The wood deck diagonal bracing is installed for lateral support in this installment of the sagging deck repair series. This project is continued from How to Build a Code Compliant Deck Railing – Part 2.
Wood Deck Racking, Lateral Loads and Diagonal Bracing
Lateral support in the form of diagonal bracing is required by the deck building code to resist “racking” (side movement) caused by lateral loads. Lateral forces and loads are caused by wind, live loads (people) and vibration (seismic/earthquakes). Wood deck knee bracing is commonly used because it looks nice and doesn’t obstruct the view as would K- or X-bracing. The following illustration explains wood deck racking and knee bracing:
See Figures 22 and 23 on page 13 of the Georgia Amendments Prescriptive Deck Details based on the 2012 International Residential Code for Knee, K- and X-bracing requirements. Your local building code may have specific requirements, so call your local Building Dept. or Authority Having Jurisdiction (AHJ).
How to Install Deck Post Knee Braces
Home Builder Installed Knee Bracing
The 4×4 knee bracing installed by the home builder was removed with portions of the deck rail to replace the deck post. This is the original deck construction:
The builder-installed knee bracing was toe-nailed, out of alignment and added little in the way of structural support. I plan to replace all knee bracing throughout the deck.
I would have installed new knee bracing before rebuilding the deck rail, but the rail wasn’t in a safe condition after removing several 4×4 guard posts to replace the deck post and needed to fixed first.
Install Wood Deck Knee Bracing
The Georgia Prescriptive Deck Code that I’m required to comply with calls for 2x knee bracing fastened to the deck beam and post using 1/2 inch through-bolts with washers. I could have gotten by with 2×4 knee bracing but thought that wouldn’t look right nor would be strong enough on my ~15 feet high deck. Knee bracing is installed such that it’s offset 2 feet from the deck beam and post per Figure 22 in the code:
I pre-stained, measured and sawed #2 grade pressure treated 2×6’s to make the knee braces. Working by myself it was minor challenge to hold the board and pin it in place with a single Simpson Strong-Tie SD #9 x 2-1/2 screw in each end until I could install 1/2 inch bolts:
You only get one chance at drilling a straight hole in a 6×6 deck post and instead of attempting to it drill freehand, I used the Wolfcraft Drill Guide Attachment with a 3/8 inch wood spade bit to drill pilot holes for the 1/2 inch bolts:
After drilling a 3/8 inch pilot hole with the drill guide part way into the post (the drill guide stroke is too short to go all the way through the post), I finished the hole with a 1/2 inch x 12 inch long drill bit:
The deck code requires drill holes, field cuts and notches be treated with wood preservative. I used pipe cleaners to swab a generous coat of Copper-Green Brown Wood Preservative inside the holes, then installed 1/2 in x 8 in galvanized bolts with washers at the head and nut.
The two corner post knee braces are installed in the following photo. Note that both galvanized 1/2 through-bolts are centered in the deck post and it was necessary to offset the 2nd bolt vertically so it doesn’t collide with the first bolt:
Exterior view of the new deck knee bracing:
The next knee brace is installed at the right post just below the sister block to support the improper beam splice:
Take care to locate in advance deck joists and joist hangers that may be in the way before sawing and drilling holes. It’s OK to place a bolt a bit off center or move the knee brace an inch or two to avoid framing members:
This project is continued in How to Replace Wood Deck Boards.