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Unread 03-02-2021, 11:10 AM   #1
wwhitney
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Exterior Stairs Over Finished Storage Space

Hello,

A couple questions, for a non-freezing climate:

1) Say you have PT notched stringers (~ 20" o.c.) for an exterior stairs, and you want to have (porcelain) tiled treads and risers, with everything water resistant. The idea is to manage water below the stairs, and have no waterproofing above the stringers. What is a good detail for the layers between the tile and the stringers?

E.g. sufficiently thick (how thick?) Kerdiboard to span between stringers, Kerdi band over exposed foam edges, tile with a Schluter edge profile for the stair nosing. Does that work, and/or is there a better way?

Or is this fundamentally a bad idea with PT stringers? I.e. if the water is allowed through to below, then the PT stringers will get wet, and they will move enough that it transmits through to the tile subsurface, with a significant chance of tile debonding?

2) Now consider storage space below the stairs, with drywall and electrical, but unconditioned. The waterproofing layer could be a sloped roof below the stairs (as in (1) above), or a stairstep layer following the treads and risers, or both. What is a robust choice?

Note that the stairs have sidewalls, so the waterproofing layer(s) will need to be integrated with the sidewall WRB. That makes me think the stair step water proofing is just too hard to do well and durably, with all the inside corners and changes of plane. But perhaps I'm overly pessimistic?

3) Is the following option plausible, or just crazy? A sloped roof for waterproofing (EPDM, TPO, etc). Stack up foam blocks to create a stair step substrate; bottom back corner of block is sliced at the angle of the stairs/roof, and blocks overlap each other. Tile over the foam.

Thanks,
Wayne
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Unread 03-02-2021, 09:39 PM   #2
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Exposed stairs are a bitch, Wayne, no matter how you wanna handle them. You want them weather resistant? Steel and/or poured concrete.

1. I don't see that working. What are these treads made of?

2. Robust choice? Build of steel and concrete or just poured concrete. Too late for that? Mud the entire structure, treads, risers, walls, the works with slope to drain at the horizontals. Paint it with a direct bonded waterproofing membrane and tile it all. Use lots of flexible sealant in most joints. Expect cracking, anyway.

3. No idea. Talk to some foam people. No tellin' what they can do with that stuff these days.

My opinion; worth price charged.
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Unread 03-03-2021, 11:13 AM   #3
wwhitney
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Thanks CX for your response, the hard questions are always the interesting ones.

Let me clarify the goal here a little: design details for an exterior stairs (with wood framed sidewalls) over (wood framed) storage space that will (a) keep the storage space dry (b) last 20 years or more and (c) allow for a porcelain tile finish. As far as (b) and (c) goes, if there's a possibility of minimal tile cracking over the lifetime, that's OK, as long as the tile can be replaced without compromising (a).

So basically it's a stairs, and it's a roof.

1) The finish surface would be porcelain tile, possibly a single piece per tread. The idea would be to have a subtread that is strong enough to span between stringers, and water resistant. Maybe 2" Kerdiboard with front and rear edge profiles?

2) If "expect cracking anyway" means the stairs themselves can not be expected to keep the substructure dry in the long run, then I would definitely need a sloped roof under the stairs. Is that the consensus?

Then given the sloped roof, the question is whether to provide secondary waterproofing within the stair assembly, or just to design the stair assembly to not be disturbed by water exposure. (As in (1) above, although I'm not confident about the PT stringers, or possibly (3) below)

3) So say I figure out a way to stack/adhere a stair step pattern of sufficiently strong solid EPS blocks to a sloped rubber roof. Then the question would be what detail to use between the foam and the tile.

If there's a quality thinset that will bond to EPS, the tile could be directly applied. That's fine as long as the continual exposure of the EPS to some water won't cause the tile/EPS bond to fail. Otherwise, some additional layers would be required.

Cheers, Wayne
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Unread 03-03-2021, 12:32 PM   #4
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This is not yet constructed, Wayne?

1. Not sure I'd depend upon 2" Kerdiboard to span 20" centers for a stair tread.

2. I was talking about the tile installation, not the waterproofing.

3. I don't think that's a problem at all. Isn't that done every day with sheet membranes and foam shower trays?

Again, I think you'd need to talk with some foam manufacturering people. I think you'll find our old friend Paul has gotten into that business, but I don't have a business contact for him. You might try a PM. He might have some insights for you.

My opinion; worth price charged.
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Unread 03-03-2021, 01:10 PM   #5
wwhitney
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On this project, there are existing stairs constructed with PT stringers and regular plywood treads and risers, with a failing coat of elastomeric "waterproof" paint on top. Under the PT stringers was a sloped layer of tar paper, with drywall below that. The drywall was very wet, and the plywood moldy.

So all that has to come out, but the sidewalls are existing, and it would be preferable to leave them in place. We'd open up the sidewalls enough to ensure a proper cap flashing and WRB and replace any damage. Then what goes in between the sidewalls would be all new.

1) This thread https://www.johnbridge.com/vbulletin...d.php?t=118784 makes it seems strong enough, plus the 2" stainless channel on the front and back would stiffen it. If the span is the only concern, an extra stringer could be added, or possibly horizontal supports between the stringers.

2) I feel like this is the first design bifurcation question I need to answer, whether to have a sloped roof under the stairs, or not.

3) Sure, so it seems like it would work. And stucco is applied more or less directly to EPS in EIFS applications, right? But what I'm not clear on, judging from the shower tray case, is whether one of the things that Kerdi membrane does is keep the EPS dry, and that if directly bonding tile to EPS would lead to debonding as a result of a wetting and drying cycle of the EPS.

Cheers, Wayne
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