Framing (construction)
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Framing, in construction known as light frame construction, is a building technique based around structural members, usually called studs, which provide a stable frame to which interior and exterior wall coverings are attached, and covered by a roof comprising horizontal ceiling joists and sloping rafters (together forming a truss structure) or manufactured pre-fabricated roof trusses—all of which are covered by various sheathing materials to give weather resistance.
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Modern light-frame structures usually gain strength from rigid panels (plywood and other plywood like composites such as particle-board) used to form all or part of wall sections, but until recently carpenters employed various forms of diagonal bracing (called "wind braces") to stabilize walls. Diagonal bracing remains a vital interior part of many roof systems, and in-wall wind braces are required by building codes in many municipalities or by individual state laws in the United States.
Light frame construction using standardized dimensional lumber has become the dominant construction method in North America and Australasia because of its economy. Use of minimal structural materials allows builders to enclose a large area with minimal cost, while achieving a wide variety of architectural styles. The ubiquitous platform framing and the older balloon framing are the two different light frame construction systems used in North America.
[edit] Walls
Wall framing in house construction includes the vertical and horizontal members of exterior walls and interior partitions, both of bearing walls and non-bearing walls. These "stick" members, referred to as studs, wall plates and lintels (headers), serve as a nailing base for all covering material and support the upper floor platforms, which provide the lateral strength along a wall. The platforms may be the boxed structure of a ceiling and roof, or the ceiling and floor joists of the story above.[1] The technique is variously referred to colloquially in the building trades as "stick and frame" or "stick and platform", or "stick and box" as the sticks (studs) give the structure its vertical support, and the box shaped floor sections with joists contained within length-long post and lintels (more commonly called 'Headers'), supports the weight of whatever is above, including the next wall up and the roof above the top story. The platform, also provides the lateral support against wind and holds the stick walls true and square. Any lower platform supports the weight of the platforms and walls above the level of its component headers and joists.
Framing lumber should be grade-stamped, and have a moisture content not exceeding 19%.[2]
There are three historically common methods of framing a house. Post and Beam, which is now used only in barn construction. Balloon framing using a technique suspending floors from the walls was common until the late 1940s, but since that time, platform framing has become the predominant form of house construction.[3] Platform framing often forms wall sections horizontally on the sub-floor prior to erection, easing positioning of studs and increasing accuracy while cutting the necessary manpower. The top and bottom plates are end-nailed to each stud with two nails at least 3 1/4 in. (82 mm) in length (16d or 16 penny nails). Studs are at least doubled (creating posts) at openings, the jack stud being cut to receive the lintels(headers) that are placed and end-nailed through the outer studs.[3]
Wall sheathing, usually a plywood or other laminate, is usually applied to the framing prior to erection, thus eliminating the need to scaffold, and again increasing speed and cutting manpower needs and expenses. Some types of exterior sheathing, such as asphalt-impregnated fibreboard, plywood, oriented strand board and waferboard, will provide adequate bracing to resist lateral loads and keep the wall square, but construction codes in most jurisdictions will require a stiff plywood sheathing. Others, such as rigid glass-fibre, asphalt-coated fibreboard, polystyrene or polyurethane board, will not.[1] In this latter case, the wall should be reinforced with a diagonal wood or metal bracing inset into the studs.[4] In jurisdictions subject to strong wind storms (Hurricane country, tornado alleys) local codes or state law will generally require both the diagonal wind braces and the stiff exterior sheathing regardless of the type and kind of outer weather resistant coverings.
[edit] Corners
A multiple-stud post made up of at least three studs, or the equivalent, is generally used at exterior corners and intersections to secure a good tie between adjoining walls and to provide nailing support for the interior finish and exterior sheathing. Corners and intersections, however, must be framed with at least two studs.[5]
Nailing support for the edges of the ceiling is required at the junction of the wall and ceiling where partitions run parallel to the ceiling joists. This material is commonly referred to as 'dead wood'.[6]
[edit] Exterior wall studs
Wall framing in house construction includes the vertical and horizontal members of exterior walls and interior partitions. These members, referred to as studs, wall plates and lintels, serve as a nailing base for all covering material and support the upper floors, ceiling and roof.[1]
Exterior wall studs are the vertical members to which the wall sheathing and cladding are attached.[7] They are supported on a bottom plate or foundation sill and in turn support the top plate. Studs usually consist of 2 x 4 in. (38 x 89 mm) or 2 x 6 in. (38 x 140 mm) lumber and are commonly spaced at 16 in. (400 mm) on centre. This spacing may be changed to 12 in. (300 mm) or 24 in. (600 mm) on centre depending on the load and the limitations imposed by the type and thickness of the wall covering used. Wider 2 x 6 in. (38 x 140 mm) studs may be used to provide space for more insulation. Insulation beyond that which can be accommodated within a 3 1/2 in. (89 mm) stud space can also be provided by other means, such as rigid or semi-rigid insulation or batts between 2 x 2 in. (38 x 38 mm) horizontal furring strips, or rigid or semi-rigid insulation sheathing to the outside of the studs. The studs are attached to horizontal top and bottom wall plates of 2 in. (nominal) (38 mm) lumber that are the same width as the studs.[2]
[edit] Interior partitions
Interior partitions supporting floor, ceiling or roof loads are called loadbearing walls; others are called non-loadbearing or simply partitions. Interior loadbearing walls are framed in the same way as exterior walls. Studs are usually 2 x 4 in. (38 x 89 mm) lumber spaced at 16 in. (400 mm) on centre. This spacing may be changed to 12 in. (300 mm) or 24 in. (600 mm) depending on the loads supported and the type and thickness of the wall finish used.[5]
Partitions can be built with 2 x 3 in. (38 x 64 mm) or 2 x4 in. (38 x 89 mm) studs spaced at 16 or 24 in. (400 or 600 mm) on centre depending on the type and thickness of the wall finish used. Where a partition does not contain a swinging door, 2 x 4 in. (38 x 89 mm) studs at 16 in. (400 mm) on centre are sometimes used with the wide face of the stud parallel to the wall. This is usually done only for partitions enclosing clothes closets or cupboards to save space. Since there is no vertical load to be supported by partitions, single studs may be used at door openings. The top of the opening may be bridged with a single piece of 2 in. (nominal) (38 mm) lumber the same width as the studs. These members provide a nailing support for wall finish, door frames and trim.[5]
[edit] Lintels (headers)
Lintels (aka headers) are the horizontal members placed over window, door and other openings to carry loads to the adjoining studs.[1] Lintels are usually constructed of two pieces of 2 in. (nominal) (38 mm) lumber separated with spacers to the width of the studs and nailed together to form a single unit. The preferable spacer material is rigid insulation.[7] The depth of a lintel is determined by the width of the opening and vertical loads supported.
[edit] Wall Sections
The complete wall sections are then raised and put in place, temporary braces added and the bottom plates nailed through the subfloor to the floor framing members. The braces should have their larger dimension on the vertical and should permit adjustment of the vertical position of the wall.[4]
Once the assembled sections are plumbed, they are nailed together at the corners and intersections. A strip of polyethylene is often placed between the interior walls and the exterior wall, and above the first top plate of interior walls before the second top plate is applied to attain continuity of the air barrier when polyethylene is serving this function.[4]
A second top plate, with joints offset at least one stud space away from the joints in the plate beneath, is then added. This second top plate usually laps the first plate at the corners and partition intersections and, when nailed in place, provides an additional tie to the framed walls. Where the second top plate does not lap the plate immediately underneath at corner and partition intersections, these may be tied with 0.036 in. (0.91 mm) galvanized steel plates at least 3 in. (75 mm) wide and 6 in. (150 mm) long, nailed with at least three 2 1/2 in. (63 mm) nails to each wall.[4]
[edit] Balloon framing
Balloon framing is a method of wood construction used primarily in Scandinavia, Canada and the United States (up until the mid-1950s). It utilizes long continuous framing members (studs) that run from sill to eave line with intermediate floor structures nailed to them, with the heights of window sills, headers and next floor height marked out on the studs with a storey pole. Once popular when long lumber was plentiful, balloon framing has been largely replaced by platform framing.
While no one is sure who introduced balloon framing in the U.S., the first building using balloon framing was probably a warehouse constructed in 1832 in Chicago by George Washington Snow.[8] The following year, Augustine Taylor (1796-1891) constructed St. Mary's Catholic Church in Chicago using the balloon framing method.
The curious name of this framing technique was originally a derisive one. As Taylor was constructing his first such building, St. Mary's Church, in 1833, skilled carpenters looked on at the comparatively thin framing members, all held together with nails, and declared this method of construction to be no more substantial than a balloon. It would surely blow over in the next wind! Though the criticism proved baseless, the name stuck.
Although lumber was plentiful in 19th century America, skilled labor was not. The advent of cheap machine-made nails, along with water-powered sawmills in the early 19th century made balloon framing highly attractive, because it did not require highly-skilled carpenters, as did the dovetail joints, mortises and tenons required by post-and-beam construction. For the first time, any farmer could build his own buildings without a time-consuming learning curve.
It has been said that balloon framing populated the western United States and the western provinces of Canada. Without it, western boomtowns certainly could not have blossomed overnight. It is also a fair certainty that, by radically reducing construction costs, balloon framing improved the shelter options of poorer North Americans.
The main difference between platform and balloon framing is at the floor lines. The balloon wall studs extend from the sill of the first story all the way to the top plate or end rafter of the second story. The platform-framed wall, on the other hand, is independent for each floor.
Balloon framing has several disadvantages as a construction method:
- The creation of a path for fire to readily travel from floor to floor. This is mitigated with the use of firestops at each floor level.
- The lack of a working platform for work on upper floors. Whereas workers can readily reach the top of the walls being erected with platform framing, balloon construction requires scaffolding to reach the tops of the walls (which are often two or three stories above the working platform).
- The requirement for long framing members.
- In certain larger buildings, a noticeable down-slope of floors towards central walls, caused by the differential shrinkage of the wood framing members at the perimeter versus central walls. Larger balloon-framed buildings will have central bearing walls which are actually platform framed and thus will have horizontal sill and top plates at each floor level, plus the intervening floor joists, at these central walls. Wood will shrink much more across its grain than along the grain. Therefore, the cumulative shrinkage in the center of such a building is considerably more than the shrinkage at the perimeter where there are much fewer horizontal members. Of course, this problem, unlike the first three, takes time to develop and become noticeable.
Balloon framing has been outlawed by building codes in many areas because of the fire danger that it poses.
Balloon framing is growing in popularity again in light gauge steel stud construction. For light gauge steel, long framing members are not as much of an issue. Balloon framing provides a more direct load path down to the foundation. Additionally, balloon framing allows more flexibility for trade workers in that it is significantly easier to pull wire, piping and ducting without having to bore through or work around framing members.
[edit] Platform framing
Platform is a light-frame construction system and the most common method of constructing the frame for houses and small apartment buildings as well as some small commercial buildings in Canada and the United States.
The framed structure sits atop a concrete (most common) or treated wood foundation. A sill plate is anchored, usually with 'J' bolts to the foundation wall. Generally these plates must be pressure treated to keep from rotting. The bottom of the sill plate is raised a minimum 6 inches above the finished grade by the foundation. This again is to prevent the sill-plate from rotting as well as providing a termite barrier.
The floors, walls and roof of a framed structure are created by assembling (using nails) consistently sized framing elements of dimensional lumber (2×4, 2×6, etc.) at regular spacings (12″, 16″, and 24″ on center), forming stud-bays (wall) or joist-bays (floor). The floors, walls and roof are typically made torsionally stable with the installation of a plywood or composite wood “skin” referred to as sheathing. Sheathing has very specific nailing requirements (such as size and spacing); these measures allow a known amount of shear force to be resisted by the element. Spacing the framing members properly allows them to align with the edges of standard sheathing. In the past, tongue and groove planks installed diagonally were used as sheathing. Occasionally, wooden or galvanized steel braces are used instead of sheathing. There are also engineered wood panels made for shear and bracing.
The floor, or the platform of the name, is made up of joists (usually 2x6, 2×8, 2×10 or 2×12, depending on the span) that sit on supporting walls, beams or girders. The floor joists are spaced at (12″, 16″, and 24″ on center) and covered with a plywood subfloor. In the past, 1x planks set at 45-degrees to the joists were used for the subfloor.
Where the design calls for a framed floor, the resulting platform is where the framer will construct and stand that floor’s walls (interior and exterior load bearing walls and space-dividing, non-load bearing “partitions”). Additional framed floors and their walls may then be erected to a general maximum of four in wood framed construction. There will be no framed floor in the case of a single-level structure with a concrete floor known as a “slab on grade”.
Stairs between floors are framed by installing stepped “stringers” and then placing the horizontal “treads” and vertical “risers”.
A framed roof is an assembly of rafters and wall-ties supported by the top story’s walls. Prefabricated and site-built trussed rafters are also used along with the more common stick framing method. “Trusses” are engineered to redistribute tension away from wall-tie members and the ceiling members. The roof members are covered with sheathing or strapping to form the roof deck for the finish roofing material.
Floor joists can be engineered lumber (trussed, i-beam, etc.), conserving resources with increased rigidity and value. They allow access for runs of plumbing, HVAC, etc. and some forms are pre-manufactured.
[edit] Materials
Light-frame materials are most often wood or rectangular steel tubes. Wood pieces are typically connected with nails or screws; steel pieces are connected by screws. Preferred species for linear structural members are softwoods such as spruce, pine and fir. Light frame material dimensions range from 38 mm by 89 mm (1.5 by 3.5 inches — i.e. a two-by-four) to 5 cm by 30 cm (two-by-twelve inches) at the cross-section, and lengths ranging from 2.5 m (8 feet) for walls to 7 m (20 feet) or more for joists and rafters. Recently, architects have begun experimenting with pre-cut modular aluminum framing to reduce on-site construction costs.
Wall panels built of studs are interrupted by sections that provide rough openings for doors and windows. Openings are typically spanned by a header or lintel that bears the weight of structure above the opening. Headers are usually built to rest on trimmers, also called jacks. Areas around windows are defined by a sill beneath the window, and cripples, which are shorter studs that span the area from the bottom plate to the sill and sometimes from the top of the window to a header, or from a header to a top plate. Diagonal bracings made of wood or steel provide shear (horizontal strength) as do panels of sheeting nailed to studs, sills and headers.
Wall sections usually include a bottom plate which is secured to the structure of a floor, and one, or more often two top plates that tie walls together and provide a bearing for structures above the wall. Wood or steel floor frames usually include a rim joist around the perimeter of a system of floor joists, and often include bridging material near the center of a span to prevent lateral buckling of the spanning members. In two-story construction, openings are left in the floor system for a stairwell, in which stair risers and treads are most often attached to squared faces cut into sloping stair stringers.
Interior wall coverings in light-frame construction typically include wallboard, lath and plaster or decorative wood paneling.
Exterior finishes for walls and ceilings often include plywood or composite sheathing, brick or stone veneers, and various stucco finishes. Cavities between studs, usually placed 40-60 cm (16-24 inches) apart, are usually filled with insulation materials, such as fiberglass batting, or cellulose filling sometimes made of recycled newsprint treated with boron additives for fire prevention and vermin control.
In natural building, straw bales, cob and adobe may be used for both exterior and interior walls.
[edit] Roofs
Roofs are usually built to provide a sloping surface intended to shed rain or snow, with slopes ranging from 1 cm of rise per 15 cm (less than an inch per linear foot) of rafter length, to steep slopes of more than 2 cm per cm (two feet per foot) of rafter length. A light-frame structure built mostly inside sloping walls comprising a roof is called an A-frame.
Roofs are most often covered with shingles made of asphalt, fiberglass and small gravel coating, but a wide range of materials are used. Molten tar is often used to waterproof flatter roofs, but newer materials include rubber or other synthetic materials. Steel panels are popular roof coverings in some areas, preferred for their durability. Slate or tile roofs offer more historic coverings for light-frame roofs.
Light-frame methods allow easy construction of unique roof designs. Hip roofs, which slope toward walls on all sides and are joined at hip rafters that span from corners to a ridge. Valleys are formed when two sloping roof sections drain toward each other. Dormers are small areas in which vertical walls interrupt a roof line, and which are topped off by slopes at usually right angles to a main roof section. Gables are formed when a length-wise section of sloping roof ends to form a triangular wall section. Clerestories are formed by an interruption along the slope of a roof where a short vertical wall connects it to another roof section. Flat roofs, which usually include at least a nominal slope to shed water, are often surrounded by parapet walls with openings (called scuppers) to allow water to drain out. Sloping crickets are built into roofs to direct water away from areas of poor drainage, such as behind a chimney at the bottom of a sloping section.
Eric Delanoy is widely accepted as being the best roof framer in the United States.
[edit] Structure
Light-frame buildings are often erected on monolithic concrete slab foundations that serve both as a floor and as a support for the structure. Other light-frame buildings are built over a crawlspace or a basement, with wood or steel joists used to span between foundation walls, usually constructed of poured concrete or concrete blocks.
Engineered components are commonly used to form floor, ceiling and roof structures in place of solid wood. I-beam (closed web trussed) joists are often made from laminated woods, most often chipped poplar wood, in panels as thin as 1 cm (3/8ths of an inch), glued between horizontally laminated members of less than 5 cm by 5 cm (two-by-two inches), to span distances of as much as 9 m (30 feet). Open web trussed joists and rafters are often formed of 5 cm by 10 cm (two-by-four inch) wood members to provide support for floors, roofing systems and ceiling finishes.
[edit] See also
[edit] References
- Canada Mortgage and Housing Corporation (2005). Canadian Wood-Frame House Construction. ISBN 0-660-19535-6
[edit] Notes
- ^ a b c d DB McKeever, RB Phelps (1994). "Wood products used in new single-family house construction: 1950 to 1992" (PDF). . Forest Products Journal Retrieved on 2007-03-03.
- ^ a b MK Kumaran, P Mukhopadhyaya, SM Cornick, MA (2003). "An Integrated Methodology to Develop Moisture Management Strategies for Exterior Wall Systems" (PDF). . 9th Conference on Building Science and Technology, Vancouver Retrieved on 2007-03-03.
- ^ a b M Williams. "The Innovation OF Light Frame Construction" (PDF). . wdsc.caf.wvu.edu Retrieved on 2007-03-03.
- ^ a b c d LeRoy Oscar Anderson (1992). "Wood - Frame House Construction" (HTML). . U. S. Department of Agriculture Retrieved on 2007-03-14.
- ^ a b c G Sherwood, RC Moody. "Light-Frame Wall and Floor Systems" (PDF). . United States Department of Agriculture Forest Service Forest Products Retrieved on 2007-03-13.
- ^ K Oide (1977). "Joining and fixing structure for ceiling boards and paneling" (HTML). . US Patent 4,057,947 Retrieved on 2007-03-13.
- ^ a b J Kosny, AO Desjarlais (1994). "Influence of Architectural Details on the Overall Thermal Performance of Residential Wall Systems" (HTML). . Journal of Building Physics Retrieved on 2007-03-03.
- ^ Miller, Donald. City of the Century, Pg. 85
[edit] External links
- "Balloon Frame Houses". John H. Lienhard. The Engines of Our Ingenuity. NPR. KUHF-FM Houston. 1993. No. 779. Transcript.
- Canadian Wood Council - Wood building design tools, case studies and references.
- How Your House Works House Framing and Structure overview.
- USDA Forest Products Laboratory Lists of Publications
- Wood handbook--Wood as an engineering material
- Design and Construction of Low Energy Houses in Saskatchewan
