Shrinking Joists

We have a problem with ACQ-treated southern pine joists shrinking unequally — sometimes with differences as great as 1/2 inch. The narrower joists then create a swale by pulling down the composite decking. We crown the joists before installation and check them to be sure they're in one plane before decking. Do you have any suggestions? What about steel joists?

Andy Engel, editor of Professional Deck Builder, responds: In the 25 years that I've used southern pine joists, this problem has occurred more than once. Southern pine is strong and takes preservative treatment well, but in my experience its stability is hard to predict. I have several suggestions that might help.

First, movement in lumber is aggravated by unequal drying. If you close off the air flow below a deck, you can create a situation where the bottoms of the joists will stay wet while their tops bake in the sun. Try to build decks that allow air to circulate to encourage even drying.

In most cases, lumber will take its permanent shape upon its initial drying. So a simple solution would be to seek out joists that are dried after treatment, and stored under cover. You can spot these by the letters "KDAT" (Kiln Dried After Treatment) or "ADAT" (Air Dried After Treatment).

Dry lumber may be hard to find locally, and you may have to pester lumberyards to order it. You'll still find different widths in the same stack, but at least you can cull the oddballs while building the deck, and you'll have some assurance that the joists' relative sizes will be consistent.

Along the same lines, if you build a large volume of decks, you might find it worthwhile to buy joist material in bulk, and stack it somewhere it can dry for six months or longer. Don't stack it in direct sun, or the exposed wood will dry more quickly than the protected material, which will cause the exposed wood to warp. And don't expect any drying to happen outside in northern climates during the winter. You'd have to restack the lumber with stickers between the layers and with space between the edges of the lumber to allow for air flow.

Another strategy is to use shallower joists; a 2x8, for example, will shrink less than a 2x12, reducing the depth of any potential swale. You would have to decrease the joist spans, of course, by adding beams as needed.

Finally, yes, steel joists are an option. As reported in the Products column in the January 2007 issue of PDB, at least one company (Xccent, 800/933-4748, makes steel joists for decks.

What's an ICC-ES Report?

I have been asked to provide an ICC-ES report for a composite-decking product I wish to install. What exactly is an ICC-ES report and what is it for?

Glenn Mathewson, a former deck builder and an ICC-certified Master Code Professional, Combination Inspector for Westminster, Colo., responds: An ICC-ES report is issued by the International Code Council Evaluation Service, a not-for-profit subsidiary of the ICC that provides technical evaluations of building products and methods. Reports describe the suitability of a product or method as an alternative to a prescriptive code requirement, or certify compliance with a specific code requirement.

ICC-ES reports are vital for the proper use and installation of many products, and in almost all cases supersede the manufacturer's specifications, requirements, and installation instructions. Unfortunately, I have seen cases where manufacturers published information that conflicted with their product's ES report, leading the contractor into a code violation.

It is important to note that an ICC-ES report is not specifically required by the I-codes; the ICC Evaluation Service is merely a testing agency that is often recognized and approved by local building officials. It is up to the manufacturer to choose (and pay for) the lab that evaluates its product or method. Although many manufacturers use ICC-ES because of its national recognition, "evidence of compliance" can be provided by any agency that's approved by the building official, as stated in the 2006 IRC, section R104.11.1.

While ES reports are important for the proper use and installation of many products — such as fasteners, hangers, decking anchors, manufactured guards and handrails, and pressure-treated lumber — they are especially informative when it comes to the installation of composite decking. Unlike wood, which is subject to an industry-wide grading system, composite decking can be produced with a variety of methods and compositions, and properties can vary widely between products.

Most composite decking is evaluated only for exterior use in residential occupancies, and almost all composite boards are required to span a minimum of three joists and be fastened with two fasteners per support. Other requirements, however, vary considerably. The distance that a deck board can span at angles to the joists, for example, is specific to the manufacturer.

Also, the distance a deck board can span when used as a stair tread will almost always be less than when the same material is used as decking. Stair treads are required by the IRC and IBC to resist not only the minimum uniformly distributed live load, but also a concentrated 300 lb. load on a 4-square-inch area in the middle of the tread span.

This concentrated load almost always requires the support spacing to be reduced. Some composite decking must be supported 8 inches on center when used as a stair tread. Correcting something like this after the fact would be a nightmare, pointing up the benefit to researching the ES report and product limitations before the project begins.

ES reports are free of charge and available to anyone at When searching for an ES report, use specific spelling and keywords. Sometimes the search feature can be tricky, though. If at first the report you seek doesn't come up, try again with different words.

Another great Web site that may help you find an ES-report number for a composite-decking product is the Ten Thousand Lakes ICC-chapter Web site. This local ICC chapter has put together an excellent list of information on composite decking, including ES report numbers for those products that have one. The list is easily accessed by clicking the link "Composite Decking updated" at

It's obvious that the codes cannot specifically address all products and construction methods — and the intent of the code isn't to ban the use of alternative products. The evaluations in ES reports provide for flexibility and creativity in your building by answering questions and concerns about safety.