Framing Material Basics

Home , Framing (construction), Laminated veneer lumber

FRAMING MATERIAL BASICS

As discussed in Building Knowledge University: Level 1Course 2, Lesson 3

The majority of houses in the United States are stick-framed homes built with dimensional lumber. However, as old-growth trees have become scarcer, the use of engineered materials that make efficient use of small-fiber stock has increased.

Dimensional Lumber Standard dimensional lumber is sold in one of several species groups. The main species groups sold in the U.S. are Spruce-Pine-Fir (a Canadian species), Douglas Fir-Larch, Hem-Fir, and Southern Pine (typically used for pressure- treated lumber). Each species group is available in a number of grades, but unless otherwise specified, most framing lumber required by code is a #2 grade. Lower grades may be allowed for studs (Stud Grade) and top plates (Utility- Grade).

Lumber sizes. As the name implies, dimensional lumber is cut into nominal dimensions of 2 inches thick by 4, 6, 8, 10, and 12 and more inches wide. However, the boards are then surfaced, resulting in a smaller dimension than their nominal size. After surfacing, all 2x framing materials are 1-1/2 thick. A 2x4 measures 1-1/2 by 3-1/2, and a 2 x6 is 1-1/2 by 5-1/2. Then the widths jump down slightly: A 2x8 measures1-1/2 by 7-1/4 inches, a 2x10 is 1-1/2 by 9-1/4, and a 2x12 is 1-1/2 by 11-1/4.

Lumber shrinkage. In a completed building, framing eventually dries to an average of 6-11%, depending on the climate. This drying causes the lumber to shrink across the grain. Shrinkage along the grain is negligible. The total shrinkage in a house, and that of large carrying beams in particular, can cause one part of a house to settle more than others, resulting in drywall cracks, window jams, and other problems.

Plywood and OSB Sheathing Sheathing and subflooring should consist of performance-rated structural panels. These are made from plywood or oriented-strand board (OSB), and should carry a stamp from APA-The Engineered Wood Association.

Span ratings on sheathing may have two numbers. For example, a 32/16 rating indicates the allowable roof span (32 inches) and a subflooring span (16 inches). Span ratings of 24/0, 24/16, 32/16, 40/20, and 48/24 are available.

If a sheathing panel stamp includes only one span number, the panel is for use as wall sheathing only. Sturdi-floor panels — a special APA designation for floor sheathing — may have span ratings of 16, 20, 24, 32, and 48 inches.

OSB vs. plywood. Although the strength and nail-holding ability of OSB and plywood are equal for similarly rated panels, OSB swells more when wet. Because of this, some builders avoid using OSB for subflooring, especially in kitchens or baths where exposure to wet conditions is likely. Some flooring contractors will not install hardwood flooring over OSB subflooring, even though tests have shown that it performs the same as plywood in this application.

All sheathing panels should be fastened with minimum 6d common nails — spaced 6 inches on-center at the edges of the panel and 12 inches on-center in the field. Keep nails at least 3/8 inch from edges. APA recommends that panels be gapped 1/8 inch (or the thickness of a 10d box nail).

Engineered Lumber The most common engineered lumber used on residential job sites is the I-joist. I-joists are considered a superior floor-framing member because of their stiffness (which reduces deflection and helps to eliminate bouncy floors) and ease of handling during construction. In addition, engineered beams, such as laminated

© 2005 DuPont or its Affiliates. All rights reserved. Page 2 of 5 veneer lumber (LVL), glue-laminated timbers (glulams), and Parallam (parallel- strand lumber, or PSL), are increasingly used for girder beams and headers.

Wood I-joists place the strongest material where the bending stresses are greatest — in the flanges along the top and bottom of the beam. This lets the lightweight beams span great distances with minimal deflection, providing a very stiff floor.

I-joist webs consist of plywood or OSB, making them more susceptible to water damage than sawn lumber. Therefore, I-joists should be stored under cover — particularly those with OSB webs.

Wood I-joists

Although they are light in weight, I-joists require special attention during handling and installation because they are an engineered product. Most manufacturers publish proprietary data regarding spans, fastening, blocking, and hole-cutting. Since each manufacturer's specifications are different, builders should consult current literature for the specific product they are using.

Laminated-veneer lumber, or LVL, is a strong, versatile engineered product that resembles plywood. LVL beams are typically built up from several LVL members nailed or bolted together. Bolting is preferable where the beam is side-loaded (e.g., a floor system with flush framing) or where it will be exposed to the weather. Side-loading with joist hangers can "peel" off the outer LVL members if they are not bolted together.

LVL is sold in depths of 7-1/4, 9-1/4, and 11-1/4 inches to match standard stick- framing sizes. However, LVL also is available in nonstandard sizes, such as 9- 1/2 and 11-7/8 inches, all the way up to 24 inches deep. These unusual sizes are meant to discourage builders from mixing engineered wood with sawn lumber.

Laminated Veneer Lumber

The characteristics of engineered lumber regarding swelling, shrinkage, and stability are much different than those of sawn lumber, so mixing is discouraged. That's why there's been a trend toward engineered-wood floor systems, which use a combination of LVLs and I-joists but no dimensional lumber. Nevertheless, manufacturers realize that builders will have projects where matching sawn lumber is required, especially in remodeling, so they also offer "standard" sizes.

Glulams

Glue-laminated timbers. A glulam consists of dimensional lumber face- laminated with structural adhesives. Three appearance grades are available: industrial, architectural, and premium. Design values are the same for all three grades, but the better grades have fewer surface defects. This is important in residential construction, where most glulams are left exposed. Glulams come in widths ranging from 2-1/2 to 10-3/4 inches. Depths are multiples of the lamination thickness: a glulam with eight 1-1/2-inch laminations will be 12 inches deep.

Parallel-Strand Lumber

Parallel-strand lumber. This material is sold under the brand name Parallam, and

© 2005 DuPont or its Affiliates. All rights reserved. Page 4 of 5 consists of strands of veneer that are aligned, coated with glue, and cured with a combination of pressure and microwave radiation. The result is stronger and stiffer than solid-sawn lumber and is equivalent in strength to multiple LVLs. The material is somewhat harder to cut and nail than solid lumber.

STRUCTURAL STEEL Because of the high bearing capacity of steel, it is a useful alternative for carrying heavy loads in wood framed buildings.

The common steel beam section is commonly referred to as an I-beam, but it is specified as a W or S beam. On a set of plans, a beam specified as W8x10 refers to a beam that is 8 inches tall and weighs 10 pounds per foot. Thus, a W8x20 beam would be twice as heavy as a W8x10 beam, and could carry a greater load.

Structural steel is graded by American Society of Testing and Materials (ASTM) standards. ASTM A-326 is the predominant grade in the structural steel market. Its carbon content yields a high-tensile-strength steel that is relatively easy to weld and fabricate.