|Retaining walls are
an integral part of many landscapes. Miller Nursery's landscape
designer, Craig Westfield, is our resident expert on designing retaining
walls. This article will pick his brain about the different types
of walls available to our customers.
So, why build a wall? Most walls are built for slope management
and drainage control. Many homes require a retaining wall around
the foundation to create a level planting surface. But walls can
also be built for aesthetic purposes. An island or berm can be a
wonderful perennial garden and add great value to the landscape.
Other important factors are strength, longevity, and functionality.
What kind of wall is necessary to control drainage? How high does
the wall need to be? How many years should the wall last? These
are just a few of the basic questions that must be answered before
There are a number
of factors when choosing what kind of material. Aesthetics is
the first consideration. It is very important to match the material
with the house. For example, a 1000 pound boulder is going to
look funny as part of a wall next to a two bedroom house. This
is an extreme, but it is essential to match the materials to the
building. Furthermore, each homeowner has a different opinion
of what they want. And that is probably the most important consideration,
the customer must feel comfortable with the material selection.
Then there is the burning
question, how much does it cost? It is always important to plan
a landscape with a budget in mind. A wall can become very expensive
and cut into other parts of the landscape. The three types of
materials used by Miller Nursery for residential landscapes are
timbers, segmental units, and natural stone. So with this said,
let's explore the different materials available for retaining
The first material
available for retaining walls are timbers. They are usually made
from pine trees, but other woods can be used. The wood is treated
to resist the elements and prevent rotting. Timbers are probably
the least expensive option for retaining walls, but they have
issues with longevity. The average timber wall will last anywhere
between 10 to 20 years. This will differ greatly based on how
the wall is exposed to the elements.
Timbers can split over
time. Sometimes a split is undetectable during construction but
can become worse as the wall grows older. Another disadvantage
of timbers is the inability to create a curve. Obviously, a timber
will not bend, so any change in direction must be created with
an angle cut. However, timbers do have a very natural feel as
they blend into the landscape very easily.
Another material available
for retaining walls are segmental units. These are also known
as pre fabricated concrete units. Craig uses Rockwood brand retaining
wall units for all of the segmental walls built by the Nursery.
He feels Rockwood offers the best locking system, has the most
effective drainage system, and is the easiest to install. Rockwood
offers the units in three sizes. Four inch, six inch, and eight
inch units are available. Each block has a lip on the bottom of
the block that provides a setback and acts as a locking system.
This allows the wall to step back into the hill naturally and
The first unit in the
wall is placed on a layer of crushed rock. As more layers are
added, crushed rock is placed between the blocks. This creates
better stability for the wall and also prevents water or dirt
from seeping through the wall. The very top layer of the wall
is made of cap units. This is a solid unit that creates a smooth
top layer and gives the wall a finished look. With proper construction
and reinforcements, a wall can be built to forty feet in height.
Also, Rockwood offers the units in gray, brown, sandstone, hunter
tan, and charcoal.
There are some drawbacks
to segmental units. They can be overpowering in a landscape and
put too much focus on the wall. To prevent this, it is very important
to choose a color that matches or contrasts naturally to the color
scheme of the house. There are also complaints that segmental
units have a cold feeling. This can be true, but Craig feels strongly
this can be eliminated with the right color.
A natural feel can
also be obtained with landscape boulders or limestone. Craig is
not a big fan of natural stone in a residential setting for a
number of reasons. His main point of contention is the cost. In
the past six years limestone has doubled in price. There has been
a huge demand from the commercial landscape industry which tightens
the supply and makes it unreasonable for many residential customers.
The labor to install a retaining wall with natural products is
also quite high compared to other materials because every piece
of stone is different. It is very time consuming to piece together
boulders or limestone. However, if the customer is willing to
accept the cost, natural stone adds an extra touch to the landscape.
There is no way of
reinforcing natural stone which can make the wall unstable in
the long run. It is impossible to reach the heights of the segmental
units with a natural material. Boulders and limestone also do
not have the pressure ratings of segmental units. Rockwood's product
has a compression strength of at least 4000 PSI. Limestone has
a much lower PSI.
So what are the conclusions?
Craig is a firm believer in segmental wall units. He says it is
by far the leader in strength, longevity, and aesthetics. This
comes from over ten years of experience with all three materials.
However, Craig says the final decision is ultimately up to the
customer as to the type of material they would prefer in their