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  1. #1
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    Jun 2011
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    Default Building deck on top of concrete patio

    Do you need to attach a ledger to house?Or can you attach joists to the concrete?
    Thanks for any advice

  2. #2
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    Dec 2010
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    Default Re: Building deck on top of concrete patio

    Placing the joists directly over a slab on grade (sog) isn't a great idea. You'll trap moisture below the deck boards, along with all manner of organic material which will turn into a compost heap or sess pool. The neighborhood mosquitoes will love you in the wet months.

    If you can engineer the deck so that you get some decent way to occasionally clean under the deck is great.

    If that isn't in the cards, think about adding to the SOG surface with more masonry.

  3. #3
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    Default Re: Building deck on top of concrete patio

    would it work if you hang the joists so that they dont actually lie on the slab.What did you mean by adding more masonary?Did you you mean something like cover sog with tile.
    Thanks for all your advice

  4. #4
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    Default Re: Building deck on top of concrete patio

    Putting deck joists on top of a slab is a bad idea. As Houstonremodeler explained, you will invite termites, pests, rodents, decay, and other undesirable god knows what.

    If you have room, use a ledger and posts, to raise the deck up a bit, to allow cleaning, air and ventilation under it.

    Otherwise, build it up, with tile, stone, bricks or more concrete.

  5. #5
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    Default Re: Building deck on top of concrete patio

    Quote Originally Posted by dj1 View Post
    Putting deck joists on top of a slab is a bad idea. As Houstonremodeler explained, you will invite termites, pests, rodents, decay, and other undesirable god knows what.

    If you have room, use a ledger and posts, to raise the deck up a bit, to allow cleaning, air and ventilation under it.

    Otherwise, build it up, with tile, stone, bricks or more concrete.
    We recently bought a home that has a concrete patio, but there is a ten and one half inch drop from the door to the concrete. There is no step, and I don't want to add a step because of an elderly mother-in-law who has problems using them. I want to put a deck on top of the concrete so there is no step-down.

    The patio is 8' deep and 18' wide. It's enclosed on three sides by the house and the walls are covered with vinyl siding. The concrete is in excellent shape and has a slight slope towards the yard. The opening faces east, so it's well protected from wind, rain and snow.

    My initial plan was to simply run 2X10 supports the long way (16" centers), sit them directly on the concrete and cover with 8' decking. My only reason for going that direction is because I think the floor would look better and more uniform.

    I reckoned 10" supports topped with a 1" decking would equal to approximately 10" and don't think that height will interfere with the door. If it's short by a half inch or so it won't be an issue, but if it's too high I'll rip a quarter inch or so off the supports.

    I've since been pondering the need to allow air flow beneath the deck, and was wondering if a free-standing deck sitting directly on concrete isn't such a great idea, even though it is basically protected with a roof. That's what brought me to this forum.

    Now I'm thinking about putting a 2" thick pavers or something like that beneath each support and using 8" supports. That would give me the 10" I want, give the floor good support, and keep the supports off the concrete. It would also be quick and easy, but would also be free standing.

    Am I overthinking or underthinking this?

  6. #6
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    Jan 2008
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    Default Re: Building deck on top of concrete patio

    I'd say you're thinking just right to do what you want. I'd add a dab of mortar under each paver and double-check the finished height with the materials you have in mind. With the minimal grade in the slab, I'd design as free-standing them add a few bolts to tie it to the house using something rot-free to make drainage space between deck and house.

    Phil

  7. #7
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    Jun 2011
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    Default Re: Building deck on top of concrete patio

    Poison Oak:

    If it wuz me, I'd use 8 foot joists, and run your decking boards in the 18 foot long direction. That way, you can build the header on the yard side to be removable so that should an animal get under your deck and die (or other unforeseen circumstance) you can always remove the yard side header and have access to the space under the deck. You could, for example, pull a dead weasel or snake out of there with a rake or use the hose from a wet/dry vaccuum cleaner to remove an old wasp's nest. I just don't like the idea of building an enclosed space that something can get into, but I can't get into that space to get that something outta there. I wouldn't build it that way for myself, so I'm posting here so you don't build it that way for yourself.

    Also, since your 2X8's are gonna be elevated off the ground, it'd be a good idea to attach some expanded metal to the yard side header to hang down to the ground to prevent squirrels and such critters from making a home under your deck.





    If you're not too concerned about the appearance, you could simply have a wooden 2X8 header on the house side, and just cover the ends of the joists on the yard side with expanded metal. In that case, the expanded metal would serve to also hold the ends of the 8 foot joists vertical, which is what blocking between floor joists is intended to do. Just use a few screws with washers and some perforated steel straping...



    at the end of each 8 foot joist to fasten the expanded metal to the joists. Install the expanded metal with the "long way of mesh" vertical, and try and put your screws in at the tops of some holes and bottoms of others to allow the least amount of relative movement between the expanded metal and the joists. Once the screws are tightened, the sharp edges of the expanded metal will dig into the wood end grain, and that should be plenty strong enough to hold the joists vertical.

    I'd check your local garden centers to see what you can order through them in the way of pavers. I expect you should be able to get smaller concrete pavers that would be more appropriate for your project. Or, pavers made out of something other than concrete, but that still won't rot.

    Also, to prevent any movement of your pavers, I'd be more inclined to set them in place, "caulk" around each one with a construction adhesive, tool the joints with a soapy finger to ensure the adhesive is pressed into contact with both the paver and the concrete patio it's sitting on, and just let that glue set.

    When it comes to beams, the deflection of a 2X8 is going to be almost twice as much as that of a 2X10 under the same weight. However, the deflection is proportional to the CUBE of the distance between supports. So, on the one hand, by going to 2X8 joists instead of 2X10's, you double the amount of deflection in the joists of your deck as you walk over it. However, on the other hand, by putting a middle support (paver) under your 2X8's, you effectively halve the length of your joists, which results in the deflection being reduced by the cube of 1/2, or by of 1/2 of 1/2 or 1/2, or 1/8. So, by putting another paver under the middle of the 8 foot joists, you MORE than compensate for the reduction in joist thickness, and get a much stronger deck floor. In that case, I'd expect you'd have more deflection of the deck floor as a result of the decking boards bending between the 16 inch on center joists. But, if I wuz building it, I'd put in those middle support pavers for the small incremental cost. I graduated from the U of Manitoba as a mechanical engineer in 1978, so I remember and understand the formulas on this page:

    http://www.clag.org.uk/beam.html

    If you don't understand why putting a middle paver under your 2X8 joists will greatly reduce the deflection in your 8 foot joists, post again and I'll explain it in greater detail.

    PS: Installing your 2X8 joists on 12 inch centers will also decrease the deflection of your deck floor. Since the deflection is proportional to the cube of the distance between supports, and my Microsoft Scientific Calculator tells me that the cube of 12 is 1728, whereas the cube of 16 is 4069. So, by installing your 2X8 joists on 12 inch centers instead of 16 inch centers, you reduce the deck board deflection by (4069 minus 1728)/4069 or by 58 percent, or about half of what it was at 16 inch joist centers. That is, the deck floor material will deflect about half as much if you install your 2X8 floor joists on 12 inch centers instead of 16 inch centers. With your joists on 16 inch centers, you need 14 joists. With your joists on 12 inch centers, you need 19 joists. Only you can decide if the benefit is worth the incremental cost.
    Last edited by Nestor; 06-21-2011 at 05:17 AM.

  8. #8
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    Jun 2011
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    Default Re: Building deck on top of concrete patio

    Thanks for the tips, guys.


    I talked with a couple of buds who looked it over yesterday, and they said I'm overthinking the idea. They both said to throw put it in and not worry about it. I'm grateful someone else kinda sorta validates my thinking.

    I doubt something as heavy as what I'm planning would slide, but the comment "I'd design as free-standing them add a few bolts to tie it to the house using something rot-free to make drainage space between deck and house." was what I had decided to do after posting my question. I'd decided to basically frame a "box" for the floor, so it would be easy to tie those into the house. I didn't think about putting a little something beneath the pavers. I figgered something that heavy wouldn't go anywhere, but I'd rather put a dab of something beneath them as they're being set as compared to WISHING I'd put a dab of something beneath them as they were being set.

    I did go to the link provided, but when I read "A particularly good exposition, and on which the equations given here are based, is contained in Mechanics of Materials (Fourth SI edition), by J M Gere and S P Timoshenko, Stanley Thornes, ISBN 0 7487 3998 X. Reference should be made to this work for the derivation of the equations ..... greek lower case delta symbol .... homogenous, linearly elastic materials" I realized it wasn't in English. I DID attempt to go through it, but was hopelessly lost. I understood your translation, though.

    Regarding the number of pavers (or whatever ends up being used) I was thinking more like four or five per joist, but it looks like you are talking about one in the middle of each, yah?

    "I just don't like the idea of building an enclosed space that something can get into, but I can't get into that space to get that something outta there. I wouldn't build it that way for myself, so I'm posting here so you don't build it that way for yourself."

    That's a good point and I thank you for it. I had it in my head there would be enough room to get something out from beneath it. Now I'll re-think it.

    You gave me a lot to digest, so I'm going to re-think what I had planned, especially the part about running the floor the opposite way.

    It's great to learn something via the internet as compared to learning it the hard way. I really appreciate y'all responding.

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