Introduction
Wood light-frame construction is a popular method of building homes in North America. The construction can be broken up into four sections, foundation, floors, walls, and roof. Some advantages of the building method include, utilizing smaller members to make large components and the redundancy of the entire system. Building with wood also has its advantages because it is easy to work with, displays signs over loading and is a renewable resource.
Foundation
The foundation of a lightwood frame home is used to dissipate the loads from the building into the soil. The foundation is made up of two parts a footing and a wall. The footing is the weight bearing portion of the foundation. The wall or in some cases a pier extends from the footing to the first floor of the building.
Footings are typically comprised of poured in place concrete. In colder climates where frost heave is an issue, footings must be set deeper in the ground then the frost reaches. The purpose of the footing is to take the above weight load of the building and displace it on the soil. In softer soils, or where large weight loads occur, pilings may be driven into the ground under the footings.
The foundation walls are used to transport the loading of the first floor onto the footings. They can also form the walls of basements, if soil is excavated on one side. Typicallymade of poured in place concrete, prefabricated concrete, or masonry blocks all of which have high compression strength. In some drier climates or in old construction, the walls may be made up of treated wood or large stones.
Floors
In lightwood frame construction floors are built to carry the weight loads of occupants and walls. They are made up of two components, joists and sheathing. The joists are evenly spaced and are responsible for bearing most of the load. The sheathing fills in the spaces and distributes any weight placed on the floor.
The foundation walls hold up the first floor. In most cases it becomes inefficient to span the entire distance between walls so beams will be used in addition to the walls. Beams typically rest on notches in the foundation walls and have additional posts that connect to additional footings in the center of the building. On any second and higher floors, the wood frame walls, both interior and exterior, are used to hold them up. In some cases beams are also used for upper level floors that span large openings or when there are heavy load requirements.
Joists are spaced twelve or sixteen inches apart. They can be made up of either solid sawn lumber or engineered joists. Dimensional sawn lumber comes in 2×8, 2×10, and 2×12 sizes. The load will determine the necessary size they are required to carry and the distance they span. Engineered floor joists create a higher strength to weight ratio, can span longer distances and often create a straighter finished floor. In cases where a higher strength is necessary, by possible additional loads or openings in the floor, joist can be doubled or tripled to meet the requirements.
The joists are then sheeted with plywood, oriented strand board, or solid wood planks. This allows for a continuous floor, where walls, occupants, and other items can be placed anywhere. The sheeting must be able to displace any point loads onto the joists, while minimizing bending. All joints must be offset and have a spleen or tongue and groove to ensure strength is not compromised. In some cases were the final occupants wish to have very little bending of the sheeting between joists, a subfloor of additional sheeting, or a thin concrete layer may be used.
Walls
The above ground walls of a lightwood framed home are what make up the living space of the finished home. Typically eight feet in height however can also be taller where desired. The wall’s structural supports are the framing members or wall studs. Exterior walls are also sheeted for additional shear strength. The entire wall can be built laying flat on the floor, tilted up, and then fastened into place. Walls may need temporary supports until the entire building is complete.
Wall studs that carry the majority of the loads are made up of 2 by 4 or 2 by 6 sized members. The later is used on exterior walls to allow for added insulation. The studs are evenly spaced every sixteen inches. They are also oriented to allow the maximum rigidity in the wall. One or two members lay flat at both the bottom and top of the wall. This again allows any loads to be distributed onto the floor below.
Although only applicable to exterior walls, they are sheeted on their outside face. This gives the wall some shear strength and prevents it from deforming from its original rectangular shape. The sheeting can be a variety of materials including, plywood, oriented strand board, exterior gypsum, and rigid foam. The type of sheeting required may depend on the building’s final cladding.
Roof Systems
There are two types of roofs, nominally flat and sloped. Nominally flat roofs are built in a similar manner to floors, however have a small slope towards drains. Flat roofs must also be able to withstand heavy snow and water loads, as it does not shed them easily.
Sloped roofs are more common on light wood framing, and are built using trusses. From as structural perspective these roofs are more interesting. The purpose of the truss is to direct loads from the roof portion down to the walls. Through careful calculations all the elements can work together to maximize the strength of the truss. The trusses are usually spaced twenty-four inches, and are sheeted with a plywood product. Waterproofing elements such as shingles are then placed on top of the sheeting to shed water.
Advantages of Light- Frame Wood Construction
Light-frame wood construction is popular method of building homes across Canada. Sawn lumber, plywood and engineered products are readily available coast to coast. This makes this building method efficient and cost effective, when compared to other housing construction. Also due to the repetitiveness and simplicity of the method, architects and carpenters can use charts and guidelines to determine member sizes and requirements. This again is cost effective as engineers are not required on every home.
The actual elements that make up large homes are relatively small. Dimensional lumber and plywood products can all be stacked and transported by regular trucks. Materials can be moved around sites with all terrain forklifts, and if necessary by manual labour. This is favorable as large walls, and floors can be assembled on site, from smaller easy to handle elements.
Light-framed wood houses are very redundant systems. If an element is removed or compromised it is unlikely that a catastrophic failure will occur. For example walls are made up of several vertical studs. If a window opening was cut in without proper support overtop, it is unlikely that the entire wall will fail.
Advantages of Using Wood as a Building Material
Building with wood is quite simple. Handsaws or electric saws can efficiently cut members to length. Also fastening can be accomplished quickly with nails or screws. This allows the actual construction of lightwood frame houses to be completed without any expensive specialized equipment. This helps keep costs down, and allows the methods to be widely used.
Wood is a good material for buildings as it displays many signs well before failure. Deformation, excessive bending, cracking, and cracking noises will usually tell the occupants that there is a structural issue with the building. Although the building may originally be designed to carry the required loads, over time rot, decomposition, and termites may undermine the structure. Again the warning devices that come with wood are useful in the long term.
Of the many common construction methods, lightwood framing is the only one that utilizes renewable resources. When wood from properly managed forests is used, carbon dioxide is removed from the atmosphere. At the end of a buildings life, wood fibers can be recycled into new products or if placed in a landfill will decompose quickly as they are an organic product.
Conclusion
Wood light-framing is a method that has a long history of success. As products and technologies continue to advance, the building method will also adapt. In the future light-frame construction, with dynamic changes will continue to succeed.