Adding to basement


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Old 05-11-15, 05:01 PM
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Adding to basement

I have a 2 story house built in 1997. The basement has a walkout sliding door near the edge of the house. . Outside the door is a patio that is 12 feet wide. From there the ground slopes upward to the far edge of the house. At that point, there is about 2 feet of exposed foundation. I would like to flatten the ground across the whole back of the house in order to pour a slab directly against the existing foundation. My concern is that while building the house I saw that under the sliding door, the footer/foundation was much deeper, so does that mean I can not dig the rest of the ground to the level of the patio without exposing the footing? Or is the footing farther below the basement floor. The builder said the ground couldn't be leveled across the back to make the patio wider but didn't explain why. I would really like to get more usable space by leveling it and pouring a pad and at a later date enclosing it and making it part of the basement which I will finish in the future.
Any insight on this would be appreciated. I live in Ky so I believe the frost line is about 18 inches.
Thank you.
 
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Old 05-11-15, 05:51 PM
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You can't (shouldn't) build on top of a pad. If you plan to build on it, you would have to remove the existing pad (since it's not on a footing), excavate about 30" below the existing pad and pour a 12" thick perimeter footing. (Usually you want to match the depth of the existing footing.) The top of the footing needs to be at/below your frost level. You would then pour a new pad on top of that. (yes, this is just like the basement floor is)

I would discourage you from doing any of this until the day comes when you are ready to build. The problem will be that unless you want a step, your cement pads should be level with one another. If your cement pads are level with one another, you will probably get water in the basement through (under/around) the patio door opening when it rains.
 
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Old 05-11-15, 06:21 PM
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Ok, thanks. That makes a lot of sense and clears things up for me a bit.
So, when ready to build, you say I need a perimeter footing, so that would be poured adjacent, basically touching the existing footer?
 
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Old 05-11-15, 07:15 PM
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yes you need a perimeter footing and the new and old footings will usually be tied together with rebar. You usually will drill into the footing a few inches and pound some short pieces of rebar in, then if you need to extend the rebar through the new footing you can tie new rebar to those short pieces. All the rebar gets encased in the footing when the new concrete is poured. You don't need a new footing along the existing wall of the house... only the 3 new sides of the proposed addition.

I think the thing I left out in the earlier post was that if the top of your footing is 18" below grade, you would need a foundation wall on those 3 sides to bring everything up above grade (usually 6" above grade is a good number to shoot for, since the sill plate of your wall needs to be at least 6" above the ground) The pad doesn't actually sit right on the footing. Not sure if that part was unclear.
 
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Old 05-12-15, 03:36 AM
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Wow, you have been really helpful. Thank you so much.
So, I understood the part about bringing the foundation wall above grade. At that point I have 3 new foundation walls and the existing wall surrounding my new area. So the floor gets poured against those, and the existing outside wall also becomes an inside wall?
I just wanted to be sure that there wouldn't be a support issue with the existing wall since now it has earth covering it on the outside and when finished will be exposed in the new addition.
 
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Old 05-12-15, 04:38 AM
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As long as you have an existing full basement in that area, shouldn't be a problem. If your existing basement floor steps up to a crawlspace... that would be a problem.
 
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Old 05-13-15, 05:47 AM
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No. I have a full basement. I greatly appreciate your information and your quick responses.
 
 

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