When we design and build a stone retaining wall, our goal isn't
merely to change grade or hold back an embankment; we also aim
to enhance the landscape. The retaining wall shown on these
pages, for instance, which we built around a new in-ground
swimming pool, provides seating and helps define a roomlike
space in the backyard. The wall was necessary because of the
slope of the yard, but locating it between the pool and the
house gave us the opportunity to create a landscape design as
beautiful as it is practical.
Like the majority of the stone walls and fences we build, this
one was installed "dry," which means that no mortar was used.
And while it isn't exceptionally tall, stone retaining walls
can be virtually any height (though a practical limit is about
8 feet); built right, they will last almost forever — or
at least until the bulldozers come and redevelop the
On the other hand, if improperly built, even a low wall can
quickly collapse into a pile of rock. What follows are the
techniques we use to make retaining walls last as long as the
stone they're built from.
Native Stone Is Best
While it's possible to build a stone retaining wall out of
practically any type of rock, we prefer to use native stone.
There are two reasons for this: A native stone will look like
it belongs in the landscape, and shipping costs will be
Here in the Northeast, we can choose from a number of stones,
including Pennsylvania bluestone, limestone, and fieldstone;
all can be found within a five-hour truck drive and are
available in various shapes, sizes, and colors.
To keep the cutting and fitting in check, we like to use stone
with a flat bottom and a clean face. That way, all we need to
do is dress the front of the stone with a hammer (to make it
look hand-cut) and set it, which creates less waste and saves
on labor and materials cost. Most sedimentary stones available
in our part of the Northeast, including limestone and the
sandstones (such as bluestone), fit this bill.
Of course, each variety of stone works differently, and some
need to be worked with a hammer more than others for a good
fit. Some stone masons lay up walls without ever picking up a
hammer; others — using special stone hammers and chisels
— cut every stone.
I'm somewhere in between; I allow the site and the stone to
tell me which approach works best. In certain cases, a wall
will fit the site better visually if the stone is left in its
natural state; other times, a more refined and worked stone
To build this particular wall, we used a Pennsylvania flagstone
called Colonial Wall Stone. It's actually a quarrying byproduct
that would probably be discarded if not for its excellent
qualities as a drywall-building stone. We buy it by the ton,
paying about $150 to $200 per ton delivered; each pallet is
filled with stones with (we hope) at least one flat face. The
stones fit together quickly with little cutting, which allows
us to work not only artistically but efficiently.
On a good day, an experienced waller and helper can hand-cut
and fit about two tons, or roughly 25 to 30 face square feet
(measured on the finished face of the wall).
Accounting for Existing Soil
Because we do much of our work in new construction, where cuts
and fills are common, we need to determine where excavation has
taken place to prevent our walls from moving or settling after
We've found that freeze/thaw cycles do a good job of settling
disturbed subsoils, so as a rule of thumb we insist on waiting
at least one winter before building on any new-construction
sites. While this policy occasionally costs us a project, I
tell impatient clients that the only thing I can guarantee if
the site isn't properly settled is that the wall will settle
If the site has been filled with more than a foot of soil, and
if the client does not want to wait a winter to start the
project, it's possible to settle the soil with water. Under
these circumstances — or if there's any doubt about soil
compaction — we excavate to create a sunken area; this is
where we plan to build. Before doing any work, we set up soaker
hoses and direct the homeowner to run the water for one hour
per day over two or three weeks. The excavation holds the water
in the area and allows it to gradually percolate into the
ground, settling the soil fully.
Sometimes a site will settle more than expected, in which case
we just fill back in with crushed stone that's been compacted
in 3-inch lifts with a plate compactor. It's extremely
important to have close to 100 percent compaction, or the wall
Once we're sure that the subsoil is fully compacted and won't
settle, we can go ahead and roughly lay out the wall and cut
the grade (see Figure 1). In most cases, we use a simple laser
transit for layout, marking straight lines and elevations with
string and stakes and constantly checking our drawings to make
sure we are staying within the parameters specified in the
Figure 1. The first step
in constructing any kind of stone wall is verifying that
disturbed soils have completely settled; only then should rough
grading and layout begin. Here, the yellow line painted on the
ground indicates the approximate position of the new
Of course, I always leave room for changes in my designs and
make adjustments in the field as necessary. Rarely does a final
installation exactly match the original plan.
For laying out straight walls, we simply measure and mark a few
key points and connect the dots. For curved walls like this
one, I mark out on the plan the various center points of the
radii for each curved section. Then, using a large screwdriver
that's been inserted through the steel loop at the beginning of
a roll-up tape measure and shoved into the ground, I measure
out and paint exact curves for the base materials. Once the
base is installed and compacted, I do the same for the wall
Prepare the Base
While many stone-building books indicate that it's okay to
place the base stones of a wall directly on excavated ground,
this works only in a frost-free climate.
In our area, where we expect a frost depth of 42 inches, we
need to account for frost heave, which is one of a stone wall's
biggest enemies (see "Frost Heave", below). The only way to
minimize frost heave is to reduce the presence of water within
the wall. To that end, we prepare a stable, well-drained base
and make sure that water has a way to drain out (Figure 2).
Frost heave occurs when, due to insufficient
drainage, excessive water is trapped in the
ground behind or beneath a retaining wall.
Freezing causes the ground to expand, pushing
the wall up or outward. When the ground thaws
and the soil settles, the wall often returns to
its original position, but individual stones
may shift position. Over time, as this cycle
repeats itself, the shifting of the stones can
weaken the wall.
In more extreme cases, freezing backfill
material pushes the wall horizontally. When the
ground thaws, the material settles down, but
the wall itself remains in the same position as
when the soil was frozen. Over the course of
years, this freeze/thaw cycle continues to push
the wall outward. Gradually, even a wall that
was battered into the slope at an angle will
begin to lean away from it; eventually, the
Figure 2. A stone
retaining wall should be built on a stable foundation and have
adequate drainage. Battering the face of the wall helps it
resist soil pressure.
Sizing the base. Generally, the
retaining wall's height determines how wide its base needs to
be. In most cases, retaining walls are about half as wide at
their base as they are tall. Since this wall's maximum height
is about 4 feet, the base of the wall is approximately 2 feet
How far the base extends below grade can vary from 6 inches to
as much as 2 feet, and is determined by the size of the wall
and the type of subsoil.
In general, well-compacted, gravelly soils that drain well and
lock together tightly require a minimum base depth. In sandy
soils, we'll dig a little deeper before running our plate
compactor, and in clay we try to provide maximum base depth.
Clay soils expand when they're wet, so they should be wet when
compacted; if you build on dry clay, it will heave the wall
when it eventually does get wet.
On this site, the soil was a combination of clay and gravel,
so we dug a trench about a foot deep for the wall base.
Sometimes, especially when the soil is wet, the stone base
material can "blow" into the soil, compromising the integrity
of the base. Therefore, we usually line our base trenches with
landscape fabric after we've compacted the trench, to separate
the base stone from the soils. Fabric isn't critical when
working with gravelly soils — but it can't hurt,
When in doubt, we always err on the side of having too much
After excavating the trench, compacting the soil as necessary,
and placing the landscape fabric, we install a base layer of
crusher run, a mix of crushed stone and stone dust that locks
up solid when compacted.
Sometimes we'll also use plain compacted crushed stone, but we
never use pea gravel, as it acts like ball bearings. Crushed
stone locks in tight when it's been compacted and drains well,
two very important features of a wall base.
To ensure that we get as close to 100 percent compaction as
possible, we run our walk-behind plate compactor over the
crushed stone until the machine "hops," which indicates that
the stone is completely compacted. (To see how this feels, run
the compactor for a second or two on a solid surface, like an
asphalt roadway; since the compactor cannot tamp this surface
any further, it will "hop.")
Drainage. Water trapped behind or under a wall
can move the wall by hydrostatic pressure as well as by frost
heave. Dry-stacked stone walls naturally allow water to weep
out from the wall face, but we also provide drainage with
perforated 4-inch pipe wrapped in landscape fabric and placed
at the base of the wall (Figure 3).
Figure 3. A stable,
well-drained base of crushed stone will help prevent the wall
from settling, and minimize damage from freeze/thaw cycles. A
4-inch perforated daylight drainpipe has been placed at the
base of the wall; it's shown here wrapped in fabric and covered
In most cases, we run the pipe to daylight away from the wall,
but if we don't have the necessary pitch, we'll run the pipe to
a dry well located a good distance from the wall. Pipe is not
critical with most stone walls because there is plenty of room
for water to weep through. But since this type of pipe is so
inexpensive, it never hurts to use it.
Because a retaining wall has to withstand the pressures of the
slope it is retaining, we typically dig the area behind the
wall back to its angle of repose — the angle at which a
slope is self-sustaining for a given type of soil. This angle
is heavily dependent on the soil type but, as a very loose
rule, it averages about 45 degrees.
With the final height of this poolside wall as our guide, we
dug into the slope so that the bottom of the cut was 2 feet
back from the back surface of the wall (to allow for crushed
stone at the base of the wall). The distance from the top of
the wall to the cut line was 2 feet plus the height of the
After the excavated slope was covered with landscape fabric,
all of this excavated area was filled with #2 crushed stone (a
large crushed stone of up to 2 inches in size) to allow for
drainage (Figure 4).
Figure 4. To reduce
soil pressure, the ground has been excavated behind the
retaining wall at an angle of about 45 degrees, then backfilled
with #2 crushed stone to enhance drainage. Lining the
excavation with landscape fabric helps keep soil from washing
into the crushed stone, which would diminish its ability to
In cases where plantings will be used, we usually backfill to
within about 12 or 18 inches of the top, lay down another layer
of landscape fabric to keep the soil from filtering down into
the crushed stone, and then fill in with topsoil.
Laying Up the Wall
We keep three basic principles in mind when building with
stone: battering, through-stones, and "1 over 2, 2 over
Battering. When building a
freestanding stone fence, battering — stepping back the
face of the wall slightly as you go higher — moves the
center of gravity to the center of the wall, making it more
stable in the event of ground movement. But in a retaining
wall, the stakes are much greater. Battering a retaining wall
dramatically reduces the amount of force exerted on that wall
by the soil and backfill materials.
Like most of our retaining walls, this wall was battered back
approximately 10 percent, or about one inch per foot. To
achieve consistency in battering a curved wall, we use a spirit
level to check the plumb as we build up. But there's no
substitute for experience, and we "eye" the wall a lot to be
sure it looks right.
Through-stones. A stone fence
essentially consists of two walls built back-to-back;
through-stones extending through the structure from front to
back tie together the two sides. On this short retaining wall,
through-stones weren't really necessary, as the mass of the
wall and the battering help to keep it from falling away from
the retained slope.
But we did create benches in the wall that serve a similar
function; we placed the 6-foot-long slabs of stone so that they
would project about 18 inches from the face of the wall (Figure
5). Each bench is like the proverbial tip of the iceberg; with
more than 4 feet of stone buried in the wall and the gravel
beyond, you can be sure that these seats will not move.
Figure 5. Three
6-foot-long stone slabs were cantilevered about 18 inches out
from the wall at roughly 18 inches above finished grade to
create informal seating. The weight of the stones, the backfill
material, and the wall above the slabs will help hold them
securely in place.
On taller walls, we'll occasionally use geogrid — a
netlike landscape textile — to separate the soil layers
and to tie the wall to the slope, much as a deadman does.
Geogrid is commonly used with segmental block retaining walls.
While we didn't use it for this short wall, it's relatively
inexpensive and a lot less costly than rebuilding a wall.
1 over 2, 2 over 1. Dry-stacked
walls rely on two forces of nature: gravity (or mass), and
friction. To maximize both forces, we always use as large a
stone as we can. To increase friction, it's important to
overlap joints as much as possible (Figure 6). One stone
sitting on top of two will apply gravity and friction to both,
creating a much stronger wall. By avoiding long, vertical
seams, which act like fault lines and become points of failure,
we essentially tie the whole wall into itself.
Figure 6. When placing
the stones, it's best to avoid vertical joints and try to
overlap each stone over two others. Here, the worker had to
align two joints to make room for electrical conduit that will
eventually supply built-in lighting fixtures.
As the wall goes up, we're careful to lay up stones so that
their longest edges project into the wall — rather than
orienting them parallel to the wall's face — even when
the stones have especially clean and beautiful faces. This
technique results in a stronger wall.
When we build our walls, there is always rubble left over from
cutting or breaking rock. We use this "waste" as infill,
tightly packing the rubble behind the face of the retaining
wall. And we place the infill carefully — rather than
just tossing it in — so that it doesn't settle over time.
We also use pieces of rubble to shim the face stones into
position, leveling them and locking them in place (Figure
Figure 7. Resembling a
triangle in cross section, the retaining wall is carefully
backfilled with unusable stone, rubble from cuts, and crushed
stone. Members of the author's crew rely on spirit levels and a
trained eye to ensure that the curved wall remains both level
and consistently battered back about 10 degrees from the
vertical while they lay up stone.
The most common way to cope — or cap — a wall is
to select stones large enough to span the wall's width. Then
the stones must be carefully cut to fit, leveled, and placed on
top of the wall. The tighter the fit, the less water these
coping stones will allow into the wall.
We often use flagstone to cap our stone fences and retaining
walls (Figure 8). Ranging in thickness from about 11/2 to 3
inches, flagstone (sometimes called "stand-up" because it's
stood up on pallets as it's quarried) is typically used for
paving pathways, walkways, and patios. We buy it by the
tractor-trailer load; when we take delivery we separate out the
really thick pieces and save them for coping stones. Since
short retaining walls are invariably used as seating, we make
sure the caps are large and stable. Indeed, their smooth,
consistent surface makes for a nice seat-wall or
Figure 8. Flat, smooth
coping stones cap the retaining wall. Mushroom-shaped
low-voltage lighting fixtures mounted in the wall beside the
stone benches provide illumination for the poolside sitting
We usually use shim-rocks to level up coping stones as we
place them, but we sometimes use mortar too; we place it in
such a way that it does not show along the face of the fence or
between the coping stones. Although the mortar separates from
the stone over time, it still acts as a perfectly formed shim
stone, supporting the decorative coping stones.
To build this wall, we used about 18 tons of Colonial Wall
Stone, 5 cubic yards of crusher run base, 1,000 square feet of
landscape fabric, and roughly 23 cubic yards of #2 crushed
stone. Labor time was about 130 man-hours, and the total
installed price was approximately $11,000.Bruce Zaretskyowns Zaretsky & Associates, a
design/build firm based in Rochester, N.Y.