I-Joists

Wood I-joists are a structural engineered wood product often used for joists and rafters in residential and commercial construction.

I-joists are made by gluing solid sawn lumber or laminated veneer lumber (LVL) flanges to a web made of plywood or oriented strand board (OSB). I-joists exhibit uniform stiffness, strength and are lightweight.

The "I" shape of these products gives a high strength-to-weight ratio. For example, wood I-joists 9-1/2" (241mm) deep and 26'-3" (8m) long weigh only about 55 lbs. (25kg), depending on the flange size. This means that they can be installed without machines, giving advantages in labor and cost.

Some wood I-joists are manufactured with knockouts in their webs for installation of wires. The knockout holes also provide ventilation when the joists are used in cathedral-type ceilings.

Holes for plumbing and mechanical ductwork are drilled easily through the web, but must be located according to the manufacturer's recommendations.

The wide flanges provide a good fastening surface for sheathing, and the product can be worked using common wood working tools. However, the flanges should never be notched or drilled and all special cuts, such as bird's mouth bearing cuts, must follow the manufacturer's recommendations.

The availability of wood I-joists used as floor joists and as deep, roof joists that allow for extra insulation has made them popular.  They are also an economical alternative to open web steel joists.

Several different wood I-joists are available and feature different designs and materials the joint between the flange and the web is critical and is typically patented by each manufacturer. Flanges are commonly made of laminated veneer lumber (LVL), visually graded lumber, or MSR lumber.

The webs are made of either oriented strand board (OSB) or plywood. Web panel joints are glued and mated by several methods such as butting of square panel ends, scarfing of the panel ends, and shaping of either a toothed or tongue-and-groove joint. The use of longer OSB panels is gaining acceptance as a means of lowering the number of end-to-end panel joints in the web.

Sizes

Wood I-joists are manufactured in a variety of sizes. Long lengths have finger-jointed flanges. Butt jointing or toothed, tongue-and-groove or scarf configurations are used to splice the webs. Lengths are limited by transportation restrictions to about 66 ft. (20m).

The depths of wood I-joists range from 9-1/2" (241mm) to 20" (508mm) with special orders up to 30" (762mm) deep. Flange depths are commonly 1-1/2" (38mm) and common flange widths vary from 1-3/4" (45mm) to 3-1/2" (89mm). Web thickness varies from 3/8" (9.5mm) to 1/2" (12.7mm).

Rim Board

Rim board is used to surround I-joist floor-joists. Rim boards mount between sills and wall plates. In addition to transferring lateral and vertical loads to the frame of the house, RIM Board can be ordered to match the depth of I-Joist framing members—something that conventional lumber typically does not match.

Quality Control

Some manufacturers make proprietary products with design and quality values that are unique to the manufacturing processes and raw materials. Other manufacturers produce products that meet the requirements for APA-The Engineered Wood Association performance rated I-joists. In either case, regular inspection by an independent testing agency is required to ensure quality.

Before commercial production begins, each manufacturer's product is extensively tested to determine its engineering properties. After that, random sample are tested to ensure the joists meet quality and strength standards.

In the U.S, the International Conference of Building Officials (ICBO) has published a document titled "Acceptance Criteria for Prefabricated Wood I-Joists," to establish criteria to evaluate wood I-joists. The Council of American Building Officials (CABO) has issued product evaluations for many kinds of wood I-joists. Also, the recent ASTM Standard D5055, Standard Specification for Establishing and Monitoring Structural Capacities of Prefabricated Wood I-Joists, outlines procedures for establishing strength values and controlling the quality of wood I-joists.

For Canada, the Canadian Construction Materials Centre (CCMC) has issued product evaluations for many of the wood I-joists marketed in Canada.

Fire Safety

Most of the leading manufacturers have conducted fire tests for common floor and roof assemblies to determine the fire performance of their products.

These evaluations are usually done by accredited certifying agencies and are applicable only to the specific I-joist and other assembly components being evaluated.

Information regarding the fire performance of assemblies incorporating wood I-joists is available from the manufacturers. The information is also available in listing books of the accredited certifying agencies such as Underwriters' Laboratories Inc. (ULI) in the U.S., Underwriters' Laboratories of Canada (ULC) or Warnock Hersey Professional Services Ltd. in Canada.

Tips

General guidelines for wood I-joists:

• Manufacturers' catalogues and evaluation reports are the primary sources for design, typical installation details and performance characteristics
  
• Typical considerations needed for product specification include: product availability, product sizes available (i.e. depths and lengths), availability of approved connectors, certified fire and sound assembly information, engineering and technical support provided by the manufacturer, product quality, product warranty, product acceptance and code approvals and installed cost effectiveness
  
• Wood I-joists should be protected from the weather during job site storage and installation

For more information on wood I-joists, visit:

The APA-The Engineered Wood Association