Trees, Foundations and Soils
I've known
Steve Houser for many years. I've watched his crews work on my own
trees at my home. He and his company have won many awards for the
excellence of their service. Steve was recognized this year as
Arborist of the Year by two important industry organizations, and
The Houser Cup was begun by the industry in Steve's name as an
annual award to the outstanding tree-climbing crew in Texas. His
company's climbers have been so-recognized 10 different years in
the past. Steve is very modest about his awards, but he's proud of
his people, and, make no mistake, he loves trees.
Arborilogical
Services is based in the Dallas/Fort Worth Metroplex (Dallas
number 972-442-1524 and Fort Worth number 817-849-1160).
Feeling that
many folks were sacrificing large and important shade trees and
even large shrubs in what I feared might be over-reaction to
foundation concerns, I asked Steve Houser to jot down his ideas.
His very considered response follows. We are grateful to Steve for
all that he does to help the trees of Texas, and for his work in
preparing this piece for our Webletter subscribers.
Neil Sperry
July 8,2002
Trees, Foundations and Soils
Steve Houser, December 2007
Arborilogical Services
DFW Metroplex
Foundation problems, or structural deficiencies
of homes and buildings are a common concern in North Texas.
Structural engineers and foundation experts state that there are
two types of foundation owners, those who have had foundation
problems and those who will. With an increasing number of area
foundations being poured on a daily basis and the increasing age
of existing area foundations, a rise in future deficiencies will
likely occur.
50-year-old pecan, now 24 inches in diameter, has been less than 2
feet
from the Sperry house since it was built in 1977.
Trees
and foundations can co-exist.
There are many potential reasons for foundation
problems, and the causes may vary greatly from site to site. The
soil in your immediate neighborhood may be one type. But
area-wide, the soil types could include clay, sand, sandy loam,
rock, or a combination of these materials. Soil diversity can
result in a diverse local plant palette, which is great for
plant enthusiasts but not always great for foundations. Due to
the variance of soil types and conditions at any single site, it
is not possible to offer any one response to a problem. The
subject of foundation deficiencies is broad, and it requires
different areas of expertise. Experts can include Soil
Scientists, foundation repair companies, Hydrologists,
Structural Engineers and Consulting Arborists. Gathering input
from each specialist is wise but it can lead to differences of
opinion and confusion. In some cases, the professional opinions
may be based on conflicting or outdated research. Deciding which
remedy is best suited to a specific circumstance is sometimes a
difficult task.
When trees are growing close to a foundation
the opinion of a Consulting Arborist is often requested. Many
times, trees are identified as the cause of foundation
deficiencies by one or more consulting professionals, and
removal of the plant is recommended. Consulting Arborists
usually have a basic knowledge of other professional fields, and
vice versa. A Consulting Arborist will offer what he knows to be
a fact, but at some point, advice or recommendations become a
professional opinion based on experience, common sense, and
research. The Consulting Arborist's objective is to teach the
property owner how the trees might affect foundations. Sometimes
this requires a discussion directed more toward the "whole
picture". By examining all of the related issues, arborists,
foundation experts, and others can determine if trees are a
problem or a contributor to a problem. The issues are not always
black and white. However, being informed and knowing what to
ask, can lead to the correct solution. A list of questions to
ask or issues to consider would include the following:
Site Planning and Development
Consider the planning and development and the
history of the site beginning as far back in time as records
will allow. Determine what existed on the site before it was
developed. Check for records available from city hall, the
builder/developer or others that may have been involved with the
site. In many housing developments, the site is graded and
leveled, removing or redistributing the topsoil (the good stuff
for growing plants!). Determine if your site has any topsoil
remaining. If so, measure the depth. Determine whether the soil
type and soil profile were taken into account in planning and
developing the area, or in the design of the foundation.
Determine if the builder made adjustments to the soil profile
before pouring the foundation. and if so, was the soil compacted
to industry standards? Experts agree that these practices
directly affect the integrity and life expectancy of a
foundation BEFORE being installed. North Texas property owners
who have researched their property's development have sometimes
found that the property was a construction debris dumpsite, a
filled floodplain, or a covered natural spring. Poor site
planning and development can lead to foundation problems.
Soils
Soils are often noted by the experts as the
primary factor in foundation problems. Do you know the soil type
that exists under the foundation? Is there rock underneath? If
so,what type of rock? Is it fractured and mixed with soil, or is
it a solid layer and to what depth? If the soil is black clay,
the high shrink/swell potential can be a factor. The black clay
soil in our area shrinks when it is dry, and expands when it is
wet. Sandy soils shrink and swell a smaller amount and do not
retain water like clay soils. When a foundation site contains a
mixture of clay, sand, sandy loam, or rock, it becomes more
difficult to predict how the soil will react. Most soil types,
and soil mixes can adequately support a foundation if they are
properly planned, designed, built, and MAINTAINED! Even the best
foundation, built under ideal conditions, can have difficulty if
it is not properly maintained.
Lot Completion
Consider the final lot grading. Does your lot
drain correctly? Does water stand near your foundation, or does
it drain away? Was the soil level below the bottom of the
foundation or above the brick or exterior material? Proper
drainage and final grading near a foundation are important in
avoiding future problems.
Life Expectancy
Consider the type of foundation, its design and
the site conditions. Was it designed to last forever? Many
people believe that since it is concrete, a foundation should
last forever. Almost everything has a life expectancy, including
concrete. Our area has foundations that are fifty years old or
more with few problems and some with problems only a few years
after construction.
Quality, Standards, and Workmanship
Quality issues apply to every person involved
in the construction process from the builder/developer to the
structural engineer and even the city officials that inspect and
approve of the construction practices. Poor workmanship or
inattention to the circumstances by any person or group that is
involved must be investigated. Foundation experts, structural
engineers, and others can provide their recommendations, but be
aware that a conflict of interest may exist. An unbiased opinion
of the standards utilized and the quality of construction on a
project is crucial. Since many experts in different professions
may also work for others in related fields, it is wise to ask if
the person or company has had any relationship with others
involved, or to hire firms from outside our area.
Proper Watering
The goal in watering a foundation is to provide
adequate and consistent soil moisture under and around a
foundation at all times throughout the year. Under-watering or
over-watering for any length of time can cause problems,
regardless of the soil type. Since North Central Texas has
experienced drought conditions in previous (but recent) years,
many homeowners who did not adequately water may now be paying
the price.
Foundation Types
Conceptually, a slab foundation is basically a
"slab" of reinforced concrete poured on top of the soil. A
pier-and- beam foundation contains "piers", or pillars, of
reinforced concrete with beams of various materials stretched
across the top of the piers (normally a beam of iron and/or
concrete). A slab foundation is more readily affected by the
soil under and around it as well as the soil moisture content. A
pier-and-beam foundation is slightly less affected by soil type
and moisture content, especially if the piers are resting on top
of a rock layer. There are other foundation types, but the
important question to ask is: "What type of foundation has been
installed and is it suitable to the site conditions?"
Trees
Trees can cause problems for nearby foundations
in three primary ways.
-
By physical contact with the foundation.
-
By affecting the moisture content of the
soil under or near a foundation.
-
By causing air gaps and shifting soil due
to decaying roots under or near a foundation (when a tree
dies or is removed).
When there is physical contact between a tree
and a foundation, the damage, if any, may vary greatly according
to the species of tree, the site conditions, proximity to the
foundation, the type of foundation, the portion of the tree
contacting the foundation, and the tree's size. The tree species
is important because a larger, older, longer-lived, shade tree
is a greater concern than a smaller, younger, shorter-lived
ornamental tree or shrub.
This is due in part to the extent of the root
system involved with shade trees. Most trees have root systems
contained within the top 4 to 5 feet of soil, with the absorbing
roots in the top 12 inches. The roots can extend outward beyond
three times the height of the tree. Thus, a tree with a height
of 50 feet, could have roots that grow outward 150 feet from the
trunk. If your tree has limbs near your house, you are likely to
have roots growing under or along your foundation. If you do not
water around the foundation, research shows that roots will have
a tendency to congregate and develop more rapidly under the
foundation simply because of the increased amount of available
moisture underneath.
The type of foundation is important because, as
a general rule, a pier-and-beam foundation can tolerate nearby
trees better than a slab. To understand why, consider the
portions of the tree that contact the foundation and the forces
exerted. A root, root crown, or root flare running under a
foundation can exert an upward pressure as the contacting
portion expands in diameter. The root, root crown, root flare,
or trunk, can also exert an inward pressure if they contact the
outer perimeter of the foundation as they expand in diameter.
Since a pier-and-beam foundation gains its support from piers
rather than just concrete on top of soil, they can more readily
sustain minor pressure from nearby trees. If a large shade tree
is 5 feet from a foundation, physical contact and pressure
exerted against the foundation is likely. If the tree is 15 feet
away, physical contact is likely and pressure exerted against
the foundation is possible. If the tree is 30 feet away,
physical contact is still possible, but not as likely to exert
much pressure against the foundation.
Trees can affect the moisture content of soil
under and around foundations. A large shade tree can transpire
as much as 200 gallons of water per day. Trees "transpire" or
release water into the air in the form of vapor. The water for
transpiration must come from the soil by way of the roots. This
is why trees are known to "pull" water from under a foundation,
but only when water in the surrounding soil is inadequate or
unavailable. Since water naturally moves through the soil from
high moisture areas to low moisture areas, simply not watering
at all can cause foundation problems regardless of nearby trees.
Trees can add to the problem but are usually not the primary
cause of moisture loss from soil.
When trees have died or have been removed, the
roots that remain under or near a foundation can create
problems. As the root system begins to rot and decay, air spaces
and gaps occur in the soil to replace the roots. This results in
a soil that can shift to fill the void. Many years ago,
foundation professionals would see a larger shade tree that is
close to the house and automatically recommend removal. Today,
research shows that removal of the tree is not always the
answer. Since many trees and foundations have existed together
for many years, any changes that occur do so over time and they
can be minor, compared to the removal of the tree. If a large
shade tree, that is close to a foundation is removed, more
dramatic changes can occur to the soil and foundation than if
the tree was left alone, in most cases. Consideration must be
given to the full, long-term effect of removing an existing tree
near a foundation.
When a foundation has a deficiency, many issues
must be considered. Seldom is a tree solely at fault. Cautiously
consider any recommendation to remove a tree as the answer to
foundation failure. In some cases, a tree will offer benefits
that help offset any foundation concerns such as the energy
savings obtained or any of the many other benefits of trees.
Obtain other opinions from numerous consultants in relative
fields.
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