Dry stone retaining walls add beauty and enhance the value of your property
as well as retain slopes to add usable level ground.
The design and construction
of dry stone walls are projects that can be undertaken by most homeowners with great success.
Wall construction tips
When designing any wall, it always helps to position the wall in the middle
of the slope. This gives an equal amount of soil removed that will be
needed as backfill. On uneven grades, the contour should be taken from
the bottom of the wall while all courses are laid flat creating a wall
of varying heights with a level top.
All dry stone walls should be constructed
with the thickness equal to half of the overall height. Walls should be
built leaning back toward the slope about two inches per foot of height
for stability. For walls that exceed two and one half feet in height,
the use of bonding stones and a drainage system are necessary for long-term
stability. Bonding stones stabilize a wall, and connect it to the slope
that is being retained.
Drainage behind any retaining wall of size is
critical. Hydraulic pressure is responsible for the failure of most walls.
To incorporate a drainage system in your wall, leave a twelve-inch wide
space behind the wall and lay a four inch perforated drainpipe on the
bottom and backfill with clean aggregate. Separate the drainage system
from the slope with a layer of landscape fabric, this stops soil intrusion.
The drainpipe can be directed to a lower area away from the wall reducing
the chances of erosion.
For dry stone walls that will not exceed two and
one half feet in height, elaborate drain systems are not necessary, although
landscape fabric is still necessary to stop soil on the slope from washing
through the wall creating erosion above and behind the wall. Bonding stones
should still be used for structural integrity.
Tools you will need
Shovel, small sledgehammer, string and string level, stakes to hold string,
hand tamper, safety glasses, tape measure and a builders level.
Materials you will need
For walls less than two and one half feet tall: Stone and landscape fabric.
For walls more than two and one half feet tall: Add aggregate and four-inch
drain pipe.
Terms to know
- Base stones: The broadest and thickest stones
used to make the first course or the foundation of a dry stone wall.
- Bonding stones: Longer stones turned to run
from the front face of a wall through the wall and protrude out the back
of the wall into the backfill material. These stones add structural as
well as lateral strength to a wall.
- Course: A continuous horizontal layer of stone
that maintains a constant level height.
- Hydraulic pressure: The lateral force created
by water when its natural flow is slowed or stopped.
- Cap stone: Uniform sized stones used as the
top course on a wall.
- Shim stone: A small and/or thin stone used to
level and stabilize other stones in a wall.
Calculate materials needed
Stone: Determine the length, width and height of the wall in feet. Average
the width and the height if they are not consistent. Multiply the length
x width x height to get the cubic footage. There are about 15 cubic feet
in one ton of stone so divide the cubic footage of your wall by 15 to
determine the quantity of stone needed in tons.
EXAMPLE: A 20' long wall that is 2' tall needs to be 1' wide.
20' x 1' x 2' = 40 cubic feet
40 divided by 15 = 2.67 tons
Landscape fabric: Take the height of the wall and add 1.5 feet then multiply by
the length of the wall to attain square footage.
EXAMPLE: 20'(2' +1.5') = 70 square feet.
Note: For walls more than 2.5 feet tall add 3.5 feet to the height
when figuring landscape fabric width. This allows enough
fabric to encapsulate the drainage aggregate.
Drainage aggregate: Assuming a 12" wide drainage system. Multiply the length of the wall
by the width of the drainage aggregate by the height of the wall to get
the cubic footage. There are 27 cubic feet in one cubic yard so divide
the cubic footage by 27 to get the quantity of cubic yards of drainage
aggregate needed.
EXAMPLE: 20' x 1' x 2' = 40 cubic feet
40 divided by 27 = 1.5 cubic yards
Drainpipe: The length of the drain system behind the wall for perforated pipe and
the distance to the drain area in solid drainpipe if draining elsewhere.
Backfill material: Multiply the length x width x depth of the area to be filled to attain
the cubic footage. Divide the cubic footage by 27 to get the quantity
of cubic yards needed.
Note: Soils will compact up to around 20 percent so a little extra may be necessary.
Getting started
- Dig a level bottom trench wide enough for the base stones to fit into
and about the same depth, as they are thick, then compact the bottom with
a hand tamper.
- Set stakes outside of the wall ends so a string can be tied on and leveled
with a string level above the walls maximum height.
- Lay landscape fabric covering one half of the base trench and extending
back toward the slope.
- Determine the lowest point of the trench by measuring down from the string.
- Pick out the base stones and begin laying at the lowest point of the
trench in both directions covering the landscape fabric to hold it in
place. The base course must be level and firmly installed so use shim
stones to help fill in voids and level individual stones as needed. If
shim and filler stones are in short supply, break up some whole stones
with a sledgehammer. We are definitely not building a piano here. The
correct attitude for building a dry stone wall is “If you can’t
force it, break it. If you can’t break it, your hammer is most definitely
too small!
- If the base course fades below the intended grade too far, go back to
where you started and begin the next course. Overlap the joints that are
between the stones below, and span the width of the wall with bonding
stones as often as possible. Continue this process until the base course
is complete.
- Each course of stone should be stepping back slightly to achieve a two-inch
per foot lean back toward the slope. This helps the wall retain the slope.
Use a builders level and tape measure to check your step back.
- While stacking the courses, lay the landscape fabric tight up against
the back of the wall and install and compact the backfill as you go. For
larger walls add the drainage aggregate, the pipe, and the backfill as
you go while keeping the aggregate separated from the backfill with the
landscape fabric.
- For walls more than two and one half feet tall, install bonding stones
every 18" of height about four to six feet apart.
- Uniform thickness stones should be saved for the cap course.
- Level the wall before installing the cap course and lap the landscape
fabric over the top of the wall, then trim off excess until it covers
half of the top of the wall.
- Install the cap course tightly to hold down and hide the landscape fabric.
If the wall will be in a public area, or will be used as a sitting wall,
the cap course should be mortared down or glued down with a construction
adhesive suitable for masonry.
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