Building Deck Over Slab....? - Decks & Fencing - Contractor Talk

Building Deck Over Slab....?

 
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Old 12-16-2014, 08:45 AM   #1
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Building Deck Over Slab....?


Went to look at a job yesterday where a lady wants a deck built over a slab. The slab is an outdoor patio that is actually about 8' high off the ground with block walls.

The pressure treated deck needs to sit as low as possible and I know putting PT wood on concrete is a no no as it will rot and also there will not be enough space under the decking to allow it to breathe.

My plan is to set posts just outside of the slab all the way into the ground. This will help with ventilation as the cavity between my joists will be open and no closed by the outside band.

As for the joists going over the slab I was thinking of using 2x4's with some sort of bracket to elevate them a bit off of the slab. All I can come up with is this elevated post bracket.

http://images.lowes.com/product/conv...42114079lg.jpg

Just taking these and spacing them about ever 3' or so under my 2x4 joists....

What do you guys think? Will this work?

I'd love to hear if anyone has a better idea as well.

Thank you gentlemen!
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Old 12-16-2014, 09:11 AM   #2
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Re: Building Deck Over Slab....?


If this slab is 8' off the ground ,I think the post going into the gronnd would look like crap.First thing I would do is check the slope of the slab and make sure it is away from the house.I would put posts off the slab using simpson brackets and drive anchors.
2X4's shimmed for joist are also a bad idea. Beef up your joist to keep them off the slab with no shims.
Knowing the size of the deck would help give a better answer.

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Old 12-16-2014, 09:52 AM   #3
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Re: Building Deck Over Slab....?


I agree, the post off the ground that high wouldn't look the best. But I haven't found a way to have sturdy handrails when the posts about as deep as the deck and not into the ground. I could be missing something though.

I will have to see if I can go up to a 2x6 joist but I'm not sure that will be an option or the deck will get to high.

Here is a quick image of the deck size. There is a slab under it every where except the grey area. There, the deck will be elevated 8' off the ground.

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Old 12-16-2014, 10:37 AM   #4
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Re: Building Deck Over Slab....?


Quote:
Originally Posted by SailingandSuch View Post
Went to look at a job yesterday where a lady wants a deck built over a slab. The slab is an outdoor patio that is actually about 8' high off the ground with block walls.

The pressure treated deck needs to sit as low as possible and I know putting PT wood on concrete is a no no as it will rot and also there will not be enough space under the decking to allow it to breathe.

My plan is to set posts just outside of the slab all the way into the ground. This will help with ventilation as the cavity between my joists will be open and no closed by the outside band.

As for the joists going over the slab I was thinking of using 2x4's with some sort of bracket to elevate them a bit off of the slab. All I can come up with is this elevated post bracket.

http://images.lowes.com/product/conv...42114079lg.jpg

Just taking these and spacing them about ever 3' or so under my 2x4 joists....

What do you guys think? Will this work?

I'd love to hear if anyone has a better idea as well.

Thank you gentlemen!

I could not get past the slab being 8 feet off the grade.

I have a better idea but I get paid for that ****.

Andy.
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Old 12-16-2014, 10:48 AM   #5
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Re: Building Deck Over Slab....?


pressure treated is designed to be applicable in this situation. It can sit on concrete, you just don't want debris (organic matter) piling up around it and retaining moisture. As far as venting... run your joists 3 1/2" past slab, run posts thru and red head into wall. Timberlocks in your rim to reinforce posts and HD2's if necessary.
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Old 12-16-2014, 11:16 AM   #6
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Re: Building Deck Over Slab....?


Why would treated lumber not be allowed to contact concrete?
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Old 12-16-2014, 12:01 PM   #7
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Re: Building Deck Over Slab....?


It isn't a question of whether you can or can't, it's a question of treatment level needed for the application.
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Old 12-16-2014, 12:36 PM   #8
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Re: Building Deck Over Slab....?


I put this framing on a support every 4' but this slab has many drains. Also I used composite that doesn't behave anything like real wood.

Composite would be my recommendation.

That was predictable.
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Building Deck over Slab....?-1418751424999.jpg   Building Deck over Slab....?-1418751445031.jpg   Building Deck over Slab....?-1418751460237.jpg  
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Old 12-16-2014, 02:44 PM   #9
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Re: Building Deck Over Slab....?


Quote:
Originally Posted by ScipioAfricanus View Post
I could not get past the slab being 8 feet off the grade.

I have a better idea but I get paid for that ****.

Andy.
Me too? They get the down and dirty idea for free.Then they have to pay.
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Old 12-16-2014, 06:04 PM   #10
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Re: Building Deck Over Slab....?


Quote:
Originally Posted by ScipioAfricanus View Post
I could not get past the slab being 8 feet off the grade.

I have a better idea but I get paid for that ****.

Andy.
I can personally attest to this.
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Old 12-16-2014, 07:13 PM   #11
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Re: Building Deck Over Slab....?


Two words... basket weave.
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Old 12-16-2014, 07:30 PM   #12
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Re: Building Deck Over Slab....?


Ground Contact PT....

(see your two words....and raise you an acronym)
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Old 12-16-2014, 09:06 PM   #13
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Re: Building Deck Over Slab....?


If you absolutely have to use 2x4 joists then I'd probably skip the standoffs you linked and use the plastic standoffs and block the bays a bit more, unless the BO is requiring the other standoffs for uplift reasons. You're going to want to use a LOT of those standoffs as any point loads are going to be a real problem on 2x4's.

Have you looked at using a steel frame?

Consider skipping the posts going to the ground and limit your cantilever so you can drop the posts a bit below the framing and then attach them to the block wall. I think a sleeved threaded rod would work well.
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Planer.....
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Old 12-16-2014, 09:16 PM   #14
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Re: Building Deck Over Slab....?


Thank you for all the suggestions. I think I have a few more options now for the joists.

My handrails are another story. I'm not able to easily attach the posts to the block wall do to a "overhang" of the top slab similar to a row lock of brick but just with concrete. It probably hanges over 1 1/2" or so.

In order to run the posts down a bit and fasten to the block wall I would need to put some blocking between the post and the wall in order to keep the postplumb.

An option that would be a major pain would be to just cut off the overhang, but I don't think it's feasible.....
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Old 12-16-2014, 09:36 PM   #15
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Re: Building Deck Over Slab....?


Quote:
Originally Posted by SailingandSuch View Post
In order to run the posts down a bit and fasten to the block wall I would need to put some blocking between the post and the wall in order to keep the postplumb.
.
That's what I was talking about for the threaded rod. Run the rod through the post and into the block wall with epoxy in the wall and a nut and washer on either side of the post. You could sleeve pipe onto the rod when you put it in to hide the threads between the post and the wall. Stainless or copper would look cool.
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Old 12-17-2014, 06:32 AM   #16
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Re: Building Deck Over Slab....?


Is this a composite rail? Look into timbertech secure post mount. I picked one up the other day, but haven't installed yet. Seems very sturdy.

http://timbertech.com/files/files/Se...ost_7_2014.pdf

I'll be installing directly to concrete.
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Old 12-17-2014, 07:06 AM   #17
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Re: Building Deck Over Slab....?


Quote:
Originally Posted by SailingandSuch View Post
Thank you for all the suggestions. I think I have a few more options now for the joists.

My handrails are another story. I'm not able to easily attach the posts to the block wall do to a "overhang" of the top slab similar to a row lock of brick but just with concrete. It probably hanges over 1 1/2" or so.

In order to run the posts down a bit and fasten to the block wall I would need to put some blocking between the post and the wall in order to keep the postplumb.

An option that would be a major pain would be to just cut off the overhang, but I don't think it's feasible.....
Why would it be a "major pain" to cut the concrete? I think I'd rather do that then mess around with blocking or any other half @$$ attachment you can think of. I have a diamond blade for one of my old circular saws and it would make quick work of that 1.5" overhang
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Old 12-17-2014, 07:36 AM   #18
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Re: Building Deck Over Slab....?


I would only be installing structural surface mounted posts in this application.
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Old 12-17-2014, 12:30 PM   #19
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Re: Building Deck Over Slab....?


If you use Moisture Shield composite or Azek cellular PVC you don't have to worry about ventilation underneath your decking.

I would use sleepers underneath the deck surface because Tap Cons through the decking is a bit un-slightly. ha

Bob

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