How to Build a Retaining Wall on a Slope the Right Way

To build a retaining wall on a slope, you cut a level, stepped base into the hill, set the first course below grade on compacted crushed stone, and build the wall so it leans back into the slope, with drainage stone and a pipe behind it. Taller walls also need geogrid in the soil behind and, often, a permit and an engineer's design.

Most retaining wall failures on slopes come from the parts nobody sees: a shallow or soft base, no drainage, or a first course that was never buried. Below is the process for a segmental block wall step by step, what batter means, and the mistakes to avoid.

Plan before you dig

• Decide height and purpose. How much soil will the wall hold, and what sits above it: lawn, a patio, a driveway? Loads above the wall change the design.
• Consider terraces. Two shorter walls stepped up a slope are often easier to build and drain than one tall wall.
• Check permits. Taller retaining walls, and walls holding up a driveway or structure, generally need a permit and an engineered design under the Massachusetts building code. Ask your town building department about its limits.
• Call Dig Safe. Dig Safe requires 72 hours' notice in Massachusetts before you dig, not counting weekends and holidays. Skipping this call is one of the most common and most dangerous mistakes.
• Plan where water goes. The drain pipe behind the wall needs somewhere to discharge, downhill and away from the house.

Mark the layout

Mark the face of the wall with string lines and stakes for straight runs, or paint and a hose for curves. Mark where the wall starts and ends; the ends usually turn back into the slope or step down to grade. Plan any steps into the wall at this stage.

Step the base into the slope

On a slope, the base cannot follow the grade. It is cut as a series of level steps, each one block-height higher than the last, starting at the lowest point of the wall. Allan Block's guide to stepping up the base shows the method. Each step stays fully level, and the first course on every step is buried.

Build the base

• Trench depth. Allan Block's installation guide sizes the trench as 6 inches of base material plus 1 inch of buried block for each foot of wall height. Other manufacturers publish their own rules, so follow the guide for the block you are using.
• Base material. Use crushed stone with fines that locks together when compacted. Pea gravel and round stone roll and will not hold a base.
• Compaction. Compact the base with a plate compactor and level it carefully. A soft base is the main reason walls settle and lean.
• First course. Set the first course level, front to back and side to side, and below grade. It is the course that holds the rest of the wall in place.

What batter means, and why it matters

Batter is the backward lean of a retaining wall into the slope it holds. Instead of standing straight up, each course is set slightly behind the one below it. That moves the wall's weight back toward the hill, so the soil pushing on the front has to tip the whole mass back over before the wall can lean out.

Segmental block builds batter in with a lip or pin on each unit, and the manufacturer sets the amount. Stone walls are battered by eye and with a simple frame. A wall built plumb, or one that leans out even slightly, has lost that advantage.

Drainage behind the wall

Water is the biggest force on a retaining wall. Behind every course goes a zone of clean drainage stone, with a perforated drain pipe at the base that runs to daylight or a dry well. Filter fabric between the drainage stone and the soil keeps fines from clogging it. Our post on retaining wall drainage covers the details.

Backfill, geogrid and cap

• Backfill in layers. Fill behind the drainage zone in thin layers and compact each one. Dumping and compacting all at once leaves soft soil that settles and pushes.
• Geogrid on taller walls. Geogrid is a strong mesh laid between courses and out into the compacted soil behind, which ties the soil and wall into one block. The manufacturer's charts and the engineer set where it goes.
• Cap. Glue cap units down with concrete adhesive made for the purpose, and finish the grade above the wall so water runs away from it, not over it.

Why rushing the job causes failure

The steps that take the most time, digging the base deep enough, compacting in layers, and getting the first course level, are the ones that keep a wall standing. Skipping them saves time at the start and costs a rebuild later. The order that actually saves time is careful planning and layout, then a well-built base, then a fast build above it.

Common mistakes

• No Dig Safe call before digging.
• Base too shallow, not compacted, or made of round stone.
• First course not buried, or not level.
• No drainage stone or drain pipe behind the wall.
• Clay backfill dumped in one lift.
• No geogrid on a wall tall enough to need it.
• Downspouts or surface water aimed at the wall.

Mortared block as an alternative

Where space behind the wall is tight, or the wall is part of a structure such as a raised deck, a mortared and reinforced block wall on a concrete footing below frost depth is the alternative to dry-stacked segmental block. It uses rebar and grout in the cells and weep holes for drainage. Our cinder block wall post explains that method.

When you need a pro or an engineer

Call a contractor for any wall tall enough to need geogrid, for walls with a driveway, patio or structure above them, for steep or wet slopes, and for walls along a property line or walkway. Ask the building department whether an engineered design is needed. Gerrior Masonry builds segmental block and fieldstone walls through our retaining walls service; see finished work in our landscape wall projects and on the Billerica pool deck project, where a block wall handles a change in grade.

Common questions

How deep should the base for a retaining wall be?

It depends on wall height and the block system. Allan Block's guide uses 6 inches of base plus 1 inch of buried block per foot of wall height. Always follow the manufacturer's guide for your block.

What is a batter slope ratio?

It is how far the wall steps back for each unit of height. Segmental blocks set it for you with a built-in lip. A steeper batter means a wall that leans back more.

Can I build a retaining wall on a steep slope myself?

Short garden walls on a gentle slope, yes. On a steep slope, or for any wall tall enough to need geogrid, hire a contractor and check whether you need an engineer.

Do I need a permit for a retaining wall in Massachusetts?

Often, for taller walls or walls holding up a load. Check with your town building department before you start.

If you have a slope that needs a wall, we can look at the site and explain what it will take to build it right. Gerrior Masonry is a family-owned company in Woburn, MA with over 30 years in masonry and hardscaping. We are fully insured, our work is guaranteed, and owner Joseph Gerrior oversees projects. Request a free estimate or call (781) 933-4157.