Need To Raise Carport Slab 10" To Match House Slab For Room Addition - Flooring - DIY Chatroom Home Improvement Forum

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Old 04-15-2015, 01:15 PM   #1
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Need to raise carport slab 10" to match house slab for room addition

We have a 20'x 24' carport that sits adjacent to two exterior walls of our house. We want to close in the carport area as a room addition to house, however carport is 10" lower than house slab height. Carport is at grade with surrounding soil. Want to know if I could do any of the following instead of spending $$ to pour concrete to level up the 10" difference.

1) Pour a small knee wall ( 4" wide by 10" tall) of concrete on top of existing slab perimeter and use 8x8x16 double hollow concrete blocks bumped one against another in the entire area area with holes turned sideways, so forms nearly solid concrete top. Top off with one layer of 3/4" treated T&G plywood ( structural glue to blocks) and then a second layer of 3/4 non-treated T&G plywood turned 90 degrees from the first layer, so seams are staggered. Screw top layer to second layer, with glue in between. My finished height will be about 9.5 inches which is almost the same as the needed 10". ( This process is less than half the cost of pouring a 10" slab to level up height difference.

2) Alternate idea: Simply build a 2x10 "sleeper floor" structure ( 12" oc) and top with plywood. Will provide just about 10" height. The 2x10's would be pressure treated, since they will be against the slab surface. Concerns: Termites since 2x10's are at grade and risk of eventual water damage if water seeps in between existing slab and the 2x10? However, treated should last in this condition for decades? This process is something I can do "alone", unlike pouring a slab. This idea is about 2/3 the cost of pouring 10" of concrete.

Do I need to worry about venting if I choose either method. I would put vapor barrier over existing concrete.

Would either of these ideas work well?


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Old 04-15-2015, 01:21 PM   #2
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Odds are that your carport slab is floating. This makes it unsuitable to build on. You need a proper foundation set at the correct depth for your area.
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Old 04-15-2015, 02:18 PM   #3
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None of those ideas sound good.
Post a picture of the area when you get enough post.
Most common way is to build it up at the edge of the slab with concrete block making what's called a stem wall. Google it.
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Old 04-18-2015, 09:06 AM   #4
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The only correct way to do this is to build a foundation around the entire area to be closed off. This should be what ever code is in your area. Once the area is closed off you can put #57 stone right on top of the existing slab and fill the entire area up with rocks, just leaving enough room for a 4" slab. The last time I purchased this stone (which was a few years ago) it was around $18.00 a yard, vs. $120.00 a yard for concrete. I suggest you consult with your local government, and get a permit for this project before going any further.

Last edited by VinnieVega; 04-18-2015 at 09:10 AM.
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Old 04-18-2015, 09:27 AM   #5
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If it is indeed a 'floating' slab. You will need to cut away around the perimeter. Pour a proper footing. Eihter a wall or piles depending on your area. Some would leave the old concrete slad inside the new footing if it is in good shape to act as a concrete floor.
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