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Unread 06-13-2011, 03:20 AM   #1
njw
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Tile hot tub surround

I have an exterior freestanding hot tub set into the side of a bank within a three-walled concrete structure. The structure is retaining soil and also providing a solid level surface underneath. I would like to build a minimal wooden deck on top of the concrete walls in order to support stone tile around the tub. The decking would be 12" wide all around the tub, spanning 8ft on each side. (See attached image, ignore arrows)

My plan is to build a frame of 2x8 pressure treated joists 11" o.c., anchor this onto the top of the concrete walls, then add exterior plywood, mortar bed, noble deck, thinset and tile. I'm almost sure that there are flaws with this plan so I'm hoping that the more experienced voices on this forum can weigh in and offer advice.

Some questions I had:
2x8 joists - large enough to support stone tile?
plywood base - how many layers and how thick? thinking I can get away with one 3/4 sheet since it's only 11" between "joists".
environment - thinking exterior freeze-proof slate. any gotchas for rainy Seattle or heat from proximity to hot tub?

Thanks for any suggestions. Nick
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Unread 06-13-2011, 04:36 AM   #2
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Take a look at this from the Noble company. Note that they are clear that this is not for freeze/thaw environments. They require a cement backerboard to reduce the effect of wood expansion and contraction. The backerboard must be exterior rated, too.

As for the joists: Get dry treated lumber, not pressure treated. Pressure treated will warp as it dries out. I'm not sure about the deflection rating, as I cannot tell what the span will be. Use the Deflecto tool to see for yourself. You need L/720 for stone.

Stone flooring requires 2 layers of plywood, regardless of joist spacing. If you switch to a porcelain tile, your structural worries will be much less. The joists can be L/360, and a single layer of plywood and backerboard will be sufficient.

Unless you are using Vermont slate, I'd be worried about it being "freeze-proof." Slate has many layers that can allow water to infiltrate. When it freezes, the water expands and splits the stone along the layers. A porcelain tile will eliminate that as a problem.

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Unread 06-13-2011, 12:34 PM   #3
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thanks for the advice Bob. I'd like to go with a natural stone tile, both for appearance and for slip resistance. If slate is a poor choice, what do you think about quartzite? Forgive me for asking the obvious, but what do they mean by freeze/thaw environment? Are we talking extended periods of day/night freeze/thaw cycles throughout a winter season? My instinct is that Seattle is generally too mild to worry about it especially given it's proximity to a heat source but I don't know how sensitive the products are to know if one or two big snow events per year could ruin it.
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Unread 06-13-2011, 12:46 PM   #4
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Welcome, Nick.

With your already having a nice concrete structure to work with, I'd be very reluctant to start adding wood in an exterior application for a tile installation.

Why not just frame up some forms and pour you some nice reinforced concrete wings on that tub surround and tile over that?

My opinion; worth price charged.
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Unread 06-13-2011, 12:49 PM   #5
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I don't know anything about quartzite.

If your location doesn't ever get cold enough for water to freeze, then you don't live in a freeze/thaw environment. Elevated surfaces get colder faster than those in direct contact with the ground, so a material suitable for a in-ground patio may not be suitable for a deck. If your hot tub acts as a heat source for surrounding surfaces, perhaps you need better insulation.

Historically, Seattle has had freezing weather as late as May and as early as November. Seattle gets over 2 inches of snow in December and January. I'd consider that a freeze/thaw environment if I were having to cover a warranty claim. You can do as your instincts say, but I would look for a porcelain tile that mimics slate. There are some out there that do very well. Get a frost-proof one, and one with a high coefficient of friction. Then, we can work out how to set them.
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Unread 06-13-2011, 01:04 PM   #6
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thanks again Bob. You are correct about the occasional deep freeze here. Another thread on this forum suggests quartzite could be even worse than slate for outdoor installation so porcelain is looking more attractive.

@cx: concrete wings sound interesting but I'm not sure where I'd begin on that one. Any idea how thick they'd need to be to span 8 ft to support all of the load?
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Unread 06-13-2011, 07:21 PM   #7
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Well, I imagined them sitting atop your concrete walls, Nick. If they were to span eight feet it might require a little engineering. But not overly much, think I.

Besides, you got Injineer Bob right here to handle that on your behalf. Don't need to be no steenkin' ceement injineer. A injineer isa injineer, eh?
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Unread 06-13-2011, 11:21 PM   #8
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engineer or no engineer, I'm grateful for the advice. Thanks fellas!
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Unread 06-21-2011, 06:15 PM   #9
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Hello again,

I've decided to stick with my wood joist over concrete plan. So basically I'm planning to run two 2x8's in parallel 12"o.c. for each side of the hot tub, all of it resting on top of a 3-sided concrete vault (top view attached, but ignore dimensions for joist anchors which are wrong anyway). Deflecto gives me L/827 if using 2x8's with my spacing.

Taking a look at Ditra now, here's what I come up with going with their recommendations (page 16):
2x8 dry treated SYP or Doug Fir joists
3/4" Exterior Plywood (1/4" slope for 12" width of surround)
1/2" Exterior Plywood
1/4" exterior cement backerboard applied with unmodified thinset mortar
Schluter DITRA applied with unmodified thinset mortar
3/8" frost proof quartz tile applied with unmodified thinset mortar
Does this sound like a reasonable plan of attack?

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Unread 06-21-2011, 06:34 PM   #10
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Your wood joists will/will not be down over the sides of the concrete walls?

You do/do not intend to tile the sides of the joists?
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Unread 06-21-2011, 11:17 PM   #11
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No the joists would rest on top of the walls, secured with anchor bolts and heavy brackets. No current plans to tile the sides, but it would be nice to know what I'd be up against if I did.
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Unread 06-22-2011, 05:01 AM   #12
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You would have to cover the sides with plywood, backerboard and Ditra, just like the top, except only one layer of plywood. Use Kerdi band to seal the Ditra at the corners between the top and sides and where the two sides meet. I'd allow the tile to hang below the bottom of the joists by about 1/4" to act as a drip edge.

You need to do something to keep water from entering the edge of your tile underlayment. The above will do that.
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Unread 06-22-2011, 07:57 AM   #13
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Ok any thoughts on the inside (hot tub) edge? Should I bring the Ditra down the joist or create some kind of finished edge there too?
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Unread 06-22-2011, 08:16 AM   #14
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In your drawing it appears the inside and outside are the same, Nick. I would treat them the same.

Only place I differ from Engineer Bob's plan is that I would not use Ditra on the vertical faces. I'd wrap Kerdi down over those sides. Very likely that I'd just use Kerdi on the whole thing rather than narrow strips of Ditra on the top, actually.

And that's only considering the suggestion of Schluter products. Lot of other waterproofing membranes, sheet and liquid-applied, that could be used in that application.

My opinion; worth price charged.
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Unread 06-22-2011, 01:23 PM   #15
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Thanks for the great advice. After reading some of the posts on this forum, I'm planning on following the mfr's recommendations for sealing the quartz tile and then using Laticrete SpectraLock for grout.
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