VOLUME 18, NUMBER 4
TIPS FOR SUCCESSFUL HOME OWNERSHIP
If
you see some cracks and suspect that your home is moving, take some close-up
pictures of the cracks. Take the same
set of pictures every three months and compare them. This will help you determine if there is movement actually
occurring. What we think we see can be
deceiving. If you ultimately hire a
consultant to diagnose the problems, a photographic history will be useful
information.
In This Issue
WHY CAN’T WE BUILD A PERFECT
HOUSE?
FOUNDATION TYPES
WHEN THINGS GO WRONG -
YOUR HOUSE IS MOVING AND YOU DON’T KNOW WHAT TO DO
For More Information
Call our local office or visit our Web site at:
http://www.criterium-engineers.com/
WHY CAN’T WE BUILD A PERFECT HOUSE?
This question will be an occasional YOUR HOME
topic. Whether buying a brand-new home
or a “used” home, many people expect perfection. With that expectation, they will be disappointed. There simply is no such thing as a perfect
house.
To start with, what is perfect? To each owner and builder, that probably
means something different. Is being
precisely square, level, and plumb perfect?
Are smooth, uniform, unblemished finishes perfect? Is an expected service life of 100 years
perfect? Is a virtually
maintenance-free exterior perfect? If
any of these criteria define a perfect house for you, then you will be
disappointed.
We suspect that Mother Nature and Mother Earth get
a good laugh at our futile efforts to build the perfect house. There are probably endless reasons why
building the perfect house is an impossible goal. We believe the following to be the most fundamental:
1. We build our houses on the ground. Mother Earth (the ground) is unpredictable. In this issue, we explore this topic in more
detail.
2. We build most of our houses with wood. Wood is organic. Mother
Nature did not design wood to be a predictable, reliable building
material. It is dimensionally unstable
and changes shape as it ages and as temperature and humidity change.
3. Human beings build our houses.
Human beings are not perfect.
Further, there seem to be fewer
and fewer skilled human beings available to help build houses.
4. We build our houses outside.
That’s the worst practice of all!
The same Mother Nature that gives us imperfect wood with which to build
our houses then throws unpredictable weather at us while we build.
In
this issue, we will take a look at the Mother Earth issue and determine how we
can minimize the vague and unpredictable character of the ground on which our
homes rest.
FOUNDATION TYPES
The
foundation serves several purposes, but the most important is to transfer the
weight of the house and its expected occupants (along with all their “stuff”)
to the ground under the house. A good
foundation will do this by transferring that weight uniformly to the
ground. By doing so, the risk of
differential settlement (one spot moving more than another) is minimized.
In
some parts of the country, foundations also serve to enclose basements and/or
crawl spaces. These spaces provide more
places to put our “stuff” as well as utilities such as electrical wiring,
plumbing, and heating components.
For
purposes of this discussion, we will stick to the type of foundation that does
not provide extra utility space commonly known as a slab-on-grade
foundation. Such foundations are common
throughout the country but more common in the Southern states, from east to
west. There are several basic types.
Conventionally
Reinforced Monolithic Slab Foundation -
This is the traditional slab-on-grade foundation. It consists of cast-in-place concrete poured directly on the
ground, hopefully after first compacting the soil, placing polyethylene on the
ground, and installing some steel reinforcing.
Following the pouring of the concrete, it is important that it be
“cured” well by keeping it wet and controlling the temperature. Curing concrete is somewhat like baking
bread. Both take the right combination of ambient conditions (temperature and
moisture) applied for an optimum period of time to produce the desired
results. Any significant deviation from
the proven “recipe” may lead to a completely unsatisfactory outcome.
Grade
Beams, Frost Walls, etc. - Depending on the soil and the climate it is often
desirable to add a vertical element to the perimeter of the concrete
slab-on-grade. This is done for a
variety of reasons: to stiffen the edge
of the slab, to provide more load carrying capacity under the perimeter walls
of the house, and to protect against frost penetration under the slab. Frost penetration is Mother Nature’s way of
making Mother Earth irritable and even more unpredictable.
Post-tensioned
Slabs - This type of slab is the result of more recent technological
developments. Post-tensioned slabs
include reinforcing cables within the slab that are tensioned (stretched) after
the slab has cured and effectively increase the strength of the slab (its
ability to ignore differential movements in the ground under it). Post-tensioned slabs combine onsite
construction advantages with the extra strength of “tensioned” reinforcement.
Because
our homes are built by imperfect human beings, any one (or all!) of the steps
necessary to produce a good slab-on-grade foundation may be compromised or
omitted, resulting in a marginal or inadequate concrete slab. In addition, for post-tensioned slabs, a
skilled construction crew familiar with the proper procedures is very
important. If the reinforcement is not
“tensioned” and secured properly, the slab may actually perform worse than a
conventional slab. Ultimately, the
skill and attention of the crew are very important for any system.
From
the ground up - that’s how we build our houses. However, we usually do very little to determine the
characteristics of the ground (soil) before we build. Compressive strength, water level, moisture content, grain size,
and many more variables affect the ability of the ground to support a
building. One or more of these
variables may exist at different locations under the home. As a result, we get differential settlement.
To
minimize the risk of unpredictable soil performance, we should evaluate the
soil before we build. While that may
spoil the “adventure of building,” it will minimize the agony of floor slopes
and cracks later.
Evaluating
the soil involves taking soil samples.
Samples are usually taken by boring in the earth to get a range of
samples at different, controlled depths.
Borings also ensure uniformity in the sample size which makes the
subsequent analysis more reliable.
These samples are then analyzed by a soils lab to determine bearing
capacity (how much will it hold up), plasticity index (how sensitive it is to
changes in moisture content), and more.
Soils
evaluation is usually performed by a geotechnical engineer. It is important to work with someone who is
well qualified. Soils are difficult to
evaluate and future performance is a challenge to predict.
WHEN THINGS GO WRONG - YOUR HOUSE IS MOVING AND
YOU DON’T KNOW WHAT TO DO
Consider the following:
• How bad is it? As we said, no house is perfect.
Some movement should be expected.
Generally, slopes of less than one inch in ten feet (some standards use
1/2 inch in four feet, which is similar) are considered acceptable. Cracks of 1/16 inch or less in width are
probably normal, depending on the type of slab.
• What
is the cause? While soil movement is
often the problem, a change in moisture content due to weather changes or leaky
plumbing can cause reactions in the soil that lead to movement of the
house. It is important to identify the
cause before deciding on a solution. If
it is external (water leak, change in weather, etc.), you must first eliminate
(or stabilize) the cause. Otherwise,
any repairs you undertake will soon be undone.
• Stabilize
or restore? In most cases, the most
practical goal with foundation settlement or movement is to stablize the
condition; in other words, minimize the risk of future movement. While some restoration toward a level
condition is possible, achieving a completely level slab after stabilization is
quite unlikely. As with any aspect of
constructionm, perfection is elusive.
• Which
repair system? There are many ways to
stabilize a slab. The most common is a
pier system which puts vertical supports under the edge of the concrete slab
(or perimeter grade beam) that extend down into the soil far enough to
establish a stable base. These piers
are usually spaced four to eight feet apart around the perimeter of the
slab. Many contractors use piers and
piles interchangeably. For the purpose
of this type of foundation support, the components are essentially the same
whether called piers or piles. There
are many commercial proprietary systems for this type of repair.
The foundation is one of the most important
components to help ensure a long, problem-free life for your home. Unfortunately, until we achieve some
technological breakthrough, the duo of Mother Earth and Mother Nature will continue
to enjoy wreaking havoc on our best efforts to stabilize the ground under and
around that foundation. The best results come from thoroughly evaluating the
soil before building. The more you
know, the fewer surprises will come later.
You
should consult a licensed Professional Engineer to assist with the selection of
an appropriate repair system. If you
are not sure about the significance of your problem, you should consult someone
who has the training and experience to help.
A licensed Professional Engineer with experience in soils and existing
structures is your best choice.
YOUR HOME is
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staffed exclusively by registered engineers and architects — professionals
committed to serving your needs. CRITERIUM ENGINEERS was founded in 1957.
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