Thoughts on trusting older cabin decks vs. being a buzzkill?

I’ve been in that exact spot at my uncle’s place in the Sierras. He had an old Tiger River spa on a deck that looked solid enough from the top, but once I crawled underneath with a flashlight, I saw they’d only used 2x8 joists spaced 24 inches apart. A gallon of water weighs about 8.3 pounds, so a 300-gallon tub plus four adults is easily pushing 3,500 pounds in a tiny 7x7 footprint. That’s a massive concentrated load for standard residential framing.

If I can't see the ledger board attachment or the footings, I usually stay out. I’ve seen too many stories of deck failures where the wood looked fine because it was stained every year, but the carriage bolts had rusted thru or the ledger wasn't even flashed. If you see the water in the tub swaying when the kids jump nearby, that’s a huge red flag for lateral stability. I’d rather be the "buzzkill" than the guy who ends up in the emergency room because the rim joist peeled away from the house. Trust your gut; if it feels sketchy, it probably is.

I honestly think the math-heavy approach misses the reality of how these old mountain cabins were put togather. Most of the time, those guys used massive 6x6 posts and rough-sawed timber that doesn't even match modern lumber yard dimensions. If that deck has survived decades of heavy snow loads on top of a full tub, it’s already been trough its worst-case scenario. Snow sitting three feet deep can easily exceed the weight of a 300-gallon spa, and if it hasn't buckled yet, it probably won't tonight. I'd be way more worried about the sanitation than the structure. If you've got a "brown puddle" on the cover, the chemical balance inside is probably non-existent. I’ve seen those old HotSpring Highlife models run forever, but the heaters and seals eventually get sketchy. I’m less afraid of the floor falling out and more afraid of what’s growing in the plumbing. Check the water clarity and the smell first; a structural collapse is rare, but a nasty skin infection from a neglected filter is a guarentee.

Back in the day, we didn't have all these fancy metal tension ties or structural screws. If that HotSpring has been sitting there since the 90s, I’d be checking the hangers for 'nail sickness.' Seen plenty of old decks where the galvanized coating wore off and the salt from the tub water just ate the common nails right thru. If you see those rusty streaks bleeding down the wood, she’s a goner. I’ve seen guys try to sister joists after the fact, but if the ledger's pullin' away, no amount of extra wood underneath is gonna save you from a ride to the bottom.

While the previous points regarding fastener corrosion and snow load history are certainly valid, I find myself more concerned with the specific interface of the post-to-beam connection, particularly becasue those older HotSpring models often lacked an integrated base frame that distributed the load, leading to localized crushing of the decking fibers known as compression perpendicular to the grain. If you observe any significant cupping or indentation where the tub’s weight meets the joists, it suggests the cellular structure of the timber has already been compromised by long-term hydrostatic pressure, wich essentially renders any theoretical load-bearing capacity calculations moot regardless of how substantial the original beams appeared to be.