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W H Y C A N ‘T W E B U I L D A
P E R F E C T H O U S E ?
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 construction,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 . Criterium
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