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Trimmable Open Web Floor Truss

Trusses that can be shortened by as much as 12 inches on each end

A stack of floor trusses in the factory

Open-web, or parallel flat chord trusses, represent the predominate type of floor truss used in homes. They typically consist of a wood top and bottom chord, usually 2x4 material, and wood web materials connected at joints with metal plates. A few manufacturers use steel webs. One advantage of open web member over dimension lumber or I-joists, is that the open space between web members allows for easier routing of utilities. One downside is that truss dimensions must be known in advance to fairly close tolerances. Manufacturers and codes generally do not permit trusses to be trimmed or altered in the field. The trimmable floor truss adds the flexibility of allowing the member to be shortened by as much as 12 inches on each end.

There are at least two types of trimmable floor trusses on the market. The first type is a hybrid of truss and I-joist technology. The main part of the truss has steel webs with top and bottom chords made from Laminated Veneer Lumber (LVL) or 2X lumber. The web material for a short distance on each end is made from OSB, effectively forming an I-joist on each end that can be trimmed as needed.

The second type of trimmable open-web floor truss is an all-wood truss. This product has a section of dimension lumber on the ends as opposed to an I-joist. It does not rely on truss plates for connections like most open-web wood trusses. Rather, the chords and webs are connected using finger-jointing technology.

The trimmable end on each type of truss can be cut in the field to add flexibility to the product that does not exist with the typical open-web floor joist. On the truss with the I-joist ends, the product can be trimmed up to 12 inches on each end. On the second type of truss with the solid wood ends, the length that can be cut is limited to 5½ inches.


Affordability

Trimmable open web trusses are similar in price to their open web and I joist counterparts.

Quality and Durability

Trimmable open web trusses provide the same structural support and rigidity that regular open web trusses and I joists provide, while allowing the contractor to make some adjustments on-site. This allows for a well pieced structure and a stronger house.


Easy

Trim-able open-web floor trusses are commercially available, although the number of distributors is currently limited. The manufacturer contacts can provide a listing of distributors, or otherwise provide ordering information. When they do become more-widely distributed, the issues associated with trim-able floor trusses should not be much different than with I-joists or wood floor trusses.

Like nearly all engineered products, the trim-able joist must be designed for each floor system. Likewise, repairs will likely need to be approved by a qualified designer. Most distributors have software to design the floor system and typically provide this service.

The amount of trim-able section of these joists is limited, typically to within 12 inches on each end, although one product limits cuts to 5½ inches. Installers must become familiar with the allowable cutting limits and with the installation of the overall product. Because this is a hybrid of I-joist or lumber and truss technology, framers should already be familiar with the basic materials.


Like most engineered systems, trim-able joists are typically sold as a package that may include all or part of the framing package and associated design services. The manufacturers claim their prices fall in the range of those seen for I-joists and wood trusses.


Not Applicable


Because trim-able open-web floor joists are an engineered product, they must be designed to the appropriate structural provisions of applicable building codes. Often, code officials will accept an approved design provided by the distributor, as is typical with I-joists and floor trusses. Manufacturers of these products have also obtained evaluation reports from various model code evaluation services. Examples include NER 502, available from the National Evaluation Service (NES) and PFC-5725 (from ICBO). NES and ICBO provide reports that are generally accepted as a code approval by most building inspection departments in the United States. On February 1, 2003, America's four building-product evaluation services officially combined their operations under the International Code Council. The four "legacy" evaluation services that came together to form ICC-ES were the National Evaluation Service, Inc.; BOCAI Evaluation Services; ICBO Evaluation Service, Inc.; and SBCCI Public Service Testing and Evaluation Services, Inc.


Not Applicable


The installation of a trim-able floor joist made with I-joists on the end is similar to the installation of an I-joist. The installer must follow installation details which often include squash blocks at center bearing points, web stiffeners, and blocking and/or reinforcement at cantilevers and other locations. Bracing perpendicular to the truss may be required depending on the span and the manufacturer.

The details for the truss made with dimension lumber for the trim-able ends include the typical bridging or bracing seen with wood floor trusses. Because the ends are solid dimension lumber, the details at bearing points and cantilevers are not as complicated as with the truss that uses an I-joist on the trim-able end.


Not Applicable


Trim-able open-web floor trusses represent improvements that can increase productivity on the jobsite. They add new flexibility to the standard open-web floor truss by allowing the installer to cut them to length within certain limitations. And they do this while maintaining the benefits of open-web trusses including relatively long spans, light weight, and the ability to route larger ducts and piping through the open spaces between the webs. They also can encourage the use of engineered wood and other products that reduce the burden on forests.

Both types of trim-able trusses are made with materials that are used frequently in the homebuilding industry. They do not require different tools for their installation than are found on today's construction site. Even the truss with steel members does not introduce new tool requirements, since connections for floor sheathing and to supporting structural members are through the wood members used on the top and bottom chords.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.