Questions about decks

[Jamie Smith]

Hi all-

My wife would like a deck put on the back of our house. BUT- she wants it to have a slate floor, if possible.

The would-be-deck would be on the southern side of the house. We live near Charlotte, NC

I will have more questions, one the project gets started, but the preliminary questions are:


Is a slate surface (probably a roughed surface) feasible in this type of environment?
What would I use for the subflooring? I was thinking some sort of exterior plywood, with that cement-board on top of it.
Any "watch-out-for-this" type comments?Thanks,

-jamie

(edited, because I clicked post too soon ;) )

[Dave Falkenstein]

A couple of thoughts come to mind. By "deck" I assume you are talking about a structure that is above ground, and not a patio on the ground.

First, check the local building codes to be certain you understand what the structural requirements will be for a deck covered with slate. I'd bet you will need significant added support.

Second, the slate will need to be installed so that water runs completely off it. Perhaps a slight angle in the entire structure away from the building would be appropriate. I have worked on some decks that were covered with indoor-outdoor carpet and a couple that were floored with plywood. These decks had major rotting issues, because drainage had not been properly installed. I suppose you could use pressure-treated lumber in the supporting structure to minimize the rotting problem. Just be well aware of it.

[Paul Girouard]

I've never heard of slate being layed on a wood deck.

It would be layed on a concrete slab poured with the idea , think step layout here, of slat being the "added" finished surface.

If it's totally covered by a roof and would get only minimal weather as in the classic southern "porch" maybe you could get by with the slate layed on a plywood / ("Advance deck / special OSB sub-flooring ) with cement brd. underlayment. If so having excellent ventilation beneath would be one of many considerations to plan on.

[Jamie Smith]

I should mention here that by "slate" I mean indoor/outdoor slate tiles (12" square, or 18" square), about 3/8" thick, stuff you can buy at lowes or hd.


A couple of thoughts come to mind. By "deck" I assume you are talking about a structure that is above ground, and not a patio on the ground.

First, check the local building codes to be certain you understand what the structural requirements will be for a deck covered with slate. I'd bet you will need significant added support.


Yes, I do mean an elevated, above ground deck. I don't think that support would be a problem, since it would be the same basic structure as the house (2x10" treated joists @ 16" OC).



Second, the slate will need to be installed so that water runs completely off it. Perhaps a slight angle in the entire structure away from the building would be appropriate. I have worked on some decks that were covered with indoor-outdoor carpet and a couple that were floored with plywood. These decks had major rotting issues, because drainage had not been properly installed. I suppose you could use pressure-treated lumber in the supporting structure to minimize the rotting problem. Just be well aware of it.

I had figured that I would need some pitch away from the house. I will plan carefully for drainage, this will have to stand for a long time...

Thanks for the input!

[Jamie Smith]

I've never heard of slate being layed on a wood deck.

It would be layed on a concrete slab poured with the idea , think step layout here, of slat being the "added" finished surface.

If it's totally covered by a roof and would get only minimal weather as in the classic southern "porch" maybe you could get by with the slate layed on a plywood / ("Advance deck / special OSB sub-flooring ) with cement brd. underlayment. If so having excellent ventilation beneath would be one of many considerations to plan on.

No, I am talking about slate tile on top of cement board, which would be on top of a weatherproof sub-floor (because the underside would be exposed), on top of 2x10 joists.

Note that the main question here is whether or not this can (or SHOULD) be done-- I am definitely NOT saying that I know what I am talking about :D

[James Wellman]

Assuming you have the proper support for the weight, you might investigate using Ditra (http://www.youtube.com/watch?v=f6FyWs2WZ1k) on top of treated plywood, which would act as an uncoupling layer to help counter the stresses and movement that a deck would be subjected too. I did see something similar on deck last year when the LOML and I were checking out contractors to build our new deck. They installed a tile floor and then finished the underside of the deck with beadboard and lighting. Looked like a room addition with no roof, but they did have a retractable awning.

James

[John Eaton]

I've actually worked on a similar project: 10x12' deck - PT sub-floor with 2x10s on 12" centers (makes the floor more rigid and better supports the weight). Glue and attach the cement board directly to the joices (they were pre-seasoned for 6 months to prevent shrinkage) - we did two layers crossed/glued. Held up with 2x6s at 4' intervals. The deck has ungauged natural slate instead of square tiles. This was built over 10 years ago and I saw it last year - still solid as a rock without any cracks in the mortar.

Another, more involved alternative is to pour the substrate out of reinforced concrete - you can cantilever the deck and use piers to below ground (also poured out of concrete). The hard part is creating the forms. After the concrete sets up you can slate the surface.

I'll see if I can find some pictures of that deck mentioned above, or if I get a chance I'll go by and take some with my phone. It's a good idea to figure out the weight and have an engineer OK the design, just to make sure you're accounting for the extra load (figure it with people on it). What size are you thinking about doing?

-- John

[Jamie Smith]

Assuming you have the proper support for the weight, you might investigate using Ditra (http://www.youtube.com/watch?v=f6FyWs2WZ1k) on top of treated plywood, which would act as an uncoupling layer to help counter the stresses and movement that a deck would be subjected too. I did see something similar on deck last year when the LOML and I were checking out contractors to build our new deck. They installed a tile floor and then finished the underside of the deck with beadboard and lighting. Looked like a room addition with no roof, but they did have a retractable awning.

James

Just watched the video, looking at the site now. Thanks!

(Note that lighting won't be necessary, it will be about 2' off of the ground!)

[John Russell]

A few years ago I had a fellow do something similar, but we used quartzite instead of slate. My idea was similar to yours, put down PT plywood, cover it with cement board and lay the rock.

The fellow I hired to do this talked me out of my plan saying it was overkill. Instead, he used untreated plywood and then put down Grace Ice and Water shield over the plywood. The rock was then set in mortar on this surface. This has turned out to be a problem because of expansion and contraction and some apparent movement in the plywood.

If I were to do it over again, I would either take the approach of building a VERY, VERY sturdy deck frame and as another poster suggested, put a layer of concrete over decking as the base for the rock. If that is not possible, then using 1 1/8" pressure treated plywood covered with cement board would probably stop most of the problems I have had with the mortar cracking....

[Jamie Smith]

I've actually worked on a similar project: 10x12' deck - PT sub-floor with 2x10s on 12" centers (makes the floor more rigid and better supports the weight). Glue and attach the cement board directly to the joices (they were pre-seasoned for 6 months to prevent shrinkage) - we did two layers crossed/glued. Held up with 2x6s at 4' intervals. The deck has ungauged natural slate instead of square tiles. This was built over 10 years ago and I saw it last year - still solid as a rock without any cracks in the mortar.

Another, more involved alternative is to pour the substrate out of reinforced concrete - you can cantilever the deck and use piers to below ground (also poured out of concrete). The hard part is creating the forms. After the concrete sets up you can slate the surface.

I'll see if I can find some pictures of that deck mentioned above, or if I get a chance I'll go by and take some with my phone. It's a good idea to figure out the weight and have an engineer OK the design, just to make sure you're accounting for the extra load (figure it with people on it). What size are you thinking about doing?

-- John

If you have pics, that would be great, but don't go out of your way :)

This is encouraging, because it sounds like, if done right, it is definitely do-able.

The deck will be about 15' deep (from the house), and 22' wide.

[Paul Girouard]

IMO it will be a expensive way to get deck space and a maintenance nightmare.

I've never heard of cement board laid without a sub-floor OSB / CDX /AC ply under it either. But in thirty years of contracting you don't see every thing and different regions of the country and for that matter the world (USN) a lot of different "ways of building" are done with different levels of success.

So , like they say, never say "never":D

[Lee Koepke]

Just watched the video, looking at the site now. Thanks!

(Note that lighting won't be necessary, it will be about 2' off of the ground!)
if you are only 2' off the ground, maybe consider stepping down and supporting it on grade.

You REALLY have to consider the water implications with the kind of deck you described. It has alot of potential to leak and even PT lumber will rot out after time. You also will have quite a bit of movement in the substrate, and result in cracking of the slate.

[Jamie Smith]

if you are only 2' off the ground, maybe consider stepping down and supporting it on grade.

You REALLY have to consider the water implications with the kind of deck you described. It has alot of potential to leak and even PT lumber will rot out after time. You also will have quite a bit of movement in the substrate, and result in cracking of the slate.

We already have a slab there, but my wife wants a deck, so it is a straight shot...

[Joe Chritz]

Unless you need the space underneath you would probably be faster and easier to build up and use concrete then lay the slate directly on that. You could build it similar to a concrete porch. Use blocks, fill with sand, compact and pour a slab on top.

It can be done with wood but you are probably getting into the point where any method is going to be about the same cost factor.

Joe

[Ben Grunow]

I think the hardest detail would be how to cover the edges and still allow water to run off and not be drawn into the edge grain of the plywood substrate.

I would use 2 layers of PT plywood over a seriously strong frame (12" centers and heavy joists) with a paint on waterproof coating and then thinset some cementboard over that (no screws or nails here) to thinset the tiles to. Make sure to seal the tiles/grout regularly to keep the moisture out.

Might make the edge trim out of PVC boards (Azek/Kleer) and bed them in a good exterior caulking to protect the plywood.

Sounds expensive, and maybe even cold and hard ont the feet, but doable.