Bracing & Sizing
Best Practices: Decks - Bracing & Sizing
As a Northern Utah Home Inspector, I find that the biggest difference between decks built 15 years ago and decks built today is the need for additional bracing. This added feature creates such a stronger and safer deck, that it is important to follow the codes and best practices. This is another segment in a series provided by the Journal of Light Construction trade magazine, and while all of the information presented isn’t mandated by the building codes, several of the specifics greatly exceed the code and this is why they are deemed “Best Practices”. If you are going to build a deck, or have one built, then I highly recommend that you take the safer “Best Practices” approach. It may cost more on the initial investment of the build, but the “Best Practices” detailing will provide years of trouble free peace of mind while you enjoy time on your deck.
Journal of Light Construction (JLC) - Bracing & Sizing
While the fasteners in a ledger-rim joist connection made according to IRC specs should be adequate to resist shear forces, lateral forces may still pull the ledger and rim board away from the house. The best practice, suggested by the 2012 IRC (Figure R507.2.3), is to use hold-down tension hardware in at least two locations to tie deck joists to floor joists of the primary structure. The illustrations show suggested methods for bracing ledgers when the deck joists run either parallel to or perpendicular to solid-sawn or wood I-joist floor framing.
Sizing Posts, Beams, & Joists
Here are some rules of thumb for sizing framing members for simple deck designs. To size framing for complex designs, or decks with long spans or multiple stories, consult your building department or an engineer.
Sizing deck posts. Most decks that are 8 feet or less above grade with beam spans of 10 feet or less can be built safely with 4x4 posts, but the American Wood Council (AWC) recommends using 6x6 posts in all cases for decks up to 12 feet off the ground, primarily because they are better able to resist bowing.
The material and size for structural posts for decks taller than 12 feet, or that support multistory decks, should be determined by a professional engineer. Posts taller than 12 feet may also require lateral bracing to resist bowing.
Beam and joist sizes and spans. The Prescriptive Residential Wood Deck Construction Guide (free download with registration at awc.org), published by the American Forest & Paper Association, offers useful span tables for the wood species typically used for deck beams and joists.
The tables excerpted above show changes made as of June 1, 2013, to reflect new design values for visually graded southern pine. (Old design values are grayed-out/struck-through and shown above new values.) The full tables also show values for Douglas fir-larch, hem-fir, SPF, redwood, western cedars, ponderosa pine, and red pine, for joist sizes 2x8 through 2x12, and beam sizes 3x6 through 3x12 (or 2-2x6 through 2-2x12), 4x6 through 4x12, and 3-2x6 through 3-2x12.
Michael Chotiner is a contributing editor to JLC.
FOR MORE LATERAL LOAD INFORMATION:
- See the Simpson Strong-Tie LATERAL LOAD TECHNICAL BULLETIN
- See also Professional Deck Builder's article LATERAL LOAD CONNECTIONS