Adding to existing cement foundation


Old 09-08-04, 07:19 PM
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Question Adding to existing cement foundation

I have a cement slab in my backyard that measures 6'11" x 10'4" and has been here since the '60s. I'm buying a shed that measures 8 x 10 and I'd like to place it on the existing slab. What is the best way to increase the width to accomodate the new shed? Should I frame it out and add more cement? I've never done that. Can you give me tips on it? For example, do I pour the concrete directly onto the compacted dirt or do I lay stones first??? Or can I skip the cement and use pavers or cinder blocks? Any help is appreciated!
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Old 09-10-04, 02:52 AM
Join Date: May 2004
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I would dig down to see how thick the slab is. Hopefully it is at least 4". Dig the area you wish to add on and frame it with a 2 by 6 so the top of the form board (2 by6) is level with the top of the slab. Use a 2 by 4 across the slab to check for level. Use 2by2 stakes or rebar or steel stakes every 2' along the form board. Drill with a carbide masonry bit at an angle into the side of the existing slab and insert some 3/8" or 1/2" rebar maybe 12" in length (4" into the slab). Then repeat but angle it the other way this time so you have 2 rebar pieces at opposing angles this will help anchor the slabs together. Pack the dirt down by pounding it with a sledge hammer on a wood block. Now figure out how much concrete you need by multiplying the length * width 13"by120"by 4"?(thickness)=6240cu inches. 1 cu ft. is 12*12*12=1728cu in. So it would take about 3 and 2/3 cu ft. I would mix this in a wheel barrow by hand as its not enough to get a Uhaul load or the cement truck for. This will vary with length, width and depth and always figure 10% more than calculated. Use a piece of 2by4 to screed the wet concrete and finish it with a trowel. To get a better idea what this would look like (a picture is worth.....) go to the library and look through some books as I am sure you will find a picture. Keeping the slab from separating because of how narrow your addition is will be a problem but its a light weight shed not your house.

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