by Don Vandervort, © HomeTips
Like the human body, a house has a skeleton that gives it support, shape, and a framework for outer coverings. In fact, a house’s skeleton is called a frame.
Though some new homes utilize steel framing, most houses are made of wooden beams, floor joists, walls studs, roof rafters, and related components. To ensure the structure’s strength, these parts are sized and connected in accordance with building codes that are based on basic load-engineering principles.
It’s important to know which parts of a house’s frame are critical to its structure so that you don’t compromise its strength when remodeling. For example, if you remove part or all of a load-bearing wall without reinforcing the structure elsewhere, the floors and roof may sag and the windows and doors may stick. Even worse, part of the house may actually collapse.
Non-bearing walls may be either perpendicular or parallel to joists or rafters. They often may be identified from under the house because they’re not supported by a foundation or beam. Because they don’t support loads, they usually can be removed without compromising a structure’s strength.
The foundation and footing deliver loads from the house down to solid soil. The footing is wider than the foundation to spread out loads.
The weight of roofing materials and loads compresses roof rafters, which pull ceiling joists from each end, placing them under tension. The resulting triangle transfers roof loads out to rafter ends, where the weight is carried by supporting walls.
Standard Wood House Framing
Most houses built since the 1920s have wood-frame construction, a system of wooden wall studs, floor and ceiling joists, and other wooden members that provide the house’s structure and a framework for attaching finished surfaces.
The high cost of lumber is fueling an interest in steel and other alternatives, but wood is still, far and away, the most popular framing material. In most cases, even houses that appear to have brick or stone walls actually have wood-frame construction beneath their masonry facades.
There are two basic framing methods: platform and balloon construction. Platform construction (shown above) is much more common than balloon framing (shown at right), though balloon framing was employed in many two-story houses before 1930. With platform construction, walls sit on top of subflooring. Multi-story houses are built one level at a time, with each floor providing a platform for building the next series of walls. Nearly all contemporary houses are built using the platform construction method.
With both methods, wall studs and ceiling and floor joists occur every 16 or 24 inches, measured from center to center. These standardized layouts result in the least amount of cutting and waste in floor, ceiling, and wall materials. Most older houses have 2-by-4 wall studs spaced 16 inches on center; many newer houses have 2-by-6 wall studs either 16 or 24 inches on center to make exterior walls stronger and allow a larger cavity for insulation.
Exterior wall sheathing adds rigidity to the structure and provides a flat base for siding, stucco, or other exterior wall finishes. Older homes have diagonal sheathing—1/2-inch-thick boards nailed on the diagonal. Most newer homes have plywood or similar composite panel sheathing.
Exterior roof sheathing serves the same purposes for roofing. Most contemporary roof sheathing is either plywood or oriented-strand-board (OSB) panels; spaced wood sheathing is common for wood shingle roofs.
Conventional house walls have an inner wooden framework. This framework may or may not support part of the house, but it does support wall coverings, windows, and doors. And it provides cavities for electrical wiring, plumbing, heating ductwork, insulation, and other utilities.
Though most walls are framed with 2-by-4 wall studs, 2 by 6s are used to provide more strength and/or larger cavities to house the various utilities. Exterior walls, for example, may be framed with 2 by 6s to allow generous room for insulation; some bathroom walls are framed with 2 by 6s to allow plenty of space to accommodate large pipes.
Not all houses are framed with wooden wall studs. Some are built with metal studs, a practice adopted from commercial construction methods.
Windows have a single or double sill across the base, made up of 2 by 4s laid flat. Trimmer studs support each end of a header, and cripple studs fill in the areas above and below the openings. Cripple studs are placed on the 16- or 24- inch stud layouts.
Wherever windows, doors, or other openings occur along a wall, the regular studs are eliminated. Instead, a small beam, called a header, spans across the top of the opening.
A double top plate—two 2 by 4s or two 2 by 6s laid flat—caps the top of the studs, locking them in position, giving the wall rigidity and providing a backing for nailing on wall-covering materials. At the wall’s base, studs are nailed to a 2-by-4 or 2-by-6 sole plate to lock them to the floor.
The wall frame generally consists of 2-by-4 or 2-by-6 wall studs placed vertically every 16 or 24 inches on center. Extra studs provide nailing areas and sturdy support wherever walls intersect, such as at corners.
Floor & Ceiling Framing
Though some floors are built on a concrete slab flat on the ground, most floors are raised above the ground. Raised floors are more resilient underfoot and provide access for heating equipment, insulation, plumbing, wiring, and other mechanical equipment.
A raised floor is constructed with a wooden framework that bridges from one exterior wall to another. This framework may or may not be supported intermediately by girders, beams, or walls. On upper levels of a house, the underside of the floor framing generally serves to back ceiling materials. Ceilings are usually built just like floors, only they may be constructed of lighter materials because they’re not intended to carry the same loads.
A floor’s framework is made up mostly of wooden joists that run parallel to one another at regular intervals. Floor joists are typically 2 by 8s, 2 by 10s, or 2 by 12s; ceiling joists are usually smaller—2 by 6s or even 2 by 4s in older homes. Some newer homes have manufactured, I-beam-shaped joists.
Floor joists, spaced on regular intervals, span the areas between supports such as walls, foundations, girders, and beams. Normal spacing is 16 inches on center, though some floors may have joists on 12-inch or 24-inch centers. Joist sizing and spacing are determined by building codes, which are based on engineering requirements.
Joist headers run perpendicular to joists, capping their ends.
At foundation level, floor joists rest directly on a sill that is treated with preservative so contact with the foundation will not promote termites or rot. Exact construction and connection with wall studs depends upon the method of framing.
Subflooring provides a base for finish flooring and also serves as a platform during construction. It may be made of boards laid either at right angles or diagonally across joists. Or the subfloor may be made of plywood or other panel products that are laid perpendicular to joists.
Solid blocking or metal bridging prevents joists from twisting and helps distribute loads evenly.
Joists are spliced over beams or other supports. They may be butted end to end and connected with plywood gusset plates, or they may be lapped.
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