Advice on new deck...On top of concrete, or remove concrete?

Kyle502June 8, 2011

My wife and I are going to build a deck off of our back door, but the problem is that it opens to our concrete driveway. My question is, does it really affect the integrity of the structure to build on top of the concrete vs. digging up concrete and anchoring in the ground? Now, this is going to be a very small deck, no bigger than 11x13 feet, and that's why I ask. Not very much weight will be on it, a grill and patio furniture, and the deck itself will not weigh as much as a car, and we don't have many friends. To me, with it being such a small deck, anchoring the 4x4's to the concrete should be enough, each spaced no further than 4 ft. apart, which would be about 8 total. That would spread the weight out enough across the concrete. The drive has been there for 11 years, so settling anymore shouldn't be an issue, and it's already sloped away from the house somewhat, so water run-off shouldn't be an issue. And we would give it enough clearance above the concrete for airflow, like 2 to 3 feet or so, with lattice around the bottom or something to that effect... What do you all think? I was hoping someone has been in this same situation before... Thanks for any advice you can give.

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cruzmisl

Personally I'd build it on top of the concrete assuming its in good shape. DO NOT allow the wood to come in direct contact with the cement though. Use a galvanized post anchor. You may have to drill a hole in the driveway and use hydraulic cement to secure it but it should be fine. A TON less work too.

My 2 cents from a regular guy, not a construction expert so its worth exactly that ;-)

    Bookmark   June 9, 2011 at 4:24PM
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weedyacres

With the disclaimer that I'm not a structural guru, your risk of building on a slab without footings below are frost heave, not just settling. Your call on the risk involved and the trade-off.

You mentioned using 4x4 posts, however, and that's not current code. You should use 6x6s, and you don't need 8. Here's a photo of one I helped a friend build that's 10x10. It's free standing, which uses more posts than one done with a ledger board, and we only used 4. You can see how we did the structure with 4 posts, 2 beams, and the joists resting on the beams. 2' cantilever all around.

    Bookmark   June 10, 2011 at 3:46PM
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Kyle502

Ok, thanks guys. All of that makes sense. Thanks for the tip about the galvanized post anchor, cruzmisl. And thank you, weedyacres, for the structuring tips. Good call on the 6x6's. Going against code is the last thing we need... Nice deck, by the way.

    Bookmark   June 14, 2011 at 5:53AM
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aidan_m

"the deck itself will not weigh as much as a car"

Actually it will weigh as much as a tow truck:

11' x 13' = 143 square feet.

The absolute minimum allowable designed load for a residential deck is 50 lbs/sq ft. Many building code jurisdictions will require higher design loads. Standard for light commercial is 100.

143 x 50 = 7150 lbs. You can not assume this weight is evenly distributed over all the footings. It could be concentrated more to one area, under dynamic loads.

I would saw cut the concrete and install correct footings under each post.

    Bookmark   June 14, 2011 at 12:05PM
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concretexpert

If the current slab is at least 3000 PSI concrete you will never generate that force on that deck even with dynamic forces. If your posts are 6X6 then the weight that slab should be able to bear on one post is about 81,000 pounds dead weight. Of course you will only have about 15,000 pounds maximum distributed over 8 posts.
I wouldn't tie the deck to the house though if you are in a freeze thaw climate. The slab will raise and lower during freeze thaws. This might damage the siding or facade.
If the concrete has large voids underneath it then the load it can carry will greatly decrease, and may not be able to hold the deck without cracking. If there is wiremesh in the slab or rebar the flexural strength will be enough to span a small void. Either way the crack wont open up if mesh is properly installed.

    Bookmark   March 2, 2015 at 1:58PM
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