Flooring strength question

[[DHT]Osiris]

Putting feelers out for a question I've got that I'm a little concerned I won't get a good answer for.

I've got a solarium-style room in my place that we're looking at adding a hot tub into. The paperwork specifically cites the full weight of the tub + water at 2500lbs, and the base is 80"x70". We just moved into this place and I haven't tracked down any blueprints yet (gonna try to from whatever local zoning authorities might have that) but the room itself is tiled, with what looks to be a thinset layer, a ~1/2"-3/4" cement layer for our radiant heating system (I know that's in the room), followed by about a 1/8" layer of what I suspect is some kind of concrete backerboard, followed by another ~1/2"-3/4" of some kind of plywood or particle board. I'm able to determine these numbers based on a cutout in the flooring in a closet that let me take a snapshot of the layers.

I cannot confirm what's under these layers, but I can definitely say that there's a rather unsettling 'hollow' sound if I stomp on the tile, compared to other sections of the house, which leads me to believe there's probably a void underneath it, I'd assume with joists to support the weight. Would that be reasonable? Nowhere else in the house does it sound like that. As a side note, the room appears to be roughly 4-6" off what I believe to be the slab when looking from the outside. There's no step up into the room from within the home, which I suppose means the rest of the house (or at least the section of the house that room comes off from - it was built in two parts) has a void as well.

Actual question, would it be likely that this flooring, consisting of tile, some thinset, concrete containing some radiant heating pipies, probably backerboard, and particle board, would be able to support 2500lbs (+500 for 2-3 adults?) over a ~38 sq ft area. My math brings that out to ~80lbs per sqft. If I'm standing within a single sq ft section of the tile, I'm over double that. Am I correct in that I should not be worrying?

 

[herm0016]

there must be a way to access the crawl space if it exists. maybe drill a hole in the closet and rent an inspection camera? That is a lot of weight and i would be 100 % sure of what is under it before I installed it.

 

[Greenman]

Assuming the structure was built with some attention paid to building code, the floor system will be designed for a minimum of 40 pounds per square foot, and deflection no greater than 1/360th of the span.
If it's on sleepers resting on a concrete slab, it will easily hold the tub and as many bodies you can fit in it. Again, that's assuming some attention was paid to using the proper sub floor material.

 

[PowerEngineer]

Ha! I was just about to mention the 40 pound per square foot rule of thumb. So a normally constructed floor in a house will not support a hot tub without further support. If the floor joists are actually resting directly on the concrete slab, then as Greenman says you should be good.

 

[Greenman]

Ha! I was just about to mention the 40 pound per square foot rule of thumb. So a normally constructed floor in a house will not support a hot tub without further support. If the floor joists are actually resting directly on the concrete slab, then as Greenman says you should be good.

That 40 psf number will establish the joist size for the longest span in the structure, but there will be areas that have a much shorter span with the same size joists.

 

[drnickriviera]

Assuming the structure was built with some attention paid to building code, the floor system will be designed for a minimum of 40 pounds per square foot, and deflection no greater than 1/360th of the span.
If it's on sleepers resting on a concrete slab, it will easily hold the tub and as many bodies you can fit in it. Again, that's assuming some attention was paid to using the proper sub floor material.

Would the tub be calculated as a dead load?

 

[[DHT]Osiris]

Great info above, though upon greater inspection/consideration the room probably isn't appropriate for a hot tub anyhow (no drainage, no true venting, plus the aforementioned strength issues). I had a few people recommend the drill-a-hole approach to determine what was underneath, but it sounds like even with knowing that we'd probably spend a small fortune getting it to the place where it would need to be for this.

 

[Greenman]

Would the tub be calculated as a dead load?

Anything that isn't a permanent part of the structure is considered live load.

 

[Red Squirrel]

I would not put that much weight anywhere without having full access to the floor below to reinforce it. That's a lot of weight in a small area. Ideally I would put 4 reinforced concrete pillars that would end up being under the hot tub going down to frost level and have them hold up two beams made from 2x12's. (maybe 3-4 of them glued/screwed together) then lay some 2x12 joists that sit on top of the beams 16" OC or closer and then make the floor from 2x6's laid flat like deck boards. Or several layers of 1/2" ply might work too. You want to ensure the load gets spread out well to the beams.

If there is not much height involved it might be easier to just pour a reinforced concrete slab.

 

[Greenman]

I would not put that much weight anywhere without having full access to the floor below to reinforce it. That's a lot of weight in a small area. Ideally I would put 4 reinforced concrete pillars that would end up being under the hot tub going down to frost level and have them hold up two beams made from 2x12's. (maybe 3-4 of them glued/screwed together) then lay some 2x12 joists that sit on top of the beams 16" OC or closer and then make the floor from 2x6's laid flat like deck boards. Or several layers of 1/2" ply might work too. You want to ensure the load gets spread out well to the beams.

If there is not much height involved it might be easier to just pour a reinforced concrete slab.

The math to figure out what's required isn't that complex. It's a distributed load over a fixed area. All of it depends on what is existing and where the load ends up. None of it matters at this point because the OP decided the location of the tub was undesirable.

 

[Red Squirrel]

The math to figure out what's required isn't that complex. It's a distributed load over a fixed area. All of it depends on what is existing and where the load ends up. None of it matters at this point because the OP decided the location of the tub was undesirable.

Math isin't my thing, so I usually like to just overbuild things. 😛