Foundation for Addition


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Hello All,


I'm trying to figure out the best way to do the foundation for an addition to our 1955 home.  The existing home crawl space is minimal.  Perimeter stem wall is 18 inches with 4/6 girders resting on the sill and footings/posts at 4' O.C. .  Flooring is 2x6 T&G floor boards resting on the girders.  Not sure if that meets modern code. 


For the new foundation area I was thinking of using 2x8 joists for the floor framing and matching the existing stem wall.  I cannot use wider joist without raising the floor as I cannot lower the stem wall height due to code requiring 8" min stem top to group level.  The addition needs to span distances beyond the 12' span distance so I need to support them in some way.  Girders with posts seems reasonable, but I'm concerned about the crawl space height.   2x8 joists on 18" 16" O.C is tighter than the existing foundation which has 18+5.5 and widely spaced girders dropping it to 18".  The new foundation with 4x8 joists at 16" on center and girders would drop the bottom the height to 12" under the beams which seems very tight.  I think the code allows for 12", but is it bad practice?  Am I wasting my time worrying about this? 


I have a few ideas on increasing the clearance, but I don't know which would work best and what the cost trade offs are?


1. Move the 4x6 girder to foundation stem wall height and then attach joists with joist hangers rather than resting on top.

2. Build internal foundation footing/stem walls as needed to close 2x8 spans to 12' or less (leave access openings)

3. If legal, copy the existing floor structure.  I'm guessing the 4/6 girders and T&G flooring is likely more costly.

4. Extend the addition's stem wall and interior ground level down 6 inches.  Slope the soil away from the existing foundation 2" a foot. Excavation and concrete costs go up.




Thank you,,




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You can also just dig out the underfloor (crawl space) an extra 8" deeper and extend the footings down also or just build it as it is using 2x6 T&G with girders, make sure to check the span of the 2x6 flooring in your area. 


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Hello Parkwest, 


If I went with a slab it would need to rise 8 inches off ground level to meet the existing stem wall height. Step downs are not an option.   Not sure what the concrete costs would be for that. There will be a lot of plumbing in the slab as the new area includes a kitchen and two bathrooms.  


I didn't consider this as an option as I am not aware of any home being extended that way in our 40+ track home segment.  Many have extended the existing step down family room at slab level but this creates more step down area and  we want to ellimate the step down.


Do you think the costs would be similar?




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3 hours ago, JimAlsup said:

Sorry, meant to say 16 inches off ground level.



could perhaps do the Perimeter in 2 or3 x 8"high CMU instead then? and backfill with Gravel/Sand once plumbing is done, then 4" Slab over VB.


The generic CAD details in the CA Library may give you some more Ideas...... if Pro has them?

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I happened to talk with someone different at the city today (only had to wait 5 minutes, first and hopefully not the last time that occurs).  He said any of the ideas 1- 4 or the slab would work, but the 12 inches under the girders is also fine.  I just wouldn't want to be under the house doing anything in such a tight space.  I'm thinking of exploring #1 as a starting point and then finish the plan to a level needed to review with potential contractors.  Then I'll take it to a structural engineer. 


Thanks everybody for your help.


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