Finally started ripping up the old boards and found a massive surprise

I had a similar nightmare when I ripped up the old Kwila boards at my place in Christchurch last summer. Instead of a hot tub, the previous blokes had buried an entire corrugated iron shed and just poured the concrete footings for the new deck right trough the rusted roof. Every time we had a heavy rain, the whole area turned into a swamp because the water had nowhere to drain. Getting that fiberglass out is going to be a massive pain. When I was clearing my mess, I ended up renting a heavy-duty reciprocating saw with a bunch of carbide-tipped blades to cut the larger chunks into manageable sizes for the skip bin. Make sure you check the soil underneath once the tub is gone. If it's been sitting there holding moisture for years, you might need to excavate another 200mm and backfill with some proper drainage scoria before you put down your H4 100x100 posts. It’s alot of extra sweat, but doing it right now means you won't be dealing with those sinking corners again in five years.

I had a bit of a different situation when I redid the front porch on our place in Dunedin. Instead of a buried tub, the guys before us used old car tires filled with bags of ready-mix concrete as the main supports for the 100x100 H5 piles. It looked like a total bodge job when I first saw the black rubber sticking out of the mud, but honestly, those tires probably saved the deck from the damp ground for thirty years. When I went to clear it out, I realized the tires acutally acted like a decent damp-proof course. While your hot tub shell is just a giant bowl for water, these tires were solid as a rock. I spent a whole Saturday with a crowbar trying to shift just one before I gave up. I ended up just boxing them in with new timber and adding some extra drainage scuppers. Sometimes those 'lazy' shortcuts are so over-engineered for the wrong reasons that it's more work to fix them than to just work around the mess.

Seen that shortcut plenty of times. People think fiberglass is a permanent filler, but it just traps moisture and rots your frame. If you're rebuilding, skip the cheap pine and go straight for some 140x32 Garapa or Vitex. It'll handle the NZ humidity way better. While you've got the site open, make sure you're using Simpson Strong-Tie joist hangers instead of just toe-nailing into the ledger. Since you're allready digging out that tub, are you planning on using 125mm square H5 piles or sticking with the standard 100mm? Going bigger now prevents any future sway.

That sounds like a nightmare to haul out! I'm just starting to learn about deck building and honestly had no idea you coud even use tires or buried sheds as a foundation. Is that even legal? I was looking at those H4 posts mentioned—what does 'H4' acutally stand for? I don't know much about the diffrent wood types yet, but if I'm digging out a mess like that hot tub, how do I know if the soil is actually dry enought to start building again?