How to Build a Sandstone Block Retaining Wall: Step-by-Step

How to Build a Sandstone Block Retaining Wall: A Step-by-Step Guide

A sandstone block retaining wall is one of the most enduring and attractive ways to manage slopes, create level garden areas, and add structure to your outdoor spaces. Across Sydney, from the harbour foreshore to the Blue Mountains foothills, sandstone retaining walls have been used for over two centuries and remain the preferred choice for homeowners who want a natural, heritage-appropriate finish.

This guide walks you through the process of building a sandstone block retaining wall, from planning and preparation through to the final capping. We also cover when you need engineering involvement and how to ensure your wall stands the test of time.

Before You Start: Planning Your Sandstone Block Wall

Do You Need Engineering?

In NSW, any retaining wall over 600 mm in height generally requires structural engineering design and a construction certificate from your local council. Even walls under 600 mm may require approval if they are near a boundary, support a surcharge load (such as a driveway or structure above), or are in a bushfire or flood-prone area. Always check with your council before starting.

Choosing Your Sandstone Blocks

The type of sandstone block you choose affects both the appearance and the construction method of your wall. Popular options include:

• Rockfaced blocks: Natural split face with a rugged, traditional appearance. Ideal for heritage-style gardens
• Rough squared blocks: Roughly shaped to approximate dimensions for a slightly more uniform look while retaining natural character
• Hydrasplit blocks: Hydraulically split for consistent thickness and a clean split-face texture. Popular for contemporary retaining walls
• Sawn all round blocks: Diamond-sawn on all faces for a formal, precise finish

For a detailed comparison, see our sandstone blocks buying guide and our article on sandstone logs vs blocks.

Calculating Quantities

Measure the length and height of your wall to determine the face area in square metres. Your Stone Front sales team can help you calculate the number of blocks required based on the block size you select. Always order 5 to 10 percent extra to account for cutting and wastage.

Step-by-Step Construction Guide

Step 1: Set Out and Excavate

Mark out the wall line using string lines and pegs. Excavate a trench for the footing that is at least 300 mm deep and 150 mm wider than the blocks on each side. The trench should be dug into firm, undisturbed soil. For walls over 600 mm high, your engineer will specify the footing dimensions.

Step 2: Prepare the Foundation

Compact the base of the trench using a plate compactor. Lay a 100 mm bed of compacted road base (DGB20) as a level foundation. Check levels carefully with a spirit level or laser level. The foundation must be flat and firm, as this determines the accuracy of the entire wall.

Step 3: Install Drainage

Drainage is critical to the longevity of any retaining wall. Behind the wall, lay a 100 mm slotted agricultural drain pipe (ag pipe) at the base, wrapped in geotextile filter fabric. This pipe should fall to a discharge point at one or both ends of the wall. Backfill around the pipe with 20 mm drainage gravel to a depth of at least 300 mm behind the blocks.

Without proper drainage, hydrostatic pressure from trapped water will push against the wall and cause it to lean, crack or collapse over time. This is the single most common cause of retaining wall failure.

Step 4: Lay the First Course

Set the first course of sandstone blocks on the compacted base. Use a spirit level to check each block is level both along its length and front to back. The first course is the most important, as every subsequent course relies on it being straight and level.

For gravity retaining walls, set the blocks with a slight lean-back (batter) towards the retained soil. A typical batter is 10 to 15 mm per course (roughly 1 in 6 slope). This helps the wall resist the pressure of the soil behind it.

Step 5: Lay Subsequent Courses

Stack each subsequent course on top, staggering the vertical joints like brickwork (running bond pattern). This interlocking pattern distributes loads evenly and strengthens the wall. Depending on your block type and engineer's specification, you may need to use construction adhesive between courses or rely on the mass and batter of the blocks alone.

Continue backfilling with drainage gravel behind each course as you build up. Lay geotextile fabric between the drainage gravel and the retained soil to prevent fine particles from clogging the drainage layer.

Step 6: Capping

The top course can be capped with sandstone capping or edging pieces for a neat finish. Sawn capping provides a flat, smooth top that doubles as informal seating or a surface for potted plants. Secure the capping with construction adhesive.

Step 7: Backfill and Finish

Complete the backfill behind the wall with topsoil above the drainage gravel layer. Allow the soil to settle and top up as needed. Plant the area above and around the wall to stabilise the soil and soften the appearance.

Building Sandstone Block Steps

Sandstone block steps can be integrated into your retaining wall to provide access between levels. The principle is the same as the wall itself: a compacted base, good drainage, and staggered block placement. Each step should have a rise of approximately 150 mm to 180 mm and a tread depth of at least 300 mm for comfortable use.

Use sawn all round blocks for step treads if you want a smooth, slip-resistant surface, or rockfaced blocks for a more rustic look.

Common Mistakes to Avoid

• Skipping drainage: The number one cause of retaining wall failure. Always install ag pipe and drainage gravel
• Inadequate foundation: Building on uncompacted fill or topsoil leads to settlement and cracking
• No batter: A perfectly vertical gravity wall will eventually lean forward under soil pressure
• Ignoring engineering requirements: Walls over 600 mm need professional design. Cutting corners risks wall collapse and potential liability
• Poor block selection: Using blocks that are too small for the wall height. Heavier blocks perform better in gravity walls

When to Hire a Professional

Small garden walls under 600 mm can be a satisfying DIY project for experienced handypeople. However, for walls over 600 mm, walls supporting driveways or structures, walls on reactive clay soils, or any wall requiring council approval, engage a qualified landscaper or stonemason who has experience with sandstone construction.

For more detailed guidance on retaining wall options, read our comprehensive sandstone retaining walls guide. To source quality sandstone blocks for your project, visit Stone Front at our Sydney yard or request a free quote online.