How to Build a Stacked Stone Retaining Wall

No matter how small your yard, mowing a steep slope can be a dreadful task.

And if you're a master gardener (like the owner of this city lot), you might enjoy grooming a series of beds rather than maneuvering a mower on a horrible hill. The answer may be to replace that sloped grass yard with a series of terraced retaining walls and planting beds. Building a dry-stacked stone retaining wall like this one is time-consuming, but it does not require special masonry skills, and it can be less strenuous than working with manufactured concrete blocks, which are typically heavier than limestone.

Scott Reynolds of Creative Habitats Inc. in Minnetonka, Minnesota, transformed this sloping lot with a series of beautiful stone walls and we documented the process to show you how it's done. Scott and his crew of two tamed this bank (with the help of some power equipment) in a little more than a week. Though it might take a bit longer, a handy DIYer can tackle this project and create a timeless and enduring structure.

Designing this Retaining Wall
In this dry-stacked wall, only the top course is set in mortar; gravity holds the other stones in place. The cut Chilton stones vary in thickness and length but are a uniform 8-in. width. Chilton is a type of limestone from Wisconsin that is available at most stone yards; you could also use other types of locally quarried limestone.

For inspiration, look at stone walls in your area and check out stone suppliers' displays to find types of rock and designs that you like. This wall is built in a basic running bond pattern in which all of the stones in each row are the same height. and the joints are staggered (strengthening the wall).

"I always look at the house for patterns to mimic in the landscape. It's important to consider the entire composition." – Scott Reynolds, Creative Habitats

Scott incorporated arcs in the shape of the wall partially to give the natural-stone structure an organic, flowing design, but also to reflect curves featured in the entrance area of the house. He used a 5-ft.-radius cutout for the patio area. (The center is about 18 in. from the sidewalk's edge.) The same radius was repeated in the two curved walls on the upper level. Although the patio side of the design is different from the wall on the opposite side of the steps, an overall balance is achieved through the symmetrical placement of shrubs and plantings.

Using a scale drawing and the dimensions of the rise and span of the hill, Scott calculated the wall's surface area and ordered 20 percent more stone than the plan required to allow for waste. (He also included the buried base layers of walls in his calculations.) Before you begin designing your wall, Scott recommends these preparatory steps:

  • Obtain a survey of your property to be sure your project fits within the boundaries.
  • Dial 8-1-1 to have utilities marked.
  • Tell your neighbors about the project beforehand; they'll be more tolerant of the mess or any inconvenience and noise if they're in on the plan.
  • Check with your city regarding code requirements, setbacks, etc., and about street restrictions for deliveries of stone and gravel.
  • Determine a convenient staging area for piling extra soil and supplies.
  • Know the characteristics of your soil (sand or clay?).
  • Collect all of the tools and gear you'll need before you begin.

Laying groundwork
Cut stone is delivered in mixed lengths and thicknesses (ranging from about 2 in. to 3 in. thick), but each course of the wall must be made of stones of the same thickness. (Aesthetically and structurally, it's best to use the thickest stones for the lower courses.) To save installation time, Scott sorts the stones into piles according to their thickness in 1/4-in. increments before starting the project.

The maximum allowed height for a dry-stacked stone wall is 4 ft. Because this site's walls are less than 24 in. high, they do not require the installation of drain tile. If they were higher than 2 ft., they would have needed a perforated drainpipe under the base, and the first 12 in. of stones would need to be 18 in. wide front-to-back.

For a stone wall that is higher than 24 in., drain tile (surrounded by aggregate) must be installed.

Digging in
When terracing a hill, always work from the bottom up. The crew began by digging a 12-in.-wide x 7-in.-deep trench along the edge of the sidewalk, being careful not to disturb the soil below. They smoothed the base, making sure it was level with the horizon (which is typically not parallel with the grade of the sidewalk).

The next step is to spread a 2-in. layer of compactable three-quarter-clear gravel, which is angular and contains no crushed material. The angled surfaces of the stones help to lock them in place, preventing shifting, and the absence of crushed material allows water to drain more easily. (Scott says you can use three-quarter-minus gravel for this purpose only if you have sandy, good-draining soil.)

A power plate compactor is faster and more effective than hand tamping, and you can rent one from most home centers and rental companies. They weigh nearly 200 pounds but move along easily when running. Wear hearing and eye protection, gloves and reinforced-toe boots.

Pack the 2-in. layer using a power plate compacter, the top of a maul or a tamper. Add another 2 in. of gravel and pack again, continuing until you have a smooth, level 4-in.-thick base. (The remaining 3 in. of trench depth allows the first course of stone to be below grade.)

Regardless of their finished height, stone walls require an 8- to 10-in.-wide layer of gravel behind them. This site has clay soil, which is prone to hydrostatic pressure and can cause any retaining wall to bulge and heave, so Scott used three-quarter-clear gravel (not three-quarter-minus) behind the wall as well to allow rain and melting snow to drain.

Add gravel backfill as each course of stone is completed. This allows you to completely and easily fill any gaps between the rocks before the row is covered by the next course.

Cutting Stone
For most of the project, you can pick through the stones to find a good fit, but sometimes you need a custom shape or size. You can use these methods for cutting stone:

  • Chisel and hammer (score and tap along the score line with the chisel several times)
  • Maul (whack and hope to get lucky)
  • Diamond-blade gas saw (rental)
  • Hydraulic splitter

You (and any bystanders or helpers) should always wear eye and hearing protection when cutting or breaking stone. Some stones have visible grain lines that may indicate where they are likely to split, but in most cases your results will depend on luck.

Setting Stone
As with any wall construction, the base course sets the stage for the rest of the project. For this first layer, you can use stones that have odd shapes or different thicknesses, as long as the top surfaces are all even and level. Just pound down any high spots using the top end of a maul. Check for level front-to-back and side-to-side. (If you can't get it perfect, err on the side of a slightly higher front edge than back.) Make each course level before you start the next layer — don't plan to compensate for height differences later. As you install each stone, flip and turn it until you have a solid fit. You might need to try a different one for compatibility with neighboring stones and with the base. Set the front face of each new row 1/4 in. back from the previous row.

Each row of stones is stepped back (battered) 1/4 in. along the front faces. It's a subtle graduation, apparent only when compared with a level. Battering adds significant strength to a dry-stacked wall.

Natural stones' irregular shapes sometimes make a slight "rocking" movement unavoidable. You can compensate for this by shimming with a stone chip (called chinking) under the offending rock. Where needed, you can knock off protruding edges of stones with a maul. Try to avoid leaving gaps between neighboring stones, but where they occur, fill vertical spaces with a stone chip and gravel.

For times when a stone's fit is almost (but not quite) perfect, Scott's crew keeps a bucket of limestone chips handy to prop the gap. These shims become tightly affixed under the weight of the wall.

Use various sizes of levels to check each stone as you install it, adjusting for level front-to-back and side-to-side. Continue to backfill with gravel as you add each new course. Scott did not install landscape fabric because it tends to prevent drainage and to build hydrostatic pressure – especially in clay soil. If your soil is very sandy, you can add landscape fabric between the gravel backfill and the soil behind it. Do not lay fabric directly against the back of the wall.

For the top course (called capstones), Scott used the thinner (2-in.) stones, being careful to fit the shapes together well. Once the entire top course was in place, he folded back the row of capstones and applied mortar before pressing them in place. To prevent soil from seeping past the top layer of stone, you can pack mortar along the back of the capstone row as well.

To adhere capstones, mortar is applied along the rear half of the previous row, filling voids and helping to lock the entire system and to protect the most vulnerable layer from being disturbed.

To allow room for a layer of soil and plants, stop adding gravel backfill when you're 5 in. shy of the top of the wall. You may choose to place landscape fabric on top of the gravel before adding the soil.

Building up
The upper walls are constructed like the first: Dig a trench, pack in a 4-in.-thick gravel base and install level courses of stone with 8 to 10 in. of gravel backfill behind. Dig the trench deep enough that the top of the base course is level with (or slightly lower than) the capstone of the lower wall.

The return walls (the sections that run alongside the steps and into the hill) need to extend into the soil at least 6 in. beyond their exposed portions. The hidden portions must also be supported with packed gravel and a course of base stone.

Adding outcroppings
To help support mounds of soil and plants within each tier, Scott placed small boulders as outcroppings within the tiers. Strategically set to support the greatest elevation changes, these rocks are partially submerged in the soil to help them stay in position. (One of these boulders was a lucky find: When excavating the patio area, Scott discovered a large rock buried right in the spot where he had planned for one!)

Placing the patio
A small patio created along the sidewalk provides room for potted plants or a small sitting area. The flagstone floor was laid over a 1-in.-thick base of compacted granite sand which, like the gravel used for base in the walls, is angular rather than rounded so it locks together and stays in place. Once the 1-1/2- to 2-in.-thick flagstones were in place, gaps between them were filled with more granite sand.