Ground level deck next to a shallow slab

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Old 09-05-18, 07:59 PM
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Lightbulb Ground level deck next to a shallow slab

Hello all! I have been considering building a deck for some time now, and have been searching these forums as well as other sources for information before beginning the project.

The question I have is likely going to result in multiple answers of "don't build a deck," but I figured I'd at least see if anyone has a suggestion on how to make this work.

I'm wanting to build a ground level deck, and here in Texas all the houses have slab foundations and are built on clay. The deck would be up against the side of the house and against a patio slab which has an addition built onto it (yep, bad idea. I didn't do it though). The problem arises when a 2x6 beam or framing member has to go right next to a 3" patio slab without undermining the support of the slab. From other sources I am looking at, the toe of a foundation is at minimum 1/3 of the height of the slab and supporting soil has to be present in at least a 45 degree angle from the toe. That wouldn't work with a 2x6 beam in the way, and I'm not about to bury wood and have it rot.

The most recent idea I've had is to use a thinner wood to attach the deck boards to, and rest these supports on concrete footings or peers to support the actual load of the deck close to the patio, then excavate and build the deck normally past the critical point. Essentially I would be pouring concrete supports to 'replace' joists and hold the end of the deck right next to the slab. Yes, that would be a lot of digging and concrete, but is it even a feasible option? I would then essentially be using a frame of 2x2 wood under the deck boards to span short (16" or less) gaps between concrete footings, only for about 1.5 feet of the deck surface. I can't help but think there would be a lot of flex.

Also due to it being a ground level deck with flush beams, I will be using at least 16 supports for a deck roughly 16'x16', so what diameter footings would I need to pour? I think the tube forms come in 12", that just seems excessive.

I've considered using the little pre-formed stands, but the soil here moves way too much for them to stay put.

Any input or deck-like alternatives would be appreciated. Concrete or pavers are not an option due to the heat and bare feet.

Thank you!
 
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Old 09-05-18, 08:26 PM
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Sonotubes come in many sizes... you can probably even find 6" tubes if you go to a redi-mix plant, assuming you can't find them at your local store. But a footing will be more stable if it is conical... meaning, dig deep enough that you are below frost... maybe create a 12" wide hole, but place the form only on the top half of the hole so that concrete footing will be 12" wide where it sloshes out the bottom of the form but it will be nice and pretty at grade where it's inside the 6" tube.

Anytime you have a deck on grade you need to use pressure treated lumber that is specifically rated for ground contact. Because in all likelihood you will probably need to bury something somewhere. And its okay to do that to wood that is rated for ground contact. And yes, you would need to place piers next to the pad, keep the joist far enough away that it will not erode the pad. Tapcon a board to the side of the pad if needed. Your piers need to be spaced according to the span of the lumber you use for joists. Or, rather than using piers, pour footing beams... like a series of 6" wide sidewalks that are as long as your deck is basically. 3 of those would give you nice footings to build your deck off of. See the span tables for joists that is listed in the DCA6 deck guide. (Google it) 2x6 is the smallest dimension joist you should consider. If you go with 2x4s they should span no more than 4 ft, imo.

Also keep in mind that a deck so close to grade may mold and mildew faster than one that is elevated due to the lack of ventilation below it.
 
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Old 09-13-18, 08:30 PM
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Wow, thank you for the detailed response. I have read other places that even with ground contact rated treated lumber you have to be cautious of where it is buried because any area which has a lot of moisture will rot the wood anyway. I drew a couple little illustrations which may help with my question, although I think I will be fine just putting the boards where they should be after excavating the soil a bit. My next concern is how to ensure that moisture can drain away properly.

Because it is a ground level deck, I was already planning on using a moisture barrier to direct rain water to some french drain openings which will be below the deck, using a 1/4" over 1' slope. I am planning on then putting a layer of gravel over this to allow the water to flow freely and prevent pooling, as well as allow better ventilation under the deck. The challenge is maintaining this idea as close to the house and patio slab as I will be getting.

I have (attempted to) attach some childish napkin drawings showing my current plans. Nothing is set is stone by any means, I don't even have any materials so all is still up for discussion. I am planning on digging the pier holes and installing the tube forms, then building the frame with a few joists to stiffen it, levelling the frame in place and then pouring the piers up to the anchors which will be fastened to the 2x6 beams. I am not a fan of using 2x4 joists either, and only the one toe board will be 2x4, others will be 2x6.
 
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Old 09-13-18, 08:55 PM
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You can also Google DCA6 deck guide. Download it, and consult the tables within it for the proper footing and column size that will meet code. Some if it may not apply to your situation, but it may help.

The question I have is likely going to result in multiple answers of "don't build a deck
I agree with that consensus. You'd be better off with a paver patio. But I know how some people are bound and determined to do what they want anyway.
 
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Old 09-14-18, 09:58 AM
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The question I have is likely going to result in multiple answers of "don't build a deck,"
I think it's likely that's the reason only X responded to your questions.
 
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Old 09-14-18, 10:09 AM
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That's exactly right.

I'm not a big fan of weeds growing up through my deck or the smell of damp and mold.
 
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