Best way to fix buried posts without a full teardown?

I had the same issue with a 12x16 deck at my old place. The previous owner just sank 4x4s into the clay. Within five years, I coud literally poke a Phillips head screwdriver trough the base of the posts. Waiting for it to fail is a bad idea beacuse once it sags, it pulls the ledger board away from the house, and then you are looking at water damage in your rim joist too. I ended up jacking it up one section at a time using a 20-ton bottle jack and some temporary 4x4 shoring. I dug out the old posts, poured 12-inch Sonotube footings with Quikrete, and set them high enugh so the wood stayed well above the grade. Using Simpson Strong-Tie ABA bases made a huge difference for keeping the wood dry. It is a weekend of back-breaking work, but shifting the load to side-piers always felt like a hack to me that makes weed-whacking a nightmare later on. Better to just do it right once and sleep better when the wind picks up.

I think jumping straight to jacking and pouring is a bit premature, especially since digging 4-foot deep holes in a crawlspace is a total nightmare. I had a similar setup with 6x6 posts on a low-profile deck. Instead of a full replacement, I focused on getting the wet dirt away from the wood. I dug out about 12 inches around each post, scrubbed off the algae, and let them dry out for a full week. I used a heavy coat of Copper Green wood preservative on the underground portion and then wrapped the base in Bituthene tape. After that, I backfilled with 3/4-inch crushed stone rather than dirt to ensure drainage. It’s been eight years and those posts are still rock solid when I do the screwdriver test. If the wood isn't soft yet, you're mostly fighting moisture retention, not immediate structural failure.

Unless the deck is already sagging or out of level, you might be over-engineering the fix. If you really want peace of mind, look into somthing like the Simpson Strong-Tie FPBS44. It lets you retro-fit a base without having to rebuild the entire support structure. It’s a lot less back-breaking than trying to pour a perfect 24-inch footing while lying on your stomach in the mud.

If you're dreading the digging, you shold look into Diamond Pier foundations. I used the DP-50 model to fix a sagging porch last summer and it saved my back. You just set the concrete head and drive the steel pins in with a demo hammer—no massive holes or mixing bags of wet concrete in a cramped space.

It gets the wood away from the soil instantly and they’re rated for serious loads. Way better than trying to DIY a pour next to an existing post.

I went trough this two summers ago. Don't bother digging out the old post if it's allready rotting at the bottom. I just cut the post about 8 inches above the ground while it was jacked up, cleared the dirt, and threw down a pre-cast concrete pier on a packed gravel base. Then I put a new 4x4 on top of that.

It’s been rock solid and I didn't have to haul 20 bags of concrete under the deck. Getting a cheap laser level from the hardware store was the best $40 I spent; it showed me the deck had already dropped two inches before I even started.

If you’re jacking it up anyway, don’t skip on the hardware upgrades. Everyone focuses on the footers, but you need to check that beam-to-post connection while the weight is off. I’d swap any old nails for GRK RSS 5/16-inch structural screws to really lock it in. For the base, the Simpson CPTZ series is the pro choice—it gives you that 1-inch standoff to kill the rot cycle while staying concealed. Just make sure you’re setting a 1/2-inch Titen HD anchor into the new concrete. It’s the only way to ensure that timber stays dry and doesn't kick out once everthing settles.

I'm just starting to learn about all this and honestly didn't realize putting posts in dirt was such a big no-no. I've got that same green stuff on mine. Is that always a sign of rot, or coud it just be harmless moss? Also, I'm pretty nervous about the jacking part. Is there a specific spot you have to place the jack so you don't accidentally snap a joist? I don't know much about weight distribution, and the idea of being under there while the deck is "floating" sounds scary. Do you need special braces to keep it from tipping sideways?