Foundations: An Overview
A sturdy foundation is the first step to a well-built home.
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A foundation anchors your house to the earth, holding it up and, just as importantly, holding it down. Surprisingly, houses are as likely to blow away in a windstorm as they are to sink into the ground. But foundations do more than just hold a house: they must resist water, ice, fungus, insects, and soil gases, and they have to stand up to soil pressures that can exert far more force than the weight of the house itself.

When they are working well, foundations escape notice, quietly doing their thing below grade. But when they begin to fail, they can be the source of trouble and expense. Even a quarter inch of uneven settling can crack stucco or tile, separate brick joints, cause windows and doors to stick, or cause floors to sag and slope. And these are the small problems! Buildings on sites with unstable soils or incorrectly placed fill can have big problems -- heaved slabs, buckled walls, and foundation movement that can threaten the whole building. So, wise builders anticipate and prevent all but the slightest foundation movement, because fixing foundation problems always costs more than doing the job right in the first place.
Slab foundation
Slab-on-grade foundations are simple to build are simple to build because the floor and footing can be poured as a single unit.
Analyzing the Site
Before you select a system or make design choices, familiarize yourself with the existing conditions on site. Good soil conditions make everything easier, which is why the signs read "Clean fill wanted." Sure, bad soils can be dealt with, but to do it you'll need a plan to match the situation. If it's a remodel project, you'll need to learn as much as you can about the original foundation so that you'll know how to properly address structural and moisture concerns. How old is the foundation? Does it show signs of movement or moisture intrusion? If it's stone or block, is the mortar in good shape? If it's concrete, is it reinforced? It is waterproofed? How high is
the water table on your lot, and where does water runoff go?

When building new, it's important to investigate the soils on the site, both at the surface and below it. Is your soil primarily gravel, sand, silt, or clay? (Sand and gravel are the best, clay is generally the worst.) How deep can you dig before you hit standing water (the "water table")? Is there bedrock just below the surface, or a soft zone where people have dumped soil or debris? Any of these conditions could complicate excavation and increase the cost of the project.
Keeping Dry
Foundations are
n't just structural. They are often the basements we work, live, and store our belongings in. But basement or no, foundations need to stay dry. Leaky basements with puddles are just one problem; water vapor is another issue. Because they're always touching the moist earth, foundations can bring a lot of humidity into the home even when no actual water is leaking in. So, you should include good drainage, waterproofing, and a vapor barrier in your plans for a new foundation. In remodels (even above ground), simple steps such as maintaining your gutters and downspouts, regrading your yard to direct rainwater away from the house, and placing plastic sheeting on the floor of your crawl space can help safeguard your new structure and finishes from moisture attack.

Lastly, the foundation is the base on which everything is built, so be careful about accuracy. Taking the time to make sure the foundation is plumb, square, level, and straight will pay off later, because framing and finishing will go much more smoothly.

 

Basement foundation
A basement foundation is basically a slab foundation with walls and a floor. The walls can be either partially or fully below grade (ground level).

    

Foundation Types
The foundation system you choose, and the construction methods you will follow, depend on the site, the climate, your needs, and your budget. Whether you're building new, adding on, or remodeling, you have options for the foundation. Traditional technologies like poured concrete or concrete block are still good, but there are more recent innovations you should look at. Post-and-pier ("sonotube") techniques are quick and inexpensive for room additions and decks. Frost-protected slabs on grade conserve energy and save money by eliminating the need for a deep frost-wall. Insulated concrete forms are so lightweight that almost anyone can form up an insulated basement. And on the right site, treated lumber ("permanent wood") foundations can be ideal for building a warm, dry finished basement with no concrete, so you can work in below-freezing weather--worry-free.

For more challenging sites, you'll need a more advanced system. Oceanfront homes, houses on steep hillsides, and houses on sites with expansive clay often require special solutions. So be prepared to engage an engineer and a contractor who specializes in your chosen system.

 

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10 February, 2003

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