Decked Area Over Elevated Concrete Slab

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Jd2870

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Hi All,

As part of a small deck project I need to construct a decked landing area outside of my back door which will end in stairs that descend to the main deck area. We previously had crumbling concrete block steps that I had taken out when we re-did our driveway recently (see pic attached). The problem is that, because the way the old additions were constructed, the landing would have to be partially constructed on top of a concrete slab. I can run a ledger board on the right hand side under the door, but the other side is just the 2x4 framed wall of the garage. I would like to use anchors to either secure some posts or run something (2 2x12’s?) horizontally, but I can’t seem to find any information on whether this is to. Frost heave obviously isn’t an issue and I believe that the slab is at least 6” thick. The decked section would be fairly small: max area roughly = 10’x4.5’; 2’ or less high. I suspect my building office (NJ) will need to be sold on the idea before they approve my plans – any comments or suggestions (especially references to the IRC) would be helpful.

Thanks!
 

Sparky617

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You could build a small deck between the two walls. You'd want to install galvanized flashing behind your ledger board. You may be able to use galvanized joist hangers and fastened directly to the house band joist, again with galvanized flashing between the joist hanger and the band joist. If that is aluminum siding it would react with the chemicals in the new pressure treated wood. You can remove the bottom piece of siding below your door and tuck your flashing up under the siding.
 

bud16415

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I would first make sure all the flashing around the door is proper and then build the deck free standing on top of the pad from pressure treated wood and not attached to the house at all. With a 1” gap all around.


Then build the steps as a second unit also free standing.


That’s just what I personally would do others my have different ideas.
 

Sparky617

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I would first make sure all the flashing around the door is proper and then build the deck free standing on top of the pad from pressure treated wood and not attached to the house at all. With a 1” gap all around.


Then build the steps as a second unit also free standing.


That’s just what I personally would do others my have different ideas.

That works as well. The slab could certainly handle a small deck.
 

Jd2870

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Many thanks for the response, Sparky – My plan is to deck over that entire space and hang stairs off the open end going down to another deck space below. I agree w/ the ledger under the door and that’s what I plan to do. I anticipate having at least one step down from the door to save space on the run of stairs that will ultimately connect the landing to the deck below, so I’d be looking to fasten it onto the concrete block (thanks for the reminder on proper flashing). The problem is, however, the opposite side - the garage sits about 2 feet lower than the back room and I can’t attach a ledger to 2x4 framing. What I want to do is frame it out with posts on the garage (right) side, anchored into the concrete slab (see pic) that forms the floor of the alcove area. My concern is whether the use of posts anchored to the slab presents any code issues. Because this seems to be an unusual design, I’m sure I’m going to have to convince the code guy that it’s compliant.


post anchor.jpg
 

Jd2870

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Thanks Bud - totally free standing is an option too and may be easier. I'm just having a hard time determining if the post-on-slab construction generally runs afoul of any building code.
 

bud16415

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It might as there is the unknown of the strength of the slab and what is under it.


To me it looks like the garage was added on and then the short block wall was put in to close the area in and the slab poured.


The witness marks on the house wall suggest there were brick steps on top of the slab once that have been removed.


If this little deck will eventually lead to another lower deck I would conceder making it all one height leaving the house and going on the big deck. Then stairs to get off and on that deck if you need access to the yard.


I’m all for no steps when you don’t have to.
 

Sparky617

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You want your landing to be 6" below the door. That keeps water from splashing on the door. I like Bud's idea of a single level deck with stairs out to the yard. But it is hard to tell the existing conditions from one picture. That said, I prefer patios over decks if I'm close to grade. Paver patios are pretty DIY friendly.
 

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