When it comes to decks and especially exterior stairs, there are several critical areas that can spell the difference between safe and dangerous construction techniques. For that reason, current code requirements focus on some of those areas. In this article, we’ll look at just one detail: the prescribed method for securing the bottom newel post at the base of a stair.
A Note from the Publisher:
WARNING: POTENTIAL CONFLICT OF INTEREST!!
If you are sensitive about articles that seem to favor a particular manufacturer, then DON’T read this one!! It’s written by an employee of Simpson Strong-Tie! But after years of experience, I’ve found that some manufacturers know more about their products than anyone else; if a carpenter wants to learn the best way to install a product, sometimes the best source of information is the manufacturer. In the future, look for more carefully-screened articles from manufacturers.
I’ve seen just about everything when it comes to setting bottom newel posts.
|Depending on soil conditions, climate, wood treatment or the type of steel, wood or steel posts buried a foot or more into the ground may rot or corrode. The 4×4 post can be embedded in the ground or concrete, but frequently the post may still rot at the surface-to-ground interface.|
Another common method is setting the post on a concrete slab—with no footing or structural hardware at all. Neither method ensures a safe installation.
Newel posts are dynamically linked to stair systems, both aesthetically and structurally, which is why stringer and post installation must be planned carefully and concurrently. There are several code requirements to consider for posts and stringers.
The stringer must support a design live load of 40 psf for residential decks. Therefore, the top of the stringer must be anchored with a proper connector (keep an eye out for future TiC articles on stringer-to-deck attachment); the bottom of the stringer should rest on an appropriate footing or be anchored to the post, which itself should rest on an appropriate footing. One approach to this detail can be found in figure 34 of the American Wood Council’s (AWC) 2009 Design For Code Acceptance 6 (DCA 6-09).
One problem with the detail in Fig. 34 is the 2×4 bearing block embedded into the ground. Most 2x material is not rated for ground contact and may rot fairly quickly.
|The post could be embedded in the concrete, but once again, when the wood dries out, it shrinks and rainwater or landscaping water drains into the gap, rotting out the post.|
Of course, it may take years to rot out a buried or embedded post, and some people believe that the homeowner will replace the stairs and posts when that happens, and perhaps the entire deck. But building something and knowing it will have to be replaced, or knowing it will fail, is generally not a good practice. Furthermore, homeowners naturally assume that structures will last for years. Unless a contractor informs a homeowner that their deck has a limited lifespan, that the posts may rot and should be maintained, a homeowner will never attend to that maintenance.
Though it isn’t pictured in the Prescriptive Residential Wood Deck Construction Guide (DCA 6-09), properly secured post bases are also the best method for meeting another critical code requirement: if the post is part of a guardrail system, it must resist a lateral design load of 200 pounds.
To meet the 200-pound lateral load requirement, Simpson Strong-Tie recommends using a hold down or “Deck Tension Tie” (DTT2Z). The Deck Tension Tie must be installed to a minimum 2x riser or blocking, and it must be attached using 1/2-in. machine bolts.
If you use a post base connector with fasteners (bolts or nails) that drive through the post, these fasteners may interfere with the DTT2Z bolts. However, fastener interference can be avoided if you use a Simpson Strong-Tie ABA44Z with SD#9 x 1½ in. screws.
In either case, the Deck Tension Tie can then be attached to the riser or blocking under the steps to resist lateral or outward force. (See the DTT technical bulletin for more information.)
Keep in mind all assemblies should be approved by the authority having jurisdiction.
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Jim Mailey is the Midwest, Northeast and Mid-Atlantic market-training manager for Simpson Strong-Tie–a company that, for more than 56 years, has developed structural products to help people build safer and stronger homes, buildings, and decks. Jim joined Simpson Strong-Tie in 1992 and he has given hundreds of presentations to more than 25,000 design professionals, building officials, builders, contractors and dealers. He has developed numerous programs designed to educate industry professionals about how to install Simpson Strong-Tie products, as well as how these products meet various building code requirements. Jim is considered an expert in safe, outdoor wood deck construction and he provides economical product solutions to satisfy structural code requirements. He has written articles about deck safety and has been quoted in deck contractor and home inspector publications. His programs, entitled “Deck and Porch Framing Connections” and “Continuous Load Path-Wood Framed Structures,” review the correct and incorrect structural methods for building a deck or home, show why commonly accepted practices should not be used, and provide informative tips that anyone (from the novice to the most experienced student) will find useful. Jim earned a B.A. from Bloomsburg University in Bloomsburg, Pennsylvania, in 1980.
Simpson Strong-Tie is committed to helping customers succeed by providing exceptional products, full-service engineering and field support, product testing and training. For more information, visit the company’s website at www.strongtie.com.