Foundations

All foundations must transmit the building loads to a stable stratum of earth. There are two criteria for stability: first, the soil under the foundations should be able to receive the imposed load without more than about 2.5 centimeters (one inch) of settlement and, second, the settlement should be uniform under the entire building. It is also important that the bottom of the foundation be below the maximum winter frost level. Wet soil expands as it freezes, and repeated freeze–thaw cycles can move the building up and down, leading to possible displacement and damage. Maximum frost depth varies with climate and topography. It can be as deep as 1.5 meters (five feet) in cold continental climates and is zero in tropical and some subtropical areas. The foundation systems for low-rise residential buildings are suitable for their light loads; nearly all are supported on spread footings, which are of two types—continuous footings that support walls and isolated pad footings that support concentrated loads. The footings themselves are usually made of concrete poured directly on undisturbed soil to a minimum depth of about 30 centimeters (12 inches). If typical continuous concrete footings are used, they usually support a foundation wall that acts either as a retaining wall to form a basement or as a frost wall with earth on both sides. Foundation walls can be built of reinforced concrete or masonry, particularly concrete block. Concrete blocks are of a standard size larger than bricks and are hollow, forming a grid of vertical planes. They are the least expensive form of masonry—using cheap but strong material—and their large size economizes on the labor required to lay them. Their appearance and weathering properties are inferior to those of fired masonry, but they are satisfactory for foundation walls. In some places timber foundation walls and spread footings are used. Excavation for foundations is the most highly mechanized operation in this building type; it is done almost entirely with bulldozers and backhoes.

Structural systems

Timber frames

In these small buildings the ancient materials of timber and masonry are still predominant in the structural systems. In North America, which has abundant softwood forests, light timber frames descended from the 19th-century balloon frame are widely used. These present-day “platform” frames are made of standard-dimension timbers, usually two or four centimeters (0.75 or 1.5 inch) thick, which are joined together by machine-made nails and other metal fasteners using hand tools.

The first step is to construct a floor, which rests on the foundation wall. A heavy timber sill is attached to the wall with anchor bolts, and on top of it are nailed the floor joists, typically 4 × 28 centimeters (1.5 × 11.25 inches) and spaced 40 centimeters (16 inches) apart. The span of the floor joists is usually about 3.6 meters (12 feet), which is the common maximum length of available timbers. The floor may need intermediate supports in the form of interior foundation walls or, if there is a basement, intermediate beams of wood or steel supported by the foundation walls and columns. For longer spans, floor trusses can be made, with members joined by nail grids or nailed plywood gussets or with wood chords and diagonal metal web members. On top of the joists is nailed plywood subflooring, which forms the deck and gives lateral stability to the floor plane.

The exterior bearing walls are made of 4 × 9-centimeter (1.5 × 3.5-inch; “2 × 4”) timber verticals, or studs, spaced 40 or 60 centimeters (16 or 24 inches) apart, which rest on a horizontal timber, or plate, nailed to the floor platform and support a double plate at the top. The walls are sheathed on the outside with panels of plywood or particleboard to provide a surface to attach the exterior cladding and for lateral stability against wind. Plywood and particleboard are fabricated in panels of standard sizes. Plywood is made of thin layers of wood, rotary-cut from logs and glued together with the wood grain running perpendicularly in adjoining layers. Particleboard consists of fine wood chips mixed together in an adhesive matrix and allowed to harden under pressure. On top of the wall plate is placed either a second floor or the roof.

Since most of the roofing materials used in these buildings are not fully watertight, the roofs must have sloped surfaces to rapidly drain off rainwater. Sloped forms are created by two methods. The traditional method uses joists similar to those of floor construction to span between exterior walls. Rafters are nailed to the ends of each joist and the rafters meet at a central ridge member, forming a triangular attic space. Where no attic space is needed, it has proved more economical to span the roof with triangular trusses with interior web members. These roof trusses are usually made of narrow timbers joined by nails, glue, or metal connectors, and they are often prefabricated in a workshop. Plywood or particleboard sheathing is then nailed to the roof surfaces to receive the roofing and to provide lateral stability, making the entire frame into a rigid box.

Light timber frames are quite flammable, but small one- or two-story buildings are easy to evacuate in case of a fire, and building codes permit the use of these frames with such features as fire-resistant gypsum board on the interiors and fire-stops (short wooden members) between the studs. Timber structures are attacked by certain species of insects—such as termites and carpenter ants—as well as certain fungi, particularly in warm, moist climates. Wood can be chemically treated to discourage these attacks; other precautions include raising the timber above the ground and keeping it dry.

Enclosure systems

Enclosure systems for this building type are varied. For roofs, traditional wood shingles or, more commonly, felt asphalt shingles are used, as are semicylindrical clay tiles and standing-seam metal roofs. Rainwater from roofs is usually caught in metal gutters and directed to exterior downspouts that discharge onto splash blocks or into underground drains connected to storm sewers.

The wall surfaces of low-rise residential buildings are clad with a range of different materials. Traditional wood elements such as shingles and horizontal shiplap, or clapboard siding, are used on light timber frames as are vertical tongue-and-groove siding and boards and battens. Aluminum and vinyl sidings have been adapted from these wooden forms. Brick and stone veneer are also applied over timber and anchored to it with metal fasteners. Cement plaster, or stucco, is another traditional material used to enclose both timber and masonry structures, and its semiliquid application allows great plasticity of form. A more recent development is a very thin synthetic resin stucco applied directly to the surface of rigid plastic foam insulation.

Insulation, which slows the rate of heat transfer through the enclosure, is usually applied at all exterior building surfaces that are exposed to air. There are two major types of insulation, rigid and nonrigid. Rigid insulations are primarily plastic foams (the dead air in the foam cells is the true insulator), which vary in thickness from 2.5 to five centimeters (one to two inches). They include styrofoam, used primarily below grade behind frost walls due to its low fire resistance; urethane foam; isocyanurate foam, which has the best fire resistance; and foam glass. Nonrigid insulations are usually made of fiber—glass fiber being the most common—often with a foil-backed paper on one side. Fiber insulations are made in thicknesses up to 23 centimeters (9.25 inches). The effectiveness of an insulation material is measured in terms of its heat-transfer rate, or U-value, often expressed as the number of BTUs passing through a given unit of insulating material each hour at an expressed temperature differential across the material. Low U-values indicate good insulating properties of the material. U-value is an inverse function of thickness, so that there is a limit to the cost-effectiveness of increasing the amount of insulation on a surface. Rigid insulation panels are applied to vertical wall sheathing and the surfacing material is fastened through the insulation, or it is applied to horizontal roof decks. Glass fiber is usually applied in the spaces between wall studs and between roof joists or the bottom chords of roof trusses.

Most low-rise residential buildings have a limited number of transparent openings in their exteriors, because of the traditional requirements of interior privacy and the relatively higher cost of windows compared to opaque walls. The traditional wooden frames of domestic windows are often clad in extruded vinyl or aluminum cladding, and frames made entirely of extruded aluminum are common. Residential windows are a major means of ventilation, and there are a variety of operating actions for their movable sections: sliding or double-hung windows are still the major form, but hinged types—including casement, hopper, and awning forms—are also used. Sliding glass panel doors are also used, particularly in warmer regions. Glazing is still largely of clear glass. Double glazing, with two panes bonded to a metal tubular separator that contains a desiccant, is cost-effective in northern climates, but triple glazing is used commonly only in regions above about 55° to 60° latitude. A recent development is heat-mirror glass, in which a low-emissivity coating enhances the relative opacity of the glass to infrared radiation and slows the rate of internal heat loss in winter.