What type of foundation?

[dmlehmann]

I am in the planning phase for a building (18' x 20') in Raleigh NC. The building (1.5 stories) will be a shop (no lift) for woodworking and storage.

Originally, I was planning to use a monolithic slab foundation, but I convinced myself a footer/foundation wall and floating slab would be better. Acknowledging that I know nothing about foundations, I convinced myself that a stem wall would protect the lower part of the structure (moisture) and there would be less cracking concrete. I also like the look of the stem wall. Now, I am unable to locate the building were I had planned (leach field extends further than I realized). Changing the location will add cost (clearing trees...). So, I am revisiting the monolithic slab idea. What are the pros and cons of the different foundation types? Is there any reason not to use a monolithic slab? What would be the best design for a monolithic slab (slab thickness, reinforcement, depth on the downturned edge, psi...)?

Thanks


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[Cyberbear]

Based on your location, you may wish to run your ideas by the local building dept. and see what they say. All of my built garages are on the west coast where bigger continuous footings and thicker mono slabs are best.
I've always gone with a minimum 6 bag concrete mix, double steel rebar in the 4" slab and over size footings. I only use the code as a starting point and have never had any cracks. This is also no doubt helped by using a sprayed on concrete sealer when still green to retard drying time. Any building is only as good as it's foundation.

 

[Leaflessshadetree]

If you're thinking about putting a monolithic slab on top of the leach field I wouldn't do it.
Have you considered moving part of the leach field?

Around here the difference between a slab and a footing on a building that size is pretty much cost. Lots of garages are build on a slab with 1 to 3 rows of block for a stem wall.

 

[Gerald O]

I don't believe zoning will let you build over any portion of your leach lines, regardless of foundation type, and it wouldn't be a good idea anyway.

For my build (in Granville county) I had to cut 2 lines that passed under my building site and then make it up by adding another line further out on the property. The ends of the cut lines had to be a minimum of 5' from the foundation. Cost me about $2200 for the septic field changes. http://garagejournal.com/forum/showthread.php?t=200759

18 x 20 is a pretty small size, so it should be no problem to go with a monolithic slab based on building weight. Is your site close to level? Where a full foundation with stem wall really works better is when there is any significant slope. On a sloped site the monolithic will require some kind of masonry retaining wall anyway, so you might as well go with stem wall and floating slab in that case.

 

[dmlehmann]

Thank you for the replies and sorry for not responding sooner. I ruled out building near the leach field. That system was already repaired once (before I moved in) and I want to leave space just in case anything else needs to done to it.

I am now planning to build closer to my house - just on the other side. This area is reasonably flat (flatter than the original site), but has ~15 yellow pine trees Got a quote for $1k for the job. The foundation footprint will have include areas were stumps we're removed. I am not a contractor, but I believe the stump holes will have to filled and then compacted to be suitable for building.

Can monolithic slabs be built on disturbed ground? Also, is it a waste of money to increase the depth of the down turned edge of a monolithic slab from 12" to 16" (so it is below the frost line)? Any recommendations for a foundation contractor?

Finally, this will largely be a DIY project. So far, I am having limited luck finding framers/carpenters willing to set trusses. Any recommendations for contractors that might be willing to take smaller jobs?

Thanks

I


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[Stuart in MN]

Can monolithic slabs be built on disturbed ground? Also, is it a waste of money to increase the depth of the down turned edge of a monolithic slab from 12" to 16" (so it is below the frost line)? Any recommendations for a foundation contractor?

The frost line is 48" in Minneapolis (actually, this year I think it was closer to 72"!) and my garage is on a slab that's thickened to 12" on the edges. It works fine; I don't think there would be any advantage to going deeper in your case.

You can put one or two courses of concrete blocks around the perimeter, and build your walls on top of them - that will keep the walls up away from moisture. Mine has a single course of blocks.

The ground does need to be compacted well, I'm not a soils guy so I can't comment on what it will take to prepare disturbed earth.

 

[Gerald O]

Frostline for all of NC is 12". But how deep you go depends on the site prep. Going by code, the bottom of the footing must be no less than 12" below the undisturbed ground surface or engineered fill.

2012 NC Residential Code

You're going to need to know a lot more than what you can ask in this forum if you plan to build this yourself. Study the building codes thoroughly.

Have you checked with one of the local garage builder specialists?
Here's one I got a quote from www.carolinagaragebuilders.com
Didn't use them though because they only do monolithic and I needed a full foundation.

 

[theoldwizard1]

So you are planning on putting a monolithic slab OVER the top of a leach field ? You can't do that where I live !

 

[dmlehmann]

No. I will not build anything over or even near the leach field.


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[dmlehmann]

I know I have a lot to learn. And, I have read the code. So , I knew that there were requirements regarding building in areas with disturbed soil. That is why I commented about prepping the build suite. However, I know nothing about building a foundation and will absolutely hire a contractor for that component of the build. I am trying to learn so that I can ask appropriate questions and get a better idea of what I am truly in for.


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[dmlehmann]

I contacted Carolina garage builders a week ago. I asked for a quote for a complete build. No response to my message - so far, anyway.


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[Gerald O]

I contacted Carolina garage builders a week ago. I asked for a quote for a complete build. No response to my message - so far, anyway.


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Try calling them and asking again.

Here's another one I had out to do a quote:
http://www.hwsgarage.com/

Didn't go with them though because the GC was too cocky and didn't want to build my custom design. He'd probably do ok if all you want is a basic stock design. You can go see their display models.

 

[dmlehmann]

On my build site several trees will be removed. As I indicated earlier, I assume this action will complicate building the foundation. So what needs to be done to properly prepare the building site after tree stumps are removed? Will I need to hire an engineer to test the soil to determine if it has been properly prepped and to confirm that the footer is deep enough. Would the prep be different for a monolithic slab?


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[dmlehmann]

Thanks again for the reply. I talked with HWS Garages today. I agree with your assessment. He doesn't want custom builds, but he did point me to a guy receptive to completing specific, smaller jobs as needed.




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[captaindiode]

If you are doing woodworking, and do not plan to pull in vehicles, have you considered floor joists and a wood floor? It may not be a good idea for a workshop, but wood seems easier on the legs than concrete.

 

[dmlehmann]

I plan to store my riding mower in the building and thought concrete would hod up better. Also, as I mentioned earlier, I had to move the proposed site for the building. The new location is pretty close to the end of my driveway. So, now I am thinking that it might be smarter to put a garage door on it. Also, I was reviewing my covenants last night. They state that I can have a private garage for up to 3 cars and an accessory building of up to 250 square feet. I have an attached 2 car garage and no sheds. Several of my neighbors have an attached 2 car garage and a detached garage. Based on this, it seems like it might actually need to be a garage in order to be legal. Plus, this way I still get to build a shed for fun.


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[Notgrownup]

My footers needed to be 12x12 on a monolithic slab and were ok for 2 story... If it is for 2 story one of the quotes I got said he would use rebar....mine is a24x24 single story, no rebar, just fiber in the concrete ...but that is Greene county...
I too was worried about the slab vs footers and back filled floor but was also told for light duty like I was using and barely no freeze here that I was overthinking it also....

 

[dmlehmann]

So long as the footer dimensions met code, the county building permits office told me I didn't need anything special for the foundation. However, I wasn't expecting to remove trees and suspect that engineered fill will be required. With that in mind, I am wondering if it would be smarter to get an engineer to design my foundation and fill requirements.


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