Final assessment on the old wood deck substructure

I went through almost the same thing with my 20 year old PT frame when I pulled the old boards off two summers ago. My first thought was scrap it all, but once everything was exposed the joists were actualy in decent shape above the beam line. The bottoms where water sat against the double rim were the rough spots, so I ended up replacing the rim and a few bad board ends and sistering four joists that had soft spots near the hangers. Passed inspection and it saved me a couple grand over a full rebuild. Two things that made a big difference. I ran butyl tape over the top of every joist before the new decking went down, which keeps water from pooling on the fastener holes, the same spot that rotted mine in the first place. And I added blocking between the joists at mid span because composite is heavier than the cedar it replaced and the frame felt springy untill I did.

One thing to check on yours, most composite brands want joists at 16 inch centers or closer, and older decks framed for wood are sometimes on 24. Mine was allready 16 but a buddy had to add joists when he switched to Trex. Worth measuring before you buy anything.

I kept my frame when I resurfaced with composite three years ago, and the thing that saved me wasn't the joists themselves, it was fixing the water entry points first. My ledger had been nailed straight over old flashing that had failed, so every rain for a decade was running down behind it. I pulled the ledger, rebuilt it with proper standoff bolts and new flashing, and that did more for the frame's future than anything else I could have done.

One thing to check before you commit either way: composite decking doesn't span as far as the 5/4 wood boards you're taking off. My joists were on 16 inch centers, wich was fine for boards laid perpendicular, but when I switched part of the layout to a diagonal pattern the manufacturer wants joists closer togehter. I added a row of joists in that section while everthing was open. If your old frame was built on wider spacing, you may need new joists even if the wood is sound. I also ran self adhesive tape over the tops of every joist before the new boards went down. Cheap roll, half a day's work, and it keeps water from sitting in the screw holes and rotting the framing from the top down, which is exactly how my dad's deck died. Sounds like your call to replace makes sense given what you found, but if any part of the frame survives, that tape is the best money you'll spend.

I recall seeing a scenario where the failure wasn't actually in the lumber fibers themselves but in the connection points at the ends of the joists where the hanger meets the beam. It is important to meticulously inspect the existing hangers for signs of shear stress or localized rot around the bolts, as those points are often where movement puts the greatest strain on an aging structure. Also, check if those 12-year-old joists are 2x12s or 2x10s, as changing the depth of the wood significantly alters the required load capacity for the new composite material.

I'm looking at the supports and I'm also worried about the way everything is connected right now. It doesn't matter if the wood is fine, but I'm nervous that the old nails and screws might not be strong enough to hold the new heavier decking. I read somewhere that the old connection points might stretch or give out under new weight. When you build the new frame, what type of connection or bolt do people usally use to make sure it won't pull apart?

Totally agree with the part about the connection points. Even if the joist lumber itself looks solid, if the hangers are compromised, the whole structure is a liability. I once ran into this where the wood was fine, but the galvanized bolts holding the joist to the ledger had stripped threads from repeated movement. My fix was replacing those specific connections with much larger carriage bolts and washers to keep everything tight. That simple hardware upgrade really locked the whole frame in place.