Advice for building a gravel base on top of a sloping patio

[Tembroke]

I'm about to become a first-time hot tub owner, specifically a Caldera Salina. I plan to place it at the corner of my concrete patio, just a few inches back from the edge. Unfortunately, there's a significant slope -- from one corner to the other, it drops almost 3 inches. Based on advice I found on this forum and elsewhere, I constructed a box from 8-foot pressure treated 2x6s, cut to match the slope of the patio. Each corner is a simple butt joint using 3 deck screws, a metal corner bracket, and 8 roofing nails going into the corner bracket. I plan to fill it with pea gravel.

My question: Is there anything else I need to do to ensure that this base is effective for years to come?

For example:

• Should I get some concrete anchors and use them to hold the frame in place? Will a slope of a couple inches cause the entire frame and hot tub to slowly slide from the patio onto the adjacent dirt, or is normal gravity + friction enough to keep it in place? I don't want to do extra work and drill extra holes, but I will if it prevents a bigger hassle later.
• Should I place some 2x4s across the length or width of the frame to make it stronger? Do I need to worry about the 2x6s starting to bow out over time, and if so is this enough to prevent it? Again, I'd rather skip the extra work and complexity, but I'm worried that I'll regret it later.

Thank you so much!

[A.O.]

If you really dont want to regret things later, pour a concrete slab.

[Tembroke]

Hmmmm.... well, I've already built the wooden frame, so I could just put self-leveling concrete in there (plus some aggregate, since parts of it will be more than 2" deep). However, I've never worked with concrete before, and it seems less fool-proof than wood and gravel.

[A.O.]

Probably want the concrete at least 3" thick, preferably 4. If the top of your frame is level you can use it to screed the concrete, lots of u tube videos out there. More work to begin with and then it is done.. no more messing with it. Dont forget to install PVC to run your electric up inside the tub enclosure so you aren't tripping over it

[cranbiz]

What I would do is add 2x6 to the inside of the box, 16" on center. Then add 2x6 blocking to stiffen up the new stringers. Now you have a solid box that won't move. Fill the box with Crusher Run, tamp it down and then level off the stone. You could always use self leveling concrete to seal the crusher run in place but I don't think it's needed. That box isn't going to move, I wouldn't worry about anchoring it down.

I'm not sure I would stub the power up in the box. I would probably stub it on the perimeter, install a weatherproof junction box to the tub and then run seal tite into the spa pack.

I agree with A.O. that a slab is best but a properly constructed box with good drainage should last the lifetime on the tub and can be removed if you ever want to reclaim that area in the future.

[Tembroke]

Thanks for the advice! I called my local hot tub dealer and talked with the technician about different options. I don't want to go with concrete, because if I ever change my mind about the hot tub location then I've ruined a perfectly good patio. He suggested turning my box into a simple deck, sitting directly on the patio as before but with 2x6s every 16" and a layer of boards on top. I ripped and attached the 2x6s this afternoon, so now I just need to put a set of 2x6 boards on top, perpendicular to the joists. I think this simple 8' x 8' 3" deck will be a reasonable solution to my patio slope problem, but maybe I'm missing something.

[cranbiz]

That works too.

If supported in both directions with 2x6 decking, it certainly will be strong enough.

[Sam]

I have a good amount of experience with this.  Do not use pea gravel.  It is too "mushy".  Use 3/4" to 1" crushed rock.  It's definitely the ideal.  We've seen it all and this stuff reaches a good compaction level under it's own weight. 
Also, do not put any additional boards inside the frame.  Just do the outer perimeter.  You want the hot tub to sit on the rock (not the wood) and be uniformly supported.  You do not need to anchor it to the pad.  It won't move. 
These gravel boxes do not need to be complicated.  Just the frame and filled with rock.  No need for crusher or road base underneath.