Q&A / 

Wood I Joists

DEAR TIM: My wife and I are building a new home soon. Our builder wants to use modern wood I joists instead of regular solid wood floor joists. He says they are stronger than solid wood. Will these products save the builder money and leave my wife and I with a flimsy floor? Do wood I joists offer other advantages? Is it safe to drill holes in the joists? Is there any other alternative floor joist material? Tony P., Wahoo, NE

DEAR TONY: Congratulate your builder for catching the wave. Wood I joists made from engineered lumber are an excellent building material. 627 million linear feet of these key structural members were made in 1997 for the North American construction market. That number represents 33 percent of the total lineal footage of floor joists used in North America in that same time period. By the year 2005, it is expected that engineered wood I-joists will be used in over 50 percent of the residential homes built in the USA and Canada.

Engineered lumber is very environmentally friendly. The lumber used to make the wood I joists probably came from a tree that grew somewhere on the 241 million acres of forest land that is suitable for repeated planting of trees and commercially logged. Currently, 27 percent more timber is planted and grown each year than is harvested. To insure that future generations have plenty of wood for their houses, the forest products industry replants about 3 million trees a day!

Wood I joists resemble traditional steel I beams. After all, a floor joist is simply a beam. The wood I-joist consists of a top and bottom flange that can be made from structural composite lumber (glued lumber veneer) or regular sawn lumber. These flanges are grooved to accept a 3/8 inch thick vertical web that is made from structural plywood or oriented strand board. All of the adhesives used to make these products are designed for exterior exposure until such time as the house can be protected from the weather. It is important that your builder keeps the wood I joists as dry as possible.

If your architect and builder work together you can minimize and sometimes eliminate bearing walls, beams, and support columns. You can purchase wood I joists in depths up to 16 inches. Using 16 inch deep wood I joists spaced at 16 inches on center, you can span 26 feet 1 inch and still meet virtually every modern building code. Just about every wood I joist is stronger inch for inch than standard solid floor joists. In many instances, the use of wood I joists can lower construction costs.

Wood I joists that are made in conformance to the PRI-400 Performance Standard as written and enforced by the APA / Engineered Wood Association are your guarantee of a quality product. This standard has made life easier for designers and builders by providing uniformity within the wood I joist industry. Companies that decide to meet the stringent testing and quality control can place the PRI trademarked stamp on the web of their products. Look for this on your wood I joists.

Your builder and all sub contractors can drill holes in wood I joists. However, most of them come with knockouts or prescored holes approximately every two feet on center for small water supply pipes and electrical wiring. Make sure your builder uses these whenever possible. Large holes for plumbing drain lines and heating and cooling ducts can be cut as long as the mechanics follow easy-to-follow guidelines provided by the manufacturers. Absolutely make sure that no one drills through, cuts or notches a top or bottom wood flange. Avoid rectangular holes with sharp 90 degree corners. The inside corners of the rectangular cuts should be rounded to minimize stress on the wood I joist.

You can also use wood floor trusses instead of traditional joists and engineered lumber wood I joists. These structural members remind me of the sides of many bridges that cross rivers. Floor trusses are very open and you can readily snake pipes, wires, ducts, etc. through them. These floor joists can be built to meet just about any need. If your carpenter leaves the sheathing off the end of the floor trusses the mechanical sub contractors will be able to install pipes and ducts much easier. Once these utilities are installed, the carpenter can button up the ends of your house.

Author's Notes - August, 1998:

Three weeks after this column ran in the Staten Island Advance, I received a letter from an Assistant Fire Chief of the New York City Fire Department. This individual - speaking on behalf of the department - felt that you and I should know that wood I-joists, "...may demonstrate excellent structural characteristics under non-fire conditions, it (wood I-joist construction) has proven to be disastrous under fire conditions with early collapse in under five minutes."

The Assistant Chief's point is right on target. Typical solid-wood floor joists take much longer to burn through in a fire. This means that a fireman charging into a burning building built with regular solid-wood joists can attempt to rescue you or a loved one with a degree of confidence. Firefighters die and are seriously and permanently injured every year when they are trapped in burning building collapses. This is a serious issue. The growth in usage of engineered wood products threatens the lives of firefighters all across the nation.

I responded to the individual and promised that I would post these notes. Furthermore, I urged him to continue to push code writing officials to mandate residential fire sprinklers in certain high probability fire locations in the typical residential home. I urge you to consider installing a simplistic, yet effective, fire sprinkler system in your home if you decide to use wood I-joists or floor trusses. Read my past column about nearly-invisible residential sprinklers. Fire sprinklers not only can save property but they can save the lives of you, your family and firefighters who are constantly on guard to serve, protect, and save you.

Tim Carter


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