Electrical & Electric

Floor Construction | Floor Joists

House Floor Options

Overview: There are several different types of floor construction that is commonly used in house construction today. Home construction has come a long way from the earth floor of antiquity to modern concrete slab and modern designs of flooring using floor joists, carrying beams, and engineered I-beams.

All this construction language may be seem strange to you so let me give you some brief descriptions.

Earth Floors:

An earth floor is simply a dwelling that has no flooring other than the earth itself. Earth floors home construction was common up until the twelfth century in Europe and other locations and is still quite common in China, Africa, and regions where predominantly native Indians live through the world. Earth floors are still common in construction of out buildings such as sheds and barns in western cultures.

Wood Floors:

Today most all house floors in western cultures are either cement slab or wood structures. Wood structured floors are the most common when the house has more than one floor and in areas where the ground freezes during winter. Concrete slab construction is almost exclusively used in single story houses built in warmer climates and commercial building construction.

Floor Joists:

The structural members that bridges the foundation walls to support the first and additional levels of flooring are built on floor overlays called floor joists. The floor joists are laid out on the foundation wall at 16” intervals and pass over a center beam to the opposite foundation wall. The center beam may be a laminated wood beam or it is often a steel beam. The center beam sits on steel columns that support the middle structure of the floor. Once the floor joists are in place an outside piece of material is attached to the ends of the floor joist forming an outside box or frame to the structure.

The floor sheathing is attached to this maze of floor joists. The size of the floor joist depends on the span of the floor and the weight of the building. Typically in a one story 24 foot wide one story structure the floor joists and box beam are made from 2 X 8s. As the structure weight increase so does the size of the floor joist. Floor joists made of 2x10s and 2x12s is common in large houses with wide spans. The length of the floor joists depends upon the width of the house. If a house is 28 feet wide there are two lengths of material used to form the joist, usually sixteen feet long and spliced together on top of the carrying beam.

Engineered Floor Joists

Engineered floor joists are very common today. Rather than a solid pieces of 2 by 8,10, or 12 lumber, the engineered joist is a wood I-beam with a bottom and top piece of lumber, usually a 2x4, with a ½ inch piece of plywood glued on edge between the upper and lower piece. They are much stronger than traditional floor joist and can a bridge a much longer span without center beam supports. They usually have knock out plugs in the plywood (web) to facilitate running electrical wiring and plumbing. This saves a lot of time cutting and drilling holes for the utility systems that run in the basement. Engineered I-beams are also cheaper than the equivalent size solid piece floor joists.

In determining the size of a traditional joist and design of an engineered floor joist, the compression load and flex in the joist must be taken in consideration. If you walk through a house and you can feel the floors bounce as you walk it is a fair indication the floor joists are inadequate or the span is too great. Over time this may lead to structural failure of the floor.

If you are building your own home and need further guidance than the building plan provides, consult with your local code officers. He can advise you of code requirements of the floor joists for your home.

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