Foundation Design--Design and Modification--Home/Apartment Renovations--TECHNICAL DECISIONS

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This section on foundations is aimed at those of you who are planning an extension. Foundations are the structural elements that transfer all the loads (including the structural weight of the house) to the surrounding ground, giving the house stability and permanence. They literally root the house to the soil. A sound foundation is crucial to the project. For this reason, it's important to have a good understanding of how foundations work.

Foundations can take many forms. Early Colonial buildings were constructed on top of a few flat stones, which served to isolate the building from the ground and prevent the wood from rot ting. The settlers soon learned that this method did not provide them with a stable house. Either bad storms and uneven settling ripped the house from the ground or the walls were destroyed by rotting. Very few, if any, of these houses remain. Another type of foundation is seen in fishermen’s houses in New England’s coastal towns; these homes are built over the water resting on wooden piles—the only way in which a house could be secured to the underwater bedrock.

Because foundations are a crucial component of the structure continuum, it's important to study how they transfer the house loads into the surrounding ground. There are two basic types of loading conditions: point loads and uniformly distributed loads. A pole vaulter making his jump clearly illustrates both of these principles. While preparing to jump, he runs, grabs on to the pole, and places his entire weight on it. His weight is being transmitted to the ground at only one point. Once he completes the jump, he comes to rest lying flat on a mat. The ground now receives his weight at many points along this mat; it's being transmitted uniformly. Similarly, foundations can transmit loads to the ground at points or uniformly.

In addition, the design of the foundations and the selection of materials are determined by the type of soil on which the house is built, the topography of the site, the nature of the house (a year-round house vs. a vacation cabin), and the availability of materials and labor.

Foundations are relatively simple to design when compared to roofs or floor systems. Nonetheless, there are so many different kinds of sub surface conditions (all underground where it's impossible to see them) and other variables that you are advised to seek out the help of an engineer when designing foundations.

Foundations may be classified into three basic types: continuous foundations, piers, and piles. The most often used of the three is the continuous foundation wall. The wall, completely or partially buried in the earth beneath the house, follows the perimeter of the house and rests on a continuous base called the footing. The loads of the walls, floors, and roof are uniformly distributed along the foundation walls. These walls, in turn, evenly transfer all the loadings to the surrounding earth. Continuous foundation walls are usually built of concrete poured into formwork or of concrete blocks. Many historic houses may have stone foundation walls. These walls, although beautiful, are extremely labor-intensive and difficult to replicate.

Piers and piles are essentially fingers gripping the ground, holding the structure in place and transmitting loads to the ground at various points. Piles (used mostly for construction in wetlands or marshes) are driven, or hammered, into the earth. They are commonly made out of chemically treated tree trunks. Piers, like continuous foundations, are built on normal soils and are cast or laid in place rather than driven. They are usually constructed of poured concrete or masonry block.

FOUNDATION TYPES

Continuous Foundations

Continuous foundation walls are the best choice in the majority of cases and most likely are used to support your existing house. (They are suited to all soil types and terrains with the exception of marshlands, water, and very steep slopes.) Continuous foundations distribute the loads evenly to the earth. In addition, foundation walls help insulate the floor from drafts and serve to hide and protect the electric, heating, plumbing, and gas lines.

Continuous foundations are built in two separate stages: first the footing and then the wall. The footing (like its namesake) distributes the weight of the structure over a larger area of earth. The spreading of the foundation at the bottom puts more earth directly under the weight, which offers more resistance to the downward push. Footings are always constructed out of poured- in-place concrete. The foundation wall which rests on the footing may be constructed of poured concrete or concrete block. The height of the wall itself is determined by the depth of the frost line and whether or not there is a basement or a crawl space.

The choice between poured concrete or concrete block is contingent on whether your extension is to have a full basement or a crawl space. A poured-in-place concrete wall, constructed properly, will be stronger and is less likely to crack and leak than a block wall. The work and expense involved, however, are considerable drawbacks. Concrete block is much easier to work with than poured concrete and , in addition, is less expensive. If you are not considering a full basement (and we don't recommend a full basement for your extension), block is the best choice. Steel reinforcement should be added to both footings and concrete-block walls.

FROST LINE: Frost line is the depth to which frost penetrates and freezes the ground. Foundations placed above this line are in danger of cracking as the ground beneath them expands and moves due to freezing. Local codes will advise you of the depth of frost penetration in your particular area. The NationalBuilding Code requires foundations to be located at a minimum of one foot below the frost line.

Pier Foundations

An alternative to continuous walls, pier foundations save time, labor, and money. They are particularly suitable for steeply sloping terrain or where excavation should be kept to a minimum. A word of caution: In cold or temperate climates the exposed plumbing lines underneath the floor are in danger of freezing. In addition, the heat loss through the floor is significant (unless you insulate the floor very well). Pier footings would be suitable for those renovators planning porch ex tensions, or extensions where plumbing lines (if any) can be routed through the existing basement. They are also applicable for summer homes (where plumbing lines are drained of water during the winter) or in warm areas not subject to freezing temperatures. The other problem of pier footings is uneven settling. All piers must rest on a similar soil bed to avoid uneven settling and subsequent cracks in the structure of the house.

Piers are most commonly constructed of treated wood, poured concrete, or concrete block, and vary in shape from a simple column to a column resting on a footing. Treated-wood piers are good for light-frame buildings such as bungalows or toolhouses. They are not as durable as masonry piers and eventually will have problems of rot and decay. Many building codes don't allow their use for house construction. We don't recommend them. Masonry piers, on the other hand, whether constructed of poured concrete or concrete block, offer more stability and are not subject to rotting.

Poured-in-place concrete piers are stronger than their concrete-block counterparts. This is due to the homogeneity of concrete as a material. They do, however, require more labor and are more expensive. The expense is due to the fact that you must purchase both the lumber for the formwork and the ready-mixed concrete. One way to simplify the labor involved in poured-in- place foundations is to use spiral cardboard forms (also referred to as Sono-Tubes, a brand name). These forms come in many diameters and are placed over the footings. A tie between the footing and the pier is provided by means of vertical reinforcing bars. These tubes must be braced with lumber before the concrete is poured.

Concrete-block piers are relatively simple and inexpensive to build. Blocks are easy to handle and can be stored on the site, eliminating the problem of having to build all piers on the same day (as in the case of poured concrete). Building with concrete block can be a one-person job. Reinforce the pier with steel bars and fill the holes in the block with cement mortar.

If you are building piers with footings, you will have to purchase footing lumber and pour concrete. (Check to see if the company sells that small a quantity of concrete.) The formwork for footings, however, is very simple. Although this type of pier requires more work than a simple column pier, it will give you a considerably better distribution of loads and consequently a more stable structure. Your job can be simplified by pouring the footing and then building the pier itself out of concrete block.

Pile Foundations

Piles are particularly effective for building on marshlands or in water. They can be constructed of treated wood, poured concrete, or precast concrete. (In large-scale construction steel beams are used, but this is far too sophisticated for small- scale residential work.) Treated-wood piles are widely used for beach houses. Poured or precast concrete piles are also used, but although more durable, they are considerably more expensive. Driving or casting piles is not easy and requires heavy machinery to drive the wood or precast concrete piles into the ground or to drill the holes for pouring concrete. If your extension requires such a foundation, hire a company that specializes in driving piles.

Variations

One of the likeliest variations is that subsurface conditions may vary from one corner of the house to another. A licensed professional engineer who specializes in subsoil investigations should be consulted. He or she will do test borings of the soil to determine its bearing capacity.

We have limited our discussion to simple pier, pile, and continuous foundations. There are, of course, numerous variations and combinations of these. It is likely that your extension may have a different type of foundation than the main house. For example, a house can have a partial full basement to enclose the boiler while the rest of the building sits on a crawl space or piers. Continuous foundations can be stepped when built on sloping ground to facilitate excavation. Combinations of continuous and pier foundations are also possible. On steeply sloping sites, one of the foundation walls might serve as a retaining wall. (This involves a more complex design procedure. We suggest that, if faced with this condition, you consult a professional as insurance against costly errors.) While making design decisions, keep in mind the benefits and drawbacks of each type of foundation. Try to keep the construction work as simple as possible.

DESIGNING THE FOUNDATIONS

The size and depth of the foundations are directly related to the bearing capacity of the soil, the house loads, the underground water table, and , in cold and temperate climates, the frost line. Since the loads on residential design are light, foundation sizes are rarely determined mathematically.

In many areas, the local building code will tell you the sizes required depending on the house design and the type of soil you are dealing with. In addition, since you are working with an existing house, you already know the size of the existing foundations. Unless the house is settling badly, the foundations are cracked, or the subsoil conditions vary significantly, the size and depth of the existing foundations are probably adequate.

In the event that your local code has no requirement for foundation sizes or there is no local code, the following are some helpful guidelines. Since the width of the foundation wall is related to the wall directly above, it would vary from approximately 10” to 12”. For instance, in wood-frame construction, the outside walls will generally vary between 6” and 7” wide; therefore a width of 10” will be adequate for the foundation wall. (We don't recommend anything less than 10” since the foundation wall has to be able to sustain the pres sure from the surrounding earth.) In the case of a brick-bearing wall, which is wider, you will most likely need a 12” foundation wall.

A good rule of thumb for sizing footings is to make the footing width twice the width of the foundation wall and its depth the same as the width of the foundation wall. For example, if the foundation wall is 12” wide, the footing should be 24” wide and 12” deep. (In poor load-bearing soils such as clay, footings carrying heavy loads may have to be made wider in order to spread the load over a larger area.)

Foundation depth depends on whether you have a basement or a crawl space, the distance to suitable bearing soil, the frost line, and the under ground water table. The bottom of the footing should be above the water table to avoid leakage problems and at least 1’ below the frost line. (Consult your local code.)

It is important that you find out whether or not your house was built on fill. You also need to know whether the soil surrounding the house where you are planning your extension is also fill. If the soil is fill, you run the danger of uneven settling. The reason for this is that the existing house has had ample time to settle. The new ex tension will also settle, but at a different rate than the house. You need to bring the foundations down to undisturbed earth. To prevent damage to your existing foundations (see next section), we strongly suggest that you limit your design options to either a crawl space or piers. As a general rule, the shallower* the new foundation, the easier the job will be (in terms of both construction and danger to the existing building). The new foundation will need to be stepped down to the level of the existing foundation. The reason for this is that the earth close to the existing foundation is made up of backfill from the original excavation. If the new footing is higher than the existing one, the new foundation will be resting on fill and is likely to settle. (Don’t go below the level of the existing foundation! See discussion on protecting the existing foundation below.)

Because foundations are so critical to both the new extension and the existing building, we feel that it's worth your while to engage the services of an architect or engineer. Professional advice will help you avoid costly mistakes.

Protecting the Existing Foundation

Foundations often fail when the earth around them is disturbed. Take every precaution to en sure that the existing foundation will not be disturbed when the new ones are built. Recently there were two building collapses in New York City. In both cases, there was excavation taking place dangerously close to the existing foundations.

New foundations should not be designed to be deeper than the existing ones. There is sound logic behind this. When the new foundation is deeper than the existing one, the excavation goes further down than the level of the existing foundation. In the process some of the earth which now supports the foundation is removed or disturbed. The remaining earth is not able to sup port the loads and the foundation will fail.

*Remember that the footings must be below the frost line.

WATERPROOFING

A few words should be said about waterproofing techniques. If your building sits on piers or piles, you don’t need to concern yourself with water proofing. Houses sitting on crawl spaces are also generally exempted from the headaches of water proofing, unless they happen to rest on soils of such wetness that the local code or general local practice demands it. When the extension sits over a full basement, however, you have no choice but to deal with waterproofing (particularly if the foundation walls are built of concrete block or fall below the water table).

For average soil conditions, black asphalt, tar, or pitch, or one of a variety of other bituminous compounds (usually referred to by trade names), is applied on the exterior of the foundation walls. Although this does not offer a complete guarantee against leakage, it may help. A much better technique is membrane waterproofing. This type is used in wet soils and consists of alternate layers of hot pitch (tar or asphalt) and felt, which are carefully lapped to the exterior of the foundation wall. Membrane waterproofing offers more protection than any of the rolled-on types but is more expensive.

In addition to either of the above techniques, footing drainage pipes are placed at a slight pitch adjacent to the footing and following the perimeter of the house. A layer of gravel or crushed stone is placed all around the pipe. These pipes may be of porous cylindrical clay (without hubs) or perforated black plastic. Their function is to collect all the excess water accumulated around the footing’s perimeter and divert it (generally to a storm sewer, a dry well, or another location on the site), thus preventing water pressure from building up next to the foundations.

Consult the local code for recommendations or requirements in your particular area. Don’t forget, however, that the effectiveness of any water proofing method is directly proportional to the thoroughness of its application.

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Updated: Thursday, June 7, 2012 8:23