Freestanding deck on an existing slab

[burnie64]

I would like to build a freestanding wood deck on an existing slab. I am wondering how to attach the 4x4 PT post to a slab. I can find plenty of info when pouring footings, but not much for a slab. It will be built under an existing porch and elevated 2', size 6' x 12'. I was thinking I could use Simpson standoff bases, drill a hole in each corner and attach to the slab with 1 1/4" 3/16 concrete screws. Any other ideas?

[Redoverfarm]

burnie64  I think I would go with something a little more substantial than that sized fastner.  They make a bolt called a "wedge anchor"with a beveled end.  Common sizes are 1/4 to 3/4" in various lengths.  Drill the approiate size hole and push the bold in and when you tighten it upon it expands to hold tight.  Lowes and HD both have them.

[MountainDon]

IMO, you don't need to worry too much about what you use to hold it in place on the slab. I do worry about how you plan on making the deck stable. That is, how are you going to prevent the deck from developing a list in one direction or another, and becoming unstable.

You say it will be under another deck. Can you tie at least one side of the new deck into the supports for the overhead deck?

The other question I have is whether or not the concrete slab is sufficiently thick and reinforced to withstand the point loads from the post of the new deck?

[Redoverfarm]

You say it will be under another deck. Can you tie at least one side of the new deck into the supports for the overhead deck?

The other question I have is whether or not the concrete slab is sufficiently thick and reinforced to withstand the point loads from the post of the new deck?

Don I think he is just building a free standing deck under an existing porch roof not another deck.  It is doubtful that any porch support are substantial enough for the deck.

[MountainDon]

  I misread, misinterpreted. 

Just looking for an easy out to prevent the sway.

In the past I made a bad situation (sway) better by installing diagonal planking under a 8 foot elevated deck that had been previously built with deck planking applied square/perpendicular to the joists. It helped, but also needed more post to beam, and post to joist bracing, but the owner didn't want the angled braces intruding in the space.

[MikeT]

Why not use your basic deck block?  HD or Lowes has easy DIY plans using these blocks.  You essentially use a number of blocks so your spans (hence your point load) would not be very large. 

mt

[PEG688]

So it's 2' to the under side of the joist or 2' overall , slab to top of decking?

Mike T may be onto the right solution , IF it's 2' to the underside of the joist.

There will be beams under the joist ?? Yes / No ?

More info please. 

[CWhite]

I'll play devil's advocate here....
Why would you want to build a deck over a slab?  Concrete is forever. 
Just curious if you  have looked into utilizing the slab with an updated texture, or color, or even a fresh coating of smoother concrete or something. 
Christina

[burnie64]

The deck will be built  for a mobile home, door

is 28" from top of slab, right now there is a 3x3

platform with stairs. I thought about the deck

block, but skirting needs to go around the deck,

deck blocks would be in the way and I would

have to cantilever the deck out.

[MikeT]

Burnie64: I don't agree with your assessment that the deck blocks would prohibit you from skirting.  You are right that you might need a small cantilever, but if you are only setting blocks in just enough to handle skirting, my guess is that you would be just fine.

Also, there are two types of deck blocks:  one where the deck framing gets set into/rests on top of the blocks in a slot that has  been formed in the blocks; and another where there is a 4x4 post base is set into the block.  I would go with the latter, set in two beams appropriately sized, then build your deck frame on top of the beams.  Using this method, you should be able to get your desired height, use the posts to handle any settling that has or that could occur on the slab, and set the beams in enough so that you can handle the skirting.

I think that would work, but I will defer to PEG, Don, Glenn and the other helpful folks here who know much more than I do....

mt

[burnie64]

I plan on using this type of construction on my

12 x 6 deck, 3 posts on the 12' dimension sides,

2 at the corners on the 6' side. I would extend

the posts at 4 points for the deck railing. The

stairs will be full length of the stair side



ED: added image tags to link - MD

[glenn kangiser]

I would suggest doing the deck as you want, described above then using a few more boards - maybe extend the joists or a few 4x4 blocks for spacers, extend the deck boards on top a few inches and add one outside the posts-- trim the deck out a little wider outside the posts just as if you were cantilevering it then skirt it.

That way you can clear the blocks.


Consider that if you use blocks -- I re-read your original pst and will reply below.

[glenn kangiser]

Reading your original post again, Burnie, I would use the standoff bases as you mentioned and for fasteners, get the Tapcon screws and proper sized drill -- comes with them in some cases.

Use treated posts and same for diagonal bracing.  The Tapcons should be more than adequate to hold the bases to an existing slab.  Most of your force will be straight down and proper diagonal bracing on the legs will take care of the rest.

[Redoverfarm]

burnie64 I ran an extra 2X on the outside just for this reason.  I wanted to rock the piers.  This give me the oppurtunity to rock the piers so that the stone set under the additional 2X rim joist.  Actually it will help on the structural intergrity as well. I doubt that any skirting material is more than 1-1/2"  and it should allow you to miss the blocks at the bottom. Just a thought.

[burnie64]

Edited the pic, I dont think an extra 2X on the perimeter will get me enough clearance looking
at the dek-block photos. Extending the deckboards would mean setting the joists flush
with the top of the ledger/skirtboard and not down 1 1/2 inches as shown in the photo right? That
would work on the end opposite the stairs, but what about the side away from the house? I am using 5/4 x 6 pt decking
What joist spacing is recommend 16o.c. or 24 o.c.?

[glenn kangiser]

That's the way I would do it. 

Away from the house I would add blocking outside the posts and another deck board outside to trim it - a bit of a cantilever effect but only for looks to get room to trim over the blocks.

[Redoverfarm]

burnie64 How are you going to attach the underpinning on the bottom?  Pressure treated tapcon to the slab?  If that is the case then you could run the plate from block to block on the slab and then just tapcon the underpinning to the block itself on the corners.

I would say that you will have to carefully measure and build the deck to fit your decking to allow an overhang so that you don't end up with a narrow strip on the outside edge.  Might even have to cut the first on neck to the house to compensate to end up with a full board width on the front edge or something that will not breakoff or breakdown on that edge.  A full board even with 1/2 supported will probably hold up. Anything less will probably not work.

[burnie64]

Deckplans.com had a deck calculator using dek-blocks, what do you think about this
system it uses only 2x6 material and 4x4 posts, which I would need to get the deck up to
height. I dont like the idea of toe screwing the 2x6 joists? on top of the 4x4 posts, is there a
simpson tie that would accomplish that? It seems simpler and reduces cost.

[Redoverfarm]

Sorry burnie you lost me there. Sometimes that is not hard to do. toescrewing to 4X4?  I couldn't visualize you delimia until I saw the way the blocks were interferring. They show a cantelever on the ends.  What would prevent you from doing the same on the front.  It wouldn't be that much out there and if tied to your joist , rim joist and supported by the blocks it should work.  You could determine how far. 

[MountainDon]

I imagine that toescrewing or toe nailing the 2x6 to the 4x4 posts would work. What they call the 2x6 end boards would help stabilize them. One set of blocking across the center would help as well.

Probably the reason they do it that way is that I can't think of a simpson product that would do the job. Simpson has things lie this AC series post cap...



... but they're sized for matching timbers. You could use a pair but you'd have to figure out how furr out the missing 2 inches. Have any rough sawn real 1" thick boards?  Just need a couple pieces at each post.  Also cost a bit for that many.

[Redoverfarm]

Just a couple more sites that might help.

http://www.bestdecksite.com/introLedgerPg1.htm
http://www.extremehowto.com/xh/article.asp?article_id=60350