Building a deck on top of a slab

Posted by jtumis on September 28, 2006, 11:29 pm

I want to build an 12x16 deck on top of an 8x10 slab. Slab is in good
condition. I want to lay the floor boards across the slab. I was
going to build a frame ( with2x6's) that would be on top of the ground
and in some areas partially underground to attach the floor boards too
where the wood deck would overhang the slab. Is this real for me to
do? Would the whole deck be able to withstand freezing? Should the
support beams in and on the ground be cemented in or supported with
stone?

Posted by Eric in North TX on September 29, 2006, 7:24 am

That sounds ideal to me, up to the point where the boards touch the
ground. There surely has to be a way to do it where all the support is
anchored to the slab without resorting to putting posts in or, having
boards touch the ground.

Posted by marson on September 29, 2006, 10:21 am

jtumis wrote:

> I want to build an 12x16 deck on top of an 8x10 slab. Slab is in good
> condition. I want to lay the floor boards across the slab. I was
> going to build a frame ( with2x6's) that would be on top of the ground
> and in some areas partially underground to attach the floor boards too
> where the wood deck would overhang the slab. Is this real for me to
> do? Would the whole deck be able to withstand freezing? Should the
> support beams in and on the ground be cemented in or supported with
> stone?

this would not be a recommended approach. besides what frost action
and differential settling would do to this setup, how are you going to
fasten the decking to the concrete? you will need sleepers (don't
fasten deck boards directly to the concrete). also, dirt and crap will
get trapped under the decking on the portion that is over the slab--a
few years and you will have plants growing between the deck boards.

an 8x10 slab is not a huge deal to pull up. why not pull it up and do
the job right?

Posted by HerHusband on September 29, 2006, 11:50 am

Don't lay the deck boards directly on the slab. There's no good way to
attach them, and there wouldn't be any drainage for the water that flows
between the deck boards.

As a start, imagine the slab isn't even there, and build the deck as you
would anywhere else.

Set concrete deck blocks on your slab, and out in the yard beyond the
slab. Install short posts as needed to make things level, and build a
frame with beams and floor joists. You can use joist hangers to keep the
joists at the same level as the beams. Then attach your deck boards to
the joists.

If you don't have the space between the deck and your entry door to use
the deck blocks, you could support the floor joists with bricks or
concrete pavers instead. Adhere the brick to the slab with masonry
adhesive to keep it from moving.

If you don't even have room for the floor joists, you'll need to get
creative. You could lay pressure treated 2x4 "sleepers" on the slab,
using adhesive to keep them from moving. Then attach your deck boards to
the sleepers. Be sure to provide for drainage though, such as using short
boards for the sleepers and leaving gaps between the ends where water can
drain away. Out beyond the slab, switch to standard joists, posts, deck
blocks.

As other's posted, it may just be easier to rent a jack hammer and tear
out the old slab.

It also sounds like you might be building into a slope? If so, I'd dig
the hill back away from the deck, either increasing the angle of the
slope, or installing retaining blocks or something. You don't want the
wood to be in contact with the ground.

Good luck!

Anthony

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