Timber Retaining Wall Calculator

About the Timber Retaining Wall Calculator

Timber retaining walls are one of the most popular DIY-friendly retaining wall options for walls up to 3 feet tall. Using pressure-treated 6×6 or 6×8 landscape timbers, these walls are relatively easy to build and provide a natural, rustic look.

This calculator estimates the number of timbers, rebar or spike connectors, and deadmen ties based on your wall dimensions. Each timber is typically 8 feet long and 6 inches tall, so a 3-foot wall requires 6 courses. Timbers are connected with 12-inch galvanized spikes or 1/2-inch rebar driven through pre-drilled holes.

Whether you're building a garden terrace, leveling a slope, or creating a raised bed border, This calculator gives you an accurate material list for ordering and budgeting.

When This Page Helps

Planning a timber retaining wall requires knowing how many timbers for each course, how many spikes to connect them, and how many deadmen to anchor the wall into the hillside. This calculator figures it all out from your wall length and height.

How to Use the Inputs

1. Enter the total wall length in feet.
2. Enter the wall height in feet (max 4 ft for timber).
3. Choose the timber size (6×6 or 6×8).
4. Review the number of timbers per course and total.
5. Check the spike and rebar count for connections.
6. Note the deadmen count for anchoring.

Example Calculation

Tips & Best Practices

• Use pressure-treated timbers rated for ground contact (UC4A or higher).
• Start with a compacted gravel base trench, burying the first course halfway.
• Stagger joints between courses like brickwork for strength.
• Install deadmen (T-shaped tie-backs) every 6–8 ft on walls over 2 ft.
• Drill pilot holes for spikes to prevent splitting.
• Install drainage gravel and perforated pipe behind the wall.

Timber Retaining Wall Construction Steps

Excavate a level trench 6–12 inches deep. Add and compact 4–6 inches of gravel. Lay the first course half-buried. Drill and spike each course to the one below. Stagger joints by at least 2 feet. Install deadmen every 6–8 feet on courses 2–3.

Backfill and Drainage

Backfill behind the wall with 12 inches of crushed gravel. Install a 4-inch perforated pipe at the base, sloped to daylight at the wall ends. Cover the gravel with filter fabric to prevent soil migration, then backfill with native soil.

Cost Comparison

Timber retaining walls cost $10–$20 per sq ft of wall face (materials only). This compares to $15–$30 for block walls and $25–$50 for poured concrete. Labor adds 50–100% to material costs for professional installation.

When to Choose a Different Wall Type

Choose block or concrete for walls over 4 feet, heavy surcharge loads (driveways, structures), or permanent structures. Timber walls are best for garden terracing, landscape borders, and low retaining needs in residential settings.

Frequently Asked Questions

• How tall can a timber retaining wall be?

  Timber walls are suitable for heights up to 3–4 feet. Beyond 4 feet, timber walls lack the structural capacity and should be replaced with block, stone, or poured concrete walls. Always check local building codes.

• What are deadmen in a retaining wall?

  Deadmen are timbers that extend perpendicular from the wall face into the hillside, anchoring the wall against soil pressure. They're typically a T-shape: one timber into the hill connected to a cross-timber for holding power.

• How long do timber retaining walls last?

  Pressure-treated timbers last 15–20 years in ground contact. Older treatments like CCA last 30+ years. Modern ACQ/CA-B treatments last 15–25 years. Proper drainage significantly extends wall life.

• Do I need drainage behind the wall?

  Yes, always. Install 4–6 inches of crushed gravel backfill behind the wall with a 4-inch perforated drain pipe at the base. This prevents hydrostatic pressure, which is the #1 cause of retaining wall failure.

• Do I need a permit for a timber retaining wall?

  Most jurisdictions require a permit for retaining walls over 4 feet in total height (including buried portion). Some require permits for walls over 2 or 3 feet. Check your local building department before starting.

• What spikes or fasteners should I use?

  Use 12-inch galvanized landscape spikes or 1/2-inch rebar cut to 24-inch lengths. Place 2 spikes per timber end and one in the middle. For rebar, drill 9/16-inch holes and drive rebar through all courses.