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Discussion Starter · #1 ·
I have a single story house on a slab on grade foundation. It was built in 1953 in Minnesota. It has a 4ft block stem wall on footings for a foundation. the slab is in good condition and has no cracks.
I would like to build a new two story addition next to the existing house. they would be connected to make one large house. Ideally i would like a basement under the new section but i am unsure of how to build the basement wall next to the existing house without damaging the old foundation wall. can i use the existing wall and somehow build it deeper? will the old wall hold the weight of the second story? if i have to build a different foundation wall next to the house how do i keep it from interfering with the old wall?
Option 2: i build a slab on grade addition. but how will me new slab connect to the old house? i still need a new foundation stem wall to hold the weight of the second floor don't i? if i use the existing stem wall and tear down the wood frame wall to build a more sturdy wall there to hold the second story will the old foundation wall hold it?
Basically in both cases how do i get the old foundation wall to hold the larger weight? or how to i get a new foundation wall close enough to connect the house but far enough to not interfere?
I have done a lot of DIY remodeling before but this will be my first foray into cement work.
 

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retired framer
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Before we get into foundations with the basement would the upstairs be at the same level or are you looking at split level? Do the ceiling joists and rafters land on this wall?
If you are thinking the same level will you be opening the wall between old and new?
 

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Discussion Starter · #3 ·
Thank you for the reply Nealtw

"Before we get into foundations with the basement would the upstairs be at the same level or are you looking at split level?"
The main floor would be at the same level as the old one or near to it. the existing slab floor is about 3" above grade. not a split level.

"Do the ceiling joists and rafters land on this wall?"
Possibly. I was originally planning on that but the new addition is more or less a rectangle so i could run them the other way with a support wall down the middle. for reference the original house is a 30' square and the new addition is a 25'x50' rectangle tacked on the side in a T shape.

"If you are thinking the same level will you be opening the wall between old and new?"
yes. I am planning on removing about 8ft of it but i could do more or less.
 

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retired framer
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Thank you for the reply Nealtw

"Before we get into foundations with the basement would the upstairs be at the same level or are you looking at split level?"
The main floor would be at the same level as the old one or near to it. the existing slab floor is about 3" above grade. not a split level.

"Do the ceiling joists and rafters land on this wall?"
On the house as is now, do the rafters land on this wall?

Possibly. I was originally planning on that but the new addition is more or less a rectangle so i could run them the other way with a support wall down the middle. for reference the original house is a 30' square and the new addition is a 25'x50' rectangle tacked on the side in a T shape.

"If you are thinking the same level will you be opening the wall between old and new?"
yes. I am planning on removing about 8ft of it but i could do more or less.
Here is one possibility for the foundation.
 

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Discussion Starter · #6 ·
sorry for missing your question.
"On the house as is now, do the rafters land on this wall?"
The ceiling joists do not but the roof is a hip roof so the roof plane comes down on this wall.

I was thinking something along the line of the first picture. I grew up in a split level but my wife hates them. but since the original house slab is pretty much at grade, wouldn't the far end of the wood floor joists be below grade? how do i deal with that?
perhaps i will do a split level but put the basement 8' down and raise the upper floor high enough to get the wood frame above grade.
Is the wall at the outside of the crawl space holding a beam that is holding my joists?
 

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Discussion Starter · #7 · (Edited)
here is a sketch with some more questions. there is also a second story floor i did not draw. i am mainly asking about the support of the new floors, both first and second, near the old house. should the new second story wall sit on the old wall? or should i build a new wall to bring the support down to the first floor joists? how will i support the cantilevered portion of the first floor? it is not drawn to scale. the crawlspace will end less than 6 feet from the existing structure. the basement will extend 28 ft from the existing structure.
 

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retired framer
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sorry for missing your question.
"On the house as is now, do the rafters land on this wall?"
The ceiling joists do not but the roof is a hip roof so the roof plane comes down on this wall.

I was thinking something along the line of the first picture. I grew up in a split level but my wife hates them. but since the original house slab is pretty much at grade, wouldn't the far end of the wood floor joists be below grade? how do i deal with that?
perhaps i will do a split level but put the basement 8' down and raise the upper floor high enough to get the wood frame above grade.
Is the wall at the outside of the crawl space holding a beam that is holding my joists?
Once in a while we do the low foundation like that for the high side on a hill, with a concrete foundation we put a ledger board in the top of the concrete wall so the floor is level with the concrete just like your slab house. But that is a big basement without any windows.
 

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here is a sketch with some more questions. there is also a second story floor i did not draw. i am mainly asking about the support of the new floors, both first and second, near the old house. should the new second story wall sit on the old wall? or should i build a new wall to bring the support down to the first floor joists? how will i support the cantilevered portion of the first floor? it is not drawn to scale. the crawlspace will end less than 6 feet from the existing structure. the basement will extend 28 ft from the existing structure.
We do all our work with engineered trusses and we would support the old roof with a girder truss. See the purple in this picture is sitting on the new wall so the footer is made bigger there to take the extra weight the old rafters are hung off the girder and you can remove all the wall below if you want to.
With a spit level that girder would have a flat top and would be the end wall of the house.




With trusses you would not need any walls inside up stairs but you would need a wall or beam system down the center of the basement.
 

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here is a sketch with some more questions. there is also a second story floor i did not draw. i am mainly asking about the support of the new floors, both first and second, near the old house. should the new second story wall sit on the old wall? or should i build a new wall to bring the support down to the first floor joists? how will i support the cantilevered portion of the first floor? it is not drawn to scale. the crawlspace will end less than 6 feet from the existing structure. the basement will extend 28 ft from the existing structure.
See if this helps, I changed the direction of the joists over the curb wall
 

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If you google foundation repairs (images) you will see why we never use CMU blocks.


The red lines are the 5/8" rebar we typically put in a 4 and 8 ft wall, all subject to an engineer.
 

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Discussion Starter · #13 · (Edited)
I probably should have posted these sooner but i have the whole thing drawn up on my PC sans foundation. i drew them myself so they are not great. see attached. so the old house is the bottom of the T and the new section is the top. the drawing of the old house is fairly accurate even that nonsense on the roof is accurate. one thing though the footer would not have that step in it as the floor height changes. even though half the house has the slab 12" higher the footer should still be 48" below grade (I am guessing on that because i haven't yet dug down to find out, but 1953 in MN should have a 48" footer). i understand what you are suggesting with the joists on the first floor. i should rotate them to run parallel to the old house so that little to no weight rests on the old foundation (but that is a considerably longer run). I am not planning on using/finishing the basement ever. i never want to use it except for utilities. so columns and lack of windows are not a problem. my main concern now is how to transfer the weight of the second floor joists to the foundation safely. I was going to run 28' I joists perpendicular to the old house as that is the shorter run but that means i have a lot of weight coming down on the old house. now the ceiling heights in the new house are 2 feet higher than the old house so I have 2ft of room for a massive header across the old house side. but how do I transfer the load down to the foundation. and what does that foundation look like?
Also, since I am not planning on using the basement it might be easier to do a slab foundation. my only concern there is lack of access to plumbing for the kitchen and downstairs bathroom. I don't know much about how you set that stuff in concrete and how well that works. i would rather just have access with a basement but maybe that is an expensive want. but slab foundations have the same concerns as basements. how do i transfer the load of the second story to the foundation at the point of the two houses meeting?
 

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That changes the picture just a little. If this was our job here the engineer would want to look at the footing and foundation, he would also measure the strength of the soil that is under it.

Slab on grade is the easiest just like you have, up here , all the dirt has to be removed and after the foundation is done it is refilled with compacted sand or chips, so home owners thing about a crawlspace which get a 2" layer of concrete and has to be 5 feet head room so it is not a stretch to get to a basement.



In this picture I moved you house over 8" so the weight of the new structure is on the new foundation and footing.

Building a wall on top of the old wall with the rafters there is bad enough but adding a beam for the open space is really destructive.
So by moving it out 8" support for a girder would be the two corners or 2 beams that meet on a post built into the bathroom wall.


The girder or beams would support the floor and the wall above and you are adding nothing to the old house but the valley set for the roof.
 

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Discussion Starter · #15 · (Edited)
thank you for the help! this is just the info i needed. I will add the 8" to my plans. for the new pillar in the middle bathroom wall, does it sit on a new foundation wall or just a pillar of some kind (see pictures)? do the footings at these points need to be bigger or deeper at all? Also is 8" an important number? would 6" or 10" be ok or is there a reason for 8?
 

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thank you for the help! this is just the info i needed. I will add the 8" to my plans. for the new pillar in the middle bathroom wall, does it sit on a new foundation wall or just a pillar of some kind (see pictures)? do the footings at these points need to be bigger or deeper at all?
I was thinking slab when I did that I will take another look for the basement.

But usually is would just be a 6x6 concrete pillar on a footing that comes up level with the concrete floor. wood post to under the beam.
The bathroom upstairs will want a 2x6 wet wall that would hide up to 4 2x6s to support the upper beam.


So the stair location would to the basement would be in the same place as your plan.
 

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thank you for the help! this is just the info i needed. I will add the 8" to my plans. for the new pillar in the middle bathroom wall, does it sit on a new foundation wall or just a pillar of some kind (see pictures)? do the footings at these points need to be bigger or deeper at all? Also is 8" an important number? would 6" or 10" be ok or is there a reason for 8?
This looks about right but the post support would be level with the floor usually, I am sure the floor could be lagged to the old foundation but you would still need the post if you are using beams for the second floor.

We use a lot of truss girders here because the are much cheaper than beams most of the time.

 
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