Is this Swedish island deck build going to hold up against the weather?

The risk of rot on that siding from upsplash is definitely real, especially if the deck boards run tight against the house. To mitigate this without ripping anything out, you might want to look into installing a metal kickplate or some **Z-flashing** along the bottom of the siding. Keeping a small gap—maybe 10mm—between the house and the first deck board helps a lot with airflow and drainage. If the wood is allready close, just keep an eye on that bottom edge every season and make sure it’s heavily sealed. Using bitumen and felt on the end grains is an old-school move that actually works quite well when you can't get modern joist tape. It stops the moisture from wicking up into the vertical supports. As for those 45-degree hidden screws, the main trick is the spacing. Since you're on an island with high humidity, the wood is going to move a lot. Those fasteners usually allow for a bit of seasonal expansion, which is likely why those older decks you saw are still holding up.

A 12-person table adds a massive amount of weight, but the bigger issue is the live load when people are shifting chairs or the grandkids are running around. Since you've already added extra bracing near the door, you're ahead of the game. Most DIY decks fail because of lateral sway, not vertical collapse. If the table feels bouncy once it's loaded up, you can always add some **X-bracing** or blocking between the joists. It stiffens everthing up significantly without needing more concrete footings.

Being in a coastal enviroment, the salt air is the biggest enemy for the hardware. Even if the lumber is treated, you have to be sure your screws and joist hangers are hot-dipped galvanized or stainless. Standard zinc-plated stuff won't last five years in that salt spray. The precast concrete was a smart call for a remote spot; as long as they’re sitting on a solid bed of gravel and not just raw soil, they shouldn't shift too much during the winter.

I've been looking into those hidden screws too, but I'm worried about the wood splitting if I use softer pine. Did you have to pre-drill every single hole at that 45-degree angle or does the tool handle it all? I’m also curious about the salt air versus the concrete. Do those precast blocks ever crack from the salt, or is that mostly just a metal hardware problem? My cousin's place is near the water and they had a hell of a time with their footings shifting after a big storm, so I'm trying to figure out if I shoud stick to the precast stuff or go deeper.

Regarding the hidden fasteners, if you used a jig, they usually have a guide so you don't need to pre-drill unless you're at the board ends. In Sweden, our pine is usually forgiving, but it expands alot with the humidity.

Since you're on an island, watch for frost heave with those precast blocks. If the ground under the gravel bed holds moisture, the deck might 'walk' during winter. If the door starts sticking in spring, that’s why. Also, for coastal spots, double-check that your hardware is 316 stainless. Even hot-dipped galvanized can struggle if the salt spray is constant.