Floating deck over concrete slab?

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Old 07-05-12, 03:43 AM
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Floating deck over concrete slab?

I have a cracked, but relatively intact concrete slab patio on the back of my house. My dad, who has built many decks in his lifetime, has offered to help me build one to cover the slab (which is now an eyesore).

His idea, though, seems a bit odd to me. He wants to build a floating deck over the slab. All of the 4x4 posts will be resting on the slab with the floor joists bolted to them. The slab is not perfectly level, nor is it completely flat (because of the cracks), so all of the posts will have to be cut differently to maintain a level floor.

I am tempted just to break up the slab altogether and build from there, but that would take much more time and energy. Not to mention that I have no idea how deep it goes or what is underneath.

If anyone has heard of this floating deck idea, let me know. I found it odd enough to need to ask a second opinion. I will attach a picture as well.
 
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Old 07-05-12, 05:38 AM
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Welcome to the forums.

How high will the deck be above the concrete? Ground level decks are a bad idea, you need airflow underneath to keep the wood from rotting.

Also, the proposed idea would use this patio as a foundation and it does not seem to be up to the task.
 
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Old 07-05-12, 05:42 AM
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In this scenario, the deck boards would be 10 inches above the concrete slab. In the picture, you can see as you step out of the back door there is a concrete step. That step would be the height of the deck.
 
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Old 07-05-12, 06:19 AM
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That might be ok but why not dig a proper foundation rather than using the patio?
 
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Old 07-05-12, 06:21 AM
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Just to avoid breaking it up I guess. I can't really think of any other reason than that.


Do you think I should just go ahead and get it out of the way?
 
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Old 07-05-12, 06:26 AM
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I would - either keep it and use it or remove it a replace it would be my thought process.

Have you thought about just replacing the slab or building a paver patio instead?
 
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Old 07-05-12, 06:29 AM
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That makes sense.

I've thought about repairing or replacing it, yea. Although it may be beyond repair since there are three very long cracks that go all the way through. I want to save $$ and my dad knows how to build a wooden deck, so I get free labor.
 
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Old 07-05-12, 07:25 AM
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hypothetically, if i was to go forward with the original plan and have the posts rest directly onto the slab, would it end up being a problem??
 
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Old 07-05-12, 07:32 AM
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If the slab is unstable, as it appears to be, then your deck will have problems.

Also, I see no mention of fastening the deck to the slab, are you intending for it just to sit loose on top of the concrete? I can see it moving if you do so.
 
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Old 07-05-12, 07:43 AM
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Yes, I guess he is assuming that the weight of the deck will keep it in place. There are no plans to fasten it either to the house or to the patio underneath.

The plan was to have the 4x4 posts along the outside edge of the concrete, then have the 2x8 board running on the OUTSIDE of that, which would extend the deck about 2 inches from the edge of the concrete, which would leave me some room to put lattice to hide the slab underneath.

I am thinking I might just be spending the next few weekends taking a sledgehammer to this thing.
 
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Old 07-05-12, 10:42 AM
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If you dug and poured a good footer everywhere you need a post [break up the slab where/if needed in the middle] you could leave the slab in place. Hauling off the broken concrete might also be a big task Does water drain off of the slab ok?
 
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Old 07-05-12, 10:50 AM
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Yes, the water still drains away from the house. the slab has been basically cut into three huge pieces by the cracks.

I am thinking now about at least putting the initial posts into the ground on the outside of the slab.

If I wanted to break the concreate for just each post area, how would I do that? Sledgehammer?
 
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Old 07-05-12, 10:51 AM
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Jackhammer would be ideal, sledge would be a lot more work but would be able to get the job done.
 
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Old 07-05-12, 11:03 AM
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Personally I don't like sinking deck posts into the ground. I'd rather dig and pour a footer and then attach the 4x4 to the footer above the grade. Less chance of the 4x4 rotting and just as secure.
 
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Old 07-05-12, 11:06 AM
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Nice catch, Mark - I missed that on the first read-through.

Yes, pour a proper footer which extends above ground and into which you sink the bracket for attaching your deck posts. A proper deck is going to be a lot of work, which is why I thought about just replacing the patio.
 
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Old 07-05-12, 11:26 AM
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I'm getting lost. Dig the hole first, then poor a footer (I'm assuming you mean concrete?). Then attach the 4x4 to the footer above the grade? What does that mean?
 
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Old 07-05-12, 11:26 AM
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Because your slab is already cracked and no longer level, I think it would be a mistake to use it as a foundation for your new deck. If you really don't want to break it up and haul it off, you could put post holes OUTSIDE of the slab and just conceal it with your deck. However, that big slab is a bit of a drainage concern as well. The cracks you have will continue to get worse every winter too.

If you decide you need to demo the slab, don't do it by hand. It will take forever. Call around to some tool rental places and get yourself an electric jack hammer, or just pay a contractor to demo it.
 
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Old 07-05-12, 11:29 AM
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Yes, you use a sonotube as a form for the footer and have it actually come out of the ground 6-8" so your wood is not sitting on the ground or in water, it lasts a lot longer that way.

Again, I think a new patio would be easier.
 
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Old 07-05-12, 12:15 PM
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could i just frame out the patio and poor new concrete right over top of the old cracked concrete ??

I'm beginning to feel like I should just hire professionals.
 
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Old 07-05-12, 12:17 PM
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Maybe but I would be looking into why this one cracked so badly, there may need to be some work done underneath to make it sound.
 
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Old 07-05-12, 01:06 PM
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One last question. Is there some way I could utilize deck blocks for the few posts that will have to go on the concrete slab?

Also, what if the slab isn't PERFECTLY level, would the deck block be no use?
 
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Old 07-05-12, 01:08 PM
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Umm... what's a deck block?
 
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Old 07-05-12, 01:11 PM
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Old 07-05-12, 01:17 PM
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If the slab isn't level and you were to use a deck block you would need to set it in mortar bed to make it level..... but if the slab continues to move

If I were to leave the slab and need support for the deck, I'd break up a big enough area where I could dig down and pour a proper footer.

I'm a painter not a deck builder but I think for the most parts those deck blocks are frowned upon.
 
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Old 07-05-12, 01:30 PM
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A few things to think about:

Pouring a new slab directly on top of the old one is very likely going to result in large, reflective cracks showing up in the new concrete--essentially a repeat of what you have now, at a material cost of around $500 minimum (I guesstimated the existing slab dimensions, from the photo). You could reinforce the heck out of it with lots of rebar, which would minimize the likelihood of reappearing cracks but could easily double the foregoing material cost figure.

By breaking up the existing slab into much smaller chunks before pouring on top of it (say no larger than 6" x 6"), you will eliminate the reflective cracking issue. If you're young and tough, a day or two with a 10-lb. maul would do it. The new slab should have steel reinforcing mesh in it, and will have to be grooved into quarters to establish control joint locations, where cracks will still occur but won't be noticeable.

But here's something else to consider--the Dad Factor. Your father won't be around forever, and to have him help you build a nice deck that you and your family can enjoy for years or even decades into the future is something you can't put a price on. Should you decide to go that route, though, you will need to convince him that the floating deck idea is not a good one. Consider building it to meet the IRC guidelines, with anchored 6 x 6 posts on top of new concrete footings. And do everyone a favor and raise the elevation of the finished deck's walking surface to be just an inch or so below the house's entry door threshold.
 
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Old 07-05-12, 02:32 PM
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Maybe your dad is trying to come up with a good way to avoid digging post holes. I am sure he knows what long, hard work it is to dig some 30" to 40" holes for deck posts.

You might want to call Home Depot and friends and check the price to rent a 2-man post hole digger. I think it is about $50 a day from the one near me. I am going to use one next month when I build my new deck. Compared to digging manually, it is almost effortless. You and your dad just hold the machine in place while it digs.
 
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Old 07-05-12, 03:04 PM
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Depending on what part of Va, he might not need to go that deep with the footers.
 
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Old 07-05-12, 04:23 PM
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Ah, yes, the infamous 2-man arm-breaker (sometimes called a hole auger). If you're lucky, it will only break a wrist or two. So be very careful when using it. They like to lock up when hitting the unexpected root or buried pipe.

Ouch hurts, so be careful.
 
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