Foundation for Addition


JimAlsup
 Share

Recommended Posts

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.

 

Thoughts?

 

Thank you,,

Jim

 

newFoundation.png

Link to comment
Share on other sites

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. 

 

Link to comment
Share on other sites

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?

 

Thanks,

Jim

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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.

-Jim

Link to comment
Share on other sites

Please sign in to comment

You will be able to leave a comment after signing in



Sign In Now
 Share

  • Member Statistics

    30418
    Total Members
    9156
    Most Online
    louisewman
    Newest Member
    louisewman
    Joined
  • Similar Content

    • By AKnightClark
      Brand new user here! First question: Is there a specific order in which the tutorial videos should be watched to learn with a better A to Z process? I'm watching a ton, but they are not in a helpful order, so I'm taking 2 steps forward, 3 steps back, making error that I have to correct, etc.
       
      As for this particular topic, are there specific tutorials for creating a slab foundation that extends over a sloped terrain? I'm looking to make the footers, foundation walls, and slab. I've attached a screenshot from a CA video (unrelated) that shows the end result I'm looking for, as well as my work showing a floating slab.

    • By MN_JohnH
      I am wondering how I can set up my foundation walls and main floor so that I can have a slab on one part and a framed floor on the other that flush out at the same level.
      But I would like the top of the ICF foundation walls to have a double sill plate with the concrete poured inside of that where the slab is and the other side would have accommodations for the framing to come out at the same height. I have a simple plan attached and some cross section details of how the top of foundation wall would look.
      I know I can just build it with the slab at the same level as the house and then just have these detail drawings included in the plan but I am wondering if there is a way to actually have Chief build it this way?


      plan.plan
    • By TrimbachDesign
      Hello Chief friends,
       
      I've been struggling and searching this topic but cannot seem to come up with the information that I am looking for. Basically, I have a sloping site and want to draw the uphill stemwall (top) 2'0" higher than the downhill wall. The floor system will ledger off the uphill wall, and the uphill stemwall top should finish about 2'0" higher than subfloor elevation. Wood wall framing will then be 2'0" shorter on this uphill side. Additionally, I'll be using some retaining walls extending off of the foundation to retain the up hill fill material. 
       
      I was able to "drag up" the top of the stem walls on the uphill wall to gain this two feet of elevation. However, there are all sorts of issues with wall intersections that I cannot seem to clean up. I think that what I am after is forcing the taller wall to run through, but I can't seem to force that in my foundation drawing or 3d view. There has to be a better way... Also, now the subfloor and some trim is sticking through the stem wall. 
       
      I'm running x12 on MacOSX. 
       
      Appreciate any help you all can provide! 
       
      Thanks,
      Austin

    • By lucratect
      In my plan I need most of the foundation to be a monolithic slab except in areas where there are pipes such as the bathroom.  I am trying to figure out how to cut a hole in a foundation that I have generated in CA with the foundation tool, but when I try the "slab hole with footing" and the "Hole in floor platform" tools I get errors. "could not create hole in floor platform" and "a slab hole must be placed inside a slab".  
       
      To elaborate on the foundation, it will be piers 4ft into the ground then a normal monolithic slab and footings will be poured on top of the piers.  The footings and slab will sit about 24" above the existing terrain where the piers tops also are.  The soil cap will be course gravel and basecourse on which the slab will be poured.  The bathrooms and other rooms with piping will be to the outside of the building and there will be a crawlspace below those areas to service pipes.
       
      I am hoping someone has experience with this and can help me.  I've scoured the forums and spent hours in CA to no avail!
    • By DomaniHomeDesign
      This is the first home my team and I have created in Chief Architect (converting from SoftPlan) and we're stuck still trying to figure out if there's a way to show a foundation only plan in the layout using the "walkout" basement, level 0. We end up with the full framed walls showing on top of the 4' stem foundation walls, no interior footings for the interior bearing walls, and a stone veneer on the framed wall above that isn't supported by the foundation wall.
       
      If anyone can offer information on how to get just a true, basic foundation plan in our plan set/layout that would be fantastic! The (nearly complete) floor plan is zipped up and provided below.
      Warnier, Adam & Diana.zip