Adding a deck over an existing slab and beyond

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Old 06-24-15, 05:44 PM
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Adding a deck over an existing slab and beyond

Recently moved into a home that had a 4 1/2 foot slab across the back of the home. This slab slopes away from the house about an inch over the 4 1/2 feet. I would like to add decking over the existing slab and about 8 ft beyond that. The problem is the slope which I don't want to carry on through the new decking and the short distance from the height of the slab to the height of the back door threshold. I have about 1 1/2" from the top of the slab to the bottom of the threshold plate. I intend to use 1" or 3/4" composite decking board laying them parallel or perpendicular to the house. My preference would be parallel but would consider perpendicular if necessary.

Any advise as to what to use as joists under the decking that will be over the slab and the best way to eliminate the slope? Thickness of the joist would be only about 1/4" or 1/2" at the point closest to the house. I've added a photo the help clear up any confusion.

Thanks a lot for any suggestions.

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Old 06-24-15, 06:21 PM
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Could you leave the concrete where it is and start your new deck parallel to the outer edge?

If you taper the joists down to that thin, then the space under there will become a trash collector. Some slope up towards the house is a good thing when it comes to water.

I don't see your location so not sure what climate variations you deal with.

Bud
 
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Old 06-24-15, 06:32 PM
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Welcome to the forums! You can't lay decking flat on concrete, as it must breathe, and you need to attach it in some way. I would do as Bud suggested, and even make a short (7") step up onto the deck to allow it to have proper ventilation.
 
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Old 06-24-15, 07:54 PM
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Nothing you suggested is going to work.
Far better if you want a bigger deck to add onto what you have with a slab.
 
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Old 06-24-15, 09:08 PM
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What you want is a "sleeper" installation. This is done all the time on roof decks, but can also be done on ground level decks (I did it on mine). All the major composite manufacturers allow it (Trex, Timbertech, etc.), but read their installation guides on the minimum clearance they want under the decking. I don't think it can go as low as 1/4" or 1/2" (likely more like 1.5").

You'll need to taper the sleepers, but you don't want to completely eliminate the pitch, just reduce it. Installation guides for the composite decking will tell you what pitch they recommend, but I'm thinking something like 1/4" over 4'.

I would use 2x4 sleepers, taper them using a taper jig on a table saw, then tapcon them to the cement. You can't taper them smaller than the minimum clearance allowed by the decking manufacturer, so you'll have a small step up from your door.

You may need to shim them in places, if so, don't go over a foot or so without a shim / the sleeper touching the ground. These aren't meant to span very much.

Also, for the sleepers, you'll want to special order ground treated 2x4's. Expect to pay $.50 extra each piece for the treatment.

Lastly, while this can be done, it will likely be more work and more expensive than simply doing a stone patio over the concrete.
 
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Old 06-24-15, 09:18 PM
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...or jackhammer the slab, get a skid steer, dig 2 ft out of your back yard, then build your deck. No deck contractor would put sleepers on a pad that would be as thin as you suggest.
 
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Old 06-24-15, 09:37 PM
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A few weeks ago I read an article about using sleepers over a slab and thought that would be a solution for my situation. FYI the article is here: How to Build a Deck Over a Concrete Patio | The Family Handyman

Thanks everyone for your input. The area where the sleepers would be less that 1 1/2" or so would be under the covered portion of the patio. I have to do more precise measurements to be sure.

Reisen, when you mentioned shims, I am not sure what I would need them for. Can you give me a little more explanation?
 
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Old 06-24-15, 11:02 PM
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That's a great article, and pretty much details what I did. My patio was cracked, so I started off by leveling and grading it in low spots with top-n-bond. Then I tapered my sleepers, and tapcon'd them down.

Neither my patio, nor my lumber (especially after tapering), was perfectly flat. As I went, I had to level the sleepers. Where a sleeper was low, I shimmed it. Where it was high, I sanded it down with a belt sander (or in a few cases, planed it with a power hand planer).

A couple of tips:

- I used a 6 foot level to reach across as many sleepers as possible and level them.

- You're going to be sinking a lot of tapcons into concrete. Invest in a rotary drill, it will make the job go much faster.

- I ran my sleepers a little long, so they stuck out over the edge of the concrete pad. I then snapped a chalk line and cut them even with each other, then added a deck board as fascia. The deck board serves double duty as both fascia and blocking.
 
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Old 06-25-15, 04:28 AM
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The OP only has 1 1/2" of usable vertical space. Sleepers won't work, since deck boards are 1" thick. Decking this patio is not advised.
 
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Old 06-25-15, 09:11 AM
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Extend the patio with a new concrete slab adjacent. Abandon the deck plans, as you do not have enough height available.
 
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Old 06-29-15, 08:33 AM
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Chandler, I agree you'd need to have a step up to the deck, which IMO, is not ideal. I'm pretty sure minimum height for sleepers for most composite manufacturers is 1.5" (ie. a 2x4 on it's side).

As long as the structure below the deck was correctly graded, it could be done with a step, but that wouldn't be my first choice either (I'd build a patio).
 
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