Concrete Slab

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Old 02-06-15, 06:58 PM
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Question Concrete Slab

I want to construct a new 15'x4' concrete slab to serve as a foundation for an outdoor kitchen. The location of the new slab is 14' feet away and 18" below the top of an existing patio; I plan on building a wooden deck between them.
Rather than bringing in a bunch of fill and having to regrade my property , is it okay to form and pour a concrete slab 18" thick, assuming I'm OK with the additional cost of the concrete and steel? I've attached a drawing to paint a clearer picture. Name:  Drawing.jpg
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Old 02-06-15, 08:40 PM
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Makes no since to me for several reasons.
Fill is cheap, concrete is expensive.
Not sure what the reasoning is for that deck, but a wooden deck should not be built that close to the ground.
 
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Old 02-07-15, 04:24 AM
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Welcome to the forums!

You can't just form up and pour the slab on top of the soil. Much of fla requires the footer to be 16" below grade. As Joe said - fill is a LOT cheaper than concrete! A deck always needs to be high enough for sufficient air flow under the deck to keep it dry. Wood that stays wet invites rot and insect damage!
 
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Old 02-07-15, 05:20 AM
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I appreciate the concerns about ground level decks and I see that there is not much support for them on this forum, however, I see plans for them in numerous deck building books and on the internet so I know they exist.

I assume that using fill to create the raised patio will require either a retaining wall or sloped grade around all sides, both of which I don't want.

So for the purpose of what I'm trying to accomplish and if the correct sub-base is in place, are there any structural issues with pouring an 18" thick slab?
 
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Old 02-07-15, 05:25 AM
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As long as the ground will support the weight, you can make the slab as thick as you want. I don't understand the reluctance to pouring a footer, erecting a foundation stem wall, backfilling and then pouring a 4" slab.

Wood decks on or close to the ground exist but that doesn't mean it's a good idea! The wood [even PT] will have a drastically shorter life if it gets wet and takes a long time to dry out.
 
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Old 02-07-15, 03:40 PM
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1st off, stop reading those magazines about ground-contact decks where do you think our wives get their ideas ? ? ? if you're bound to place an 18" thick slab of concrete, that's the same thickness we used @ newark's terminal B parking apron muy concreto, eh ? suggest lo-strength cementitious road fill instead of conc & save some $ altho that doesn't appear to be any concern
 
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Old 02-07-15, 04:27 PM
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I think concept of an "island" cooking center is great. the wood provides a separation of the spaces while the ground level drops down.

The amount of concrete is not much (about the amount needed to avoid the dreaded minimal surcharge. The forming labor is free and the forms could possible uses elsewhere. That is if there access to a ready-mix truck (cheaper and no time and labor for clean-up).

Since it is isolated from the home/residence and frost in the soil is not a problem, some of the arbitrary code depths for "foundations" in the model codes do not apply. The surface of the kitchen could even be tiled. - I wish I could do something like that in Minnesota.

I would make the wood deck narrower to emphasize the "island kitchen" a highlight and avoid having too many amateur chefs close by. A 4' wide kitchen may not be enough, but concrete is cheap - an extra yard is only $100 or so.
 
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Old 02-09-15, 07:41 PM
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The reason there are frequent ground-level wooden decks in glossy magazines and internet articles is that most (if not all) of the authors have absolutely no clue about long-term performance. And besides, it keeps groceries on the tables of deck rebuilders, especially in wet climates like yours.

The 4-foot slab width is too narrow to be practical. If you subtract 2.5' for the grilling and food prep stations, that only leaves 18" of standing and working space. Not enough for normal people, unless you're a very skinny guy, who doesn't mind frequently stepping off onto the grass. I'd go at least 5' wide.

Thick slabs perform best if 2 separate horizontal mats of rebar are used, tied together with vertical stirrups. They will prevent horizontal shear cracks from occurring, which should be expected with large temperature differentials between the top and bottom of a thick concrete slab. A preferable alternative would be pouring just a 4" slab, with an integral monolithic retaining wall.
 
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Old 02-10-15, 11:23 AM
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I'm going to proceed with my my plans for a ground level deck and accept the fact that my PT framing will have a shortened lifespan.

In response to the 2 alternate suggestions of 1) stem wall and 2) a 4" slab with an integral monolithic retaining wall. I did not know what either of these were until mentioned , but now that I've researched I don't think either will work for me considering my project requirements.

I'm looking for the least invasive way to insert this new structure into my existing landscape:

1) Single concrete delivery.

2) No new fill berms or block retaining walls that would encroach on my existing walkways, landscape beds, and new deck framing.

3) Within my DIY forming and rebar abilities.

So considering my project requirements, is the 18" slab my best option ?
 
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Old 02-10-15, 12:27 PM
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Now that you have provided some new information that will go into your personal decision on the job, I have the following comments:

The amount of concrete is minimal once you get a delivery truck involved (my friend is in the concrete and aggregate business always said the concrete business is the most expensive way to sell aggregate be cause it is a very costly truck that is very pricey do delivery. Making only one trip is sensible IF there is access around your precious shrubs and landscaping and probably around your home to the back. If you have any other areas that need a hard, durable all-weather access look to see if you need any other areas to place concrete. The concrete can also be pumped over or around your landscaping.

Forming of concrete is really very simple and the remains can be used for other uses.

I would suggest using a wood deck platform on 6x6 posts (a little wider than 4') and accept lesser mess and access problems to excavate/auger post holes and the backfill and lumber movement.

A wood structure is very limber or flexible and will adjust/move frequently over time, but will work if you are as flexible as the structure. Since it is not really attached to the home most model codes do not have a legal effect except those more restrictive requirements that can always be added to any code the local officials.

Dick
 
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