Thoughts on diagonal bracing vs. extra blocking for lateral stability?

Those 45-degree knee braces on your 6x6 posts are a lifesaver if you are more than five feet off the ground. I used 2x4 pressure-treated lumber for mine and bolted them thru with 1/2-inch galvanized carriage bolts. Without them, you get that annoying lateral sway when four or five people start moving around up there. The decking boards help a little with racking, but they won't stop the whole structure from shifting over time as the wood dries and shrinks. Regarding the blocking, I wouldn't call it an either-or situation. I put solid blocking every four feet on my last build using scrap joist material and some Simpson Strong-Tie hardware. It really stops the joists from twisting and makes the floor feel like concrete. Since you still have those trenches open near the foundation, double-check that your post footers are deep enought. If the ground shifts becuase of the backfill, no amount of internal blocking will keep the deck from leaning later on.

I actually prefer skipping the diagonal braces if you can help it, especially if you ever plan on using the space under the deck for storage or a patio. Those braces are a total knee-knocker and can look pretty dated. I’ve found that installing double rows of solid blocking every 4 feet between your joists does more for the "walk feel" than those post braces ever will. When I built my 12x16 pressure-treated frame, I used 2x10 scrap for the blocks and staggered them slightly so I could end-nail through the joists.

To handle the actual sway, I swear by the Simpson Strong-Tie DTT2Z tension ties to pull the deck frame tight against the house's rim joist. If you pair those with a solid LedgerLOK pattern, the whole structure feels like part of the foundation. Once the 5/4 boards are screwed down, that surface acts like a giant diaphragm anyway. Unless you are ten feet in the air, those diagonal post braces are often just extra lumber that gets in the way of your finish work and never looks quite right.

I'm with the pro on the knee braces—they're essential for anything elevated. One small thing people forget is to alternate the direction of the braces on each post or install them in pairs. If you only brace one side, the lateral pressure can acutally cause the 6x6 to twist as it dries out, wich throws off your railing alignment later. I also found that using structural screws like TimberLoks alongside those carriage bolts helps pull the joints tighter and prevents those annoying squeaks when the deck shifts in the wind.

Pick up a few boxes of FastenMaster ThruLOKs for those knee braces. I’ve found that carriage bolts always end up with a little play in the hole as the wood dries, but these structural screws stay incredibly tight. It makes a huge difference in how much the deck moves when you're walking near the edge. While you have the framing exposed, I'd also install Simpson Strong-Tie DTT2Z tension ties. They connect the deck joists directly to the house's wall studs. Most of that 'sway' people complain about is acutally the deck slightly pulling away from the house, and these things make it feel like it's part of the foundation.

While the structural integrity of knee braces is indisputable, you must ensure that your contractor has properly integrated a lateral load connection to the house framing, specifically by utilizing tension ties that physically link the deck joists to the interior floor joists. Even with robust blocking, if you lack that positive mechanical connection thru the ledger and into the actual house structure, the entire assembly remains susceptible to lateral displacement that no amount of internal joist stiffening can truly mitigate. I whould also insist on checking the fastener schedule for your joist hangers to ensure they aren't using standard roofing nails, wich lack the requisite shear strength for such an elevated platform.