What are the best practices for a low deck built half on a concrete slab?

Question

In our current place, we moved the main access door from going into the kitchen, to entering an old laundry room converted to a mud room. In order to do so, the front door was moved over about 4', and off the existing concrete patio. The bottom of the door sill is ~11 1/2" above the existing slab, and ~12" above grade. You can see a plan view of the existing concrete patio and proposed deck below.

The plan is to make a 10'x12' deck. Why so big for an entry way? Well, it is next to the kitchen, and it would be a great spot to grill. Also, I plan on building a bench or two on the deck for kids to take off shoes, boots, etc.. before entering the house.

Since the deck is so low to the ground, I plan on building it directly on the slab, and having it free-standing (i.e.- not connected to the house, thus no ledger board). By "directly on the slab" I mean to also shim it up, say, 1/2" to allow for circulation and water flow. I plan on using 12' 2x8 joists, spaced at 12". First question- does this all seem like a sane idea?

For my second question- obviously this deck will hang over an area where there is no concrete slab. Would it be enough to place an 8" diameter, 4'deep footer at the two outside corners and in the middle of the 12' span, one joist in from the outside, at the same level as the slab? (see dotted circles in the drawing).

Any other thoughts? Am I out of my mind?

Answer 1

You didn't say where you are located, but i figured in a cold area like me in Maine. First of all, 12 inch joist spacing a a waste of time and money. 16 inches will be fine especially if you plan to use 5/4 decking of any type. If you are in a non ground freeze area, just use simple blocking to support the additional 4 foot off the concrete. Assuming you are in the frozen north, consider using 3 foot porch posts. they are pre-poured concrete pyramids with a threaded insert dead center on the top. Often easier that sonotubes and mixing concrete. If your existing slab is not underfooted with a frost wall, and has not heaved or cracked over the years, then most likely a simple 8 inch concrete block in the ground the same depth or slightly deeper than the slab will probably do just fine. Remember you are only supporting 2 foot of deck (2 ft supported by slab and 2 ft supported my new foundation) and the decking itself will be spanning 8 to 12 feet onto the existing slab. Over kill is fine, always safe, but not always a good use of time and money. Also if you new deck is 12 inches off the ground, please consider some safe egress steps and side barriers. The rise of the step should not exceed 8 1/2 inches. A 12 inch step down can be a hazard, especially at night and for younger and older folks. Check your local code or the new IRC guide lines.

Answer 2

I thought this would be a fun thing to do (and perhaps set a precedent on diy.stackexchange.com)- post a picture with the results of my question.

Thanks to the tips here on this question, as well as another I asked, I have completed my deck project. I ended up with what was described above- a ground-level deck, mostly on the slab. I took Shirlock's advice, and went with an 8" deck block at the two corners not on the slab. I compacted the earth a bit with a hand compactor. Since the deck is not attached to the house, if there is some heave, it wont do any damage, and I can always dig some concrete holes under those two corners next spring if it turns out I need them. So, without further adieu, the finished deck (yes I realize we have some funky siding.. spring 2011 project ;) ):