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STRUCTURAL INSPECTIONS & DEFECTS
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CHIMNEY INSPECTION DIAGNOSIS REPAIR
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DECK & PORCH CONSTRUCTION
DEFINITIONS of Mobile Home, Doublewide, Modular, Panelized
DEFINITIONS of ENGINEERED WOOD OSB LVL etc
DISASTER BUILDING INSPECTION & REPAIR
EARTHQUAKE DAMAGED FOUNDATIONS
FIRE DAMAGE vs MOLD DAMAGE
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FOOTING & FOUNDATION DRAINS
FOUNDATION CRACKS & DAMAGE GUIDE
FRAMING DAMAGE, INSPECTION, REPAIR
GRADING, DRAINAGE & SITE WORK
HOUSE PARTS, DEFINITIONS
INSECT INFESTATION / DAMAGE
KIT HOMES, Aladdin, Sears, Wards, Others
LOG HOME GUIDE
MOBILE HOMES, DOUBLEWIDES, TRAILERS
MODULAR HOME CONSTRUCTION
MOISTURE CONTROL in BUILDINGS
PORCH CONSTRUCTION & SCREENING
PRE-CUT & KIT HOMES
RETAINING WALL DESIGNS, TYPES, DAMAGE
ROT, FUNGUS, INSECT DAMAGE
SINKHOLES, WARNING SIGNS
STAIRS, RAILINGS, LANDINGS, RAMPS
STRAW BALE CONSTRUCTION
STRESS SKIN INSULATED PANELS
STRUCTURAL WOOD ASSESSMENT
TIMBER FRAMING, ROT
TRUSSES, Floor & Roof
WATER ENTRY in BUILDINGS
This article series defines and illustrates basic types of engineered lumber and wood products such as plywood and OSB sheathing, Laminated Veneer Lumber LVL used for beams and headers, High Density Overlay HDO and Medium Density Overlay MDO plywood, and Parallel Strand or PSL lumber. We also define and illustrate wood I-Joists used in floor and roof construction. This article series describes wood products used in construction including engineered lumber, OSB, and Plywood products. Our page top photo shows single and built-up LVL beams along with a conventional 2x4 and a built-up 2x6 beam (at left in the photo) at a wood frame construction project in New York.
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Definitions of HDO Plywood, MDO Plywood, Glulam, LVL, Microlam, PSL, and OSB Trim and Lumber: Structural Composite Lumber
Laminated veneer lumber is an "engineered wood product", developed in the 1960's, is produced by gluing together layers of wood veneers with the wood grains in parallel. Illustrated in our photographs of LVL beams in use at a Poughkeepsie NY Project, our example LVL beams are made of fifteen laminated wood plies.
Multiple laminated veneer lumber beams can be assembled to provide greater strength to carry greater loads. In our LVL beam photo (below left) you will see that a higher-capacity LVL beam may be constructed as a built-up beam using multiple individual members nailed or nailed and glued together.
Our LVL composition closeup photo (below right) shows how the ends of individual plies within a single LVL beam may overlap within the product. [Click any image to see an enlarged, detailed view].
Because individual wood ingredient defects in a laminated wood product will not extend beyond a single layer or "veneer" of wood, the overall laminated product is stronger than a piece of lumber of the same dimensions sawn from an individual tree. This approach also permits construction of large structural beams from smaller-diameter and lower-lumber-grades of trees.
As we illustrate in our LVL beam photographs (above) and as we also discuss at TRIM, EXTERIOR CHOICES, INSTALLATION:
Laminated beams such as GluLam™ (photo at above left) or microlam structural wood beams (see LVL or laminated veneer lumber earlier in this article and illustrated further below). Layers of wood are glued (laminated) together with heat, resin binder, and pressure to form a very strong structural member that can be produced in regular sizes and lengths.
Unlike plywood or OSB, LVL lumber uses wood fragments that are all oriented in the same direction to produce very stiff beams that generally have greater span capacity than sawn lumber. GluLam produces laminated wood beams, timbers, I-joists, and other engineered wood products.
See TRIM, EXTERIOR CHOICES, INSTALLATION for a discussion of use of LVL for exterior trim.
See Laminated Veneer Lumber, Overview of the Product for a nice collection of examples of use of LVL products and a brief history of this material.
Details about plywood products used in building construction for roof or wall decking or sheathing and for subfloors are at PLYWOOD Roof, Wall, Floor Decks & Sheathing. Excerpts are below.
"Plywood" or traditional plywood sheets, typically 4' x 8' in dimension and of varying thicknesses, typically from 3/16" up to 3/4" or more, is constructed of veneers of wood glued together, but each layer of wood is placed at right-angles to those on either side.
Our photo (left) shows 1/2-inch CDX plywood installed as exterior wall sheathing on a home being constructed in Minneapolis, Minnesota.
Plywood sheets are named according to the sheet thickness, the quality of its exterior or exposed veneers (A = clear, no knots, C/D - construction grade, etc), and its thickness.
Plywood products are also selected for interior or exterior use based on the weather resistance of the glues used to glue the veneers together (exterior plywood uses exterior glues), and finally on the condition of interior veneers or plies of wood.
Plywood used for floor sheathing or underlayment beneath carpeting, for example, is selected without interior voids so that someone's shoe-heel won't puncture through a clear top veneer of wood into a hidden void below.
PLYWOOD Roof, Wall, Floor Decks & Sheathing (1905 - present as a construction material in North America) is sheet material made of thin veneers of wood that are laid with wood grains in alternating direction, glued, heated, and pressed together. Interior plywood is generally glued with urea formaldehyde based glues; exterior plywood and marine plywood use phenolic formaldehyde glues and are water resistant.
Our photo (left) shows both fire-retardant plywood roof sheathing (left half of the photo and center top and bottom) and OSB roof sheathing (center of photo and right edges of photo).
While modern plywood products use a variety of glues, heat, and pressure to produce the product, plywood has been around at least since 3500 BC when a glued-veneer version was produced in Egypt. The furniture industry has had a long history of gluing thin fine-wood veneers to a cheaper wooden base - a process similar to the production of plywood itself. The invention of the rotary lathe (ca 1850) by Immanuel Nobel (1801 - 1872) is what made modern plywood possible by permitting manufacturers to cut large thin (but thicker than wood veneers) sheets of wood from logs. The wood sheets are placed at right angles to one another and glued together.
The cross-grain construction combined with glue produces a strong, uniform material that is used for both enclosure and for structural stiffness in frame construction of building walls and roofs. The properties of plywood, including its tolerance to weather exposure (marine plywood) depend on the glues and finishes used. Both softwood and hardwoods are used in plywoods, and fine wood veneer finishes are also available (for furniture use).
The first plywood made in the Western U.S. was produced by the Portland Manufacturing Company in Oregon in 1905, a company founded four years earlier by Gustav A. Carlson, F.S. Doernbecher, and M.L. Holbrook. Peter Autzen bought out Doernbecher and Holbrook. It was Autzen's son, Thomas, who made key progress solving problems with bonding the veneers together.
Originally the Portland Manufacturing company had produced fruit baskets, crates, and drums, experience which we pose gave the owners familiarity with cutting thin strips of wood (baskets) that remained flexible for use in a manufacturing process. Their plywood was exhibited at the 1905 Worlds' Fair in Portland, Oregon. Interestingly, the first exhibition of plywood as a building product in the Northwest was in a log structure, the Forestry Building in Portland, OR where it was displayed until that building burned in 1964.
In 1924 plywood sales were still primarily to door producers, but by 1928 the company had increased production and plywood was being used in automobile bodies. In that year the Pacific Coast Plywood Manufacturers, Inc. (PCPM)was formed jointly with Elliot Bay Mill Co. (Seattle), Walton Veneer Co. (Everett WA), and Washington Veneer Co. (Olympia WA). But PCPM was dissolved after the 1929 Stock Market crash.
The appeal and success of plywood as a building material are based on quite a few factors including increased construction speed (consider nailing up 4x8 sheets of plywood versus individual tongue and groove exterior building sheathing or roofing), product uniformity and strength, and the reduction of waste compared with cutting sheathing boards out of logs.
Also see FIRE RETARDANT PLYWOOD.
"PSL" or Parallel Strand Lumber is similar to LVL and is used in the same applications as LVL lumber and trim, but PSL lumber permits more defects in its veneers, creating a more random wood pattern on its visible surfaces.
"MDO" plywood (Medium Density Overlay) is constructed of a core of overlapping veneers of wood, but with a surface layer of medium density fiber. The resulting product has a smooth surface. MDO plywood can be used once as a concrete form material, but should not be re-used for that purpose.
See TRIM, EXTERIOR CHOICES, INSTALLATION for a discussion of use of MDO plywood for use on building soffits.
"HDO" (High Density Overlay) plywood is similar to MDO Plywood described just above. HDO plywood also is constructed wit a core of overlapping wood veneers, but instead of using a medium density fiber for its external surfaces, HDO plywood uses a high-density fiber exterior. HDO plywood products, having a harder exterior surface, can be re-used several time for concrete forms.
APA (The Engineered Wood Association) provides an HDO/MDO Plywood Product Guide that offers details about these products.
Details about OSB sheathing panels are at SHEATHING, OSB where we provide more details and photo examples of OSB Oriented Strand Board sheathing use. Excerpts are below.
"OSB" or Oriented Strand Board wood products is similar to LVL and PSL, but OSB is produced in sheets (typically 4' x 8' in size) and is constructed of shorter strands of wood or even (in the case of 4' x 8' sheets) wood chips that may be roughly rectangular in shape.
Developed in the 1980's, oriented strand board is an engineered wood product in which strands and flakes of wood are cut from straight, low-knot small-diameter logs, usually aspen or white birch. The wood strands and flakes of OSB are glued (or "bonded") together with a phenol-formaldehyde resin binder (forming a waterproof glue), heated (to at least 120 degF), and are compressed and flattened using pressure. Other chemical binders may be used in OSB products, and the OSB sheathing surface may have a wax coating to improve water resistance of the product.
Our photographs of OSB "plywood" below illustrate that the wood fragments glued together under heat and pressure are deliberately oriented randomly to produce greater strength than would be achieved if these small individual fragments were all oriented in the same direction
Our second OSB sheathing photo (above right) shows fire retardant treated plywood roof sheathing (photo-left) and OSB roof decking (photo-right) sheathing the roof of a multi-unit condominium building.
Properties of OSB Panels
In an OSB panel the two exterior surface layers of wood strands are oriented parallel to the long axis of the panel. In the interior OSB panel layer or "core layer" strands are oriented either randomly or across the short axis of the OSB panel. In overall thickness the ratio of face panels to core panel ranges from 40:60 to 60:40.
According to the Universite Laval and also Timberco, the dimensions of wood strands used in OSB are specified in industry standards; most producers of OSB use a combination of strands that are up to 6" long and 1" wide or from another source, 19-40 mm in width and 90 to 100mm in length.
OSB is a modern wood product that developed from earlier 1970's "waferboard" product. In 1990 the Structural Board Association was formed. By 1996 there were 38 OSB producers in North America. But unlike waferboard whose composite wood chips were place randomly, an oriented strand board product is made from wood chips that are deliberately oriented with respect to one another to provide greater strength.
As a result, modern OSB products are rated at the same strength as plywood products. OSB roofing panels are available with a perforated (breathable) foil radiant barrier affixed to the pane's interior surface.
See SHEATHING, OSB for more details and photo examples of OSB Oriented Strand Board sheathing use.
Our photo (below left) illustrates wood I-joists used in construction of building floors and roofs. You will observe that the center web of the I-joist is constructed of OSB sheathing material that we illustrated just above. Our second photo (below right) shows common lumber markings found on the solid wood top and bottom chords of wood I-joists.
Engineered wood floor trusses (photos above and below) such as I-Joists originally were constructed using a plywood web beginning in 1977, and modified by by Trus-Joist in 1969 to use laminated veneer lumber (LVL) and OSB-like laminated wood fiber web (shown in photos above left and below in combination with a steel beam).
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