Skip to Content

Why Can't we Build 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.