: October 4, 2026