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size matters?
- Thread starter nonbowler
- Start date
The perimeter footing concrete is usually much thicker. If your talking the floor only then yes.
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Fyrme
Well-known member
If you are talking about just forming up a 5" thick slab in the yard then building a stick framed building on top of it, no. You will end up with all kinds of problems with settling, heaving and cracking.
big.jim
Well-known member
does it have rebar if so i would say yes
charlestmann
Well-known member
It may not have cracks now but if it does not have a footing around the perimeter the added weight of the structure will result in cracking and vertical displacement. You would be point loading the outside 3 1/2 to 5 1/2 inches of the slab.
Fyrme
Well-known member
No.
You need a footing to set a wall on. The entire weight of the building is concentrated on a 3 1/2" 2x4 around the perimeter. That foot print needs subterranean support.
Not exactly code worthy but you could dig under the slab edges down to the frost line and up under the edge about 8+ inches. Then drill a 1/2" hole in the slab edge every 16" and run a J bolt up through the slab from the bottom and tie that to horizontal rebar in the footing, then fill it with concrete. The main problem here though would be if moisture got between the slab and the footing and froze.
Now, Here is a better idea. Saw cut a 10" three sided square on the edge of the slab every 6-8' and then post hole dig down 3' or past the frost line. Set some 5"x5" treated post in the holes and essentially you're building a reverse pole barn.
If I was hell bent on saving that slab, I'd go with the second suggestion. But honestly Unless it's a huge slab, you be saving a ton of time ripping it out and starting over.
You need a footing to set a wall on. The entire weight of the building is concentrated on a 3 1/2" 2x4 around the perimeter. That foot print needs subterranean support.
Not exactly code worthy but you could dig under the slab edges down to the frost line and up under the edge about 8+ inches. Then drill a 1/2" hole in the slab edge every 16" and run a J bolt up through the slab from the bottom and tie that to horizontal rebar in the footing, then fill it with concrete. The main problem here though would be if moisture got between the slab and the footing and froze.
Now, Here is a better idea. Saw cut a 10" three sided square on the edge of the slab every 6-8' and then post hole dig down 3' or past the frost line. Set some 5"x5" treated post in the holes and essentially you're building a reverse pole barn.
If I was hell bent on saving that slab, I'd go with the second suggestion. But honestly Unless it's a huge slab, you be saving a ton of time ripping it out and starting over.
rslaback
Well-known member
Change your garage to 24 x 22 and dig your footing around the outside of your existing slab. Pour the foundation filling in to touch the existing slab.
bad_idea
Well-known member
rslaback beat me to it. I assume you are building this shop yourself? I think adding a footing around the perimeter of the shed is something you could handle yourself.
Fyrme
Well-known member
Here's another idea. Dig a 8" trench all the way around the slab down to frost line. then form up 2x8's on the outside and form 2x4's on top of the slab and pour a short stem wall to set your walls on. I'd drill some holes horizontally into the slab and epoxy in some 1/2" rebar into them sticking out into the stem wall to help tie the two together. That way the gap doesn't grow between the stem wall and slab.
Fyrme
Well-known member
So y'all r saying the ground below the frost line is stronger than the surface? I always thought footings were 2 prevent heaving during freezing times.
Yes and yes. You are creating a larger foot print below grade on a more compacted surface than what is on the top soil.
It sounds however, you are trying to convince yourself to build on it, even after asking advice and not listening to it.
A car parked on the slab in the middle means nothing. What do you think you are doing in a garage? Take that same car and set it right on the edge of that slab with two spare tire donuts on that side. Now leave it there for, oh say, 5 years or maybe even longer. Your slab WILL eventually crack at the point of contact. I've given you three options as well as others have. Take the advice or don't. But if you want it to be there without problems in 10 years, Do it right the first time.
rslaback
Well-known member
So y'all r saying the ground below the frost line is stronger than the surface? I always thought footings were 2 prevent heaving during freezing times.
The ground below the frost line is more dense as the water never freezes and it therefore doesn't expand (when it freezes) and settle. However, the real purpose of building below the frost line is so the ground can freeze around the structure but not under it. When that happens the building theoretically doesn't move during a freeze period. An uneven heave (one part heaves more than another) caused by not building below the frost line is what causes buildings to rack and eventually sag and fall over.
Some structures are sometimes built on what is called a floating slab. However, those tend to be small structures like a corn crib or woodshed. The small size minimizes the chance or an uneven heave.
ddawg16
Well-known member
Change your garage to 24 x 22 and dig your footing around the outside of your existing slab. Pour the foundation filling in to touch the existing slab.
That is the route I would go.
Your basic problem is there is a lot more weight on those wall than you think. When you put them at the edge of the slab....chances are the slab will crack.
If you follow rs's advice....prior to the pour, I would drill holes in the edge of the existing slab and install dowel rods. This will lock the slab to the footing.
Additionally, I would build the footing with a stem wall on top...say 8". This way you get your bottom plate well above grade and drastically reduce potential wood rot problems.
Click on the Garage Build link in my signature for pics of my slab.
7th Kahuna
Well-known member
As the owner of a two story building, built largely on a foundationless 3" concrete slab, let me first of all say that in 85 years, and in earthquake county no less, it has held up remarkably well, thanks in no small part to the fact that it was built over several feet of water placed sand and gravel (read 'stream bed'). Having said that, I would NEVER suggest anyone try to recreate it. In fact if there was an easy way to add a foundation, I would. Nearly every project I take on around that property is impacted by the lack of proper support. As others have suggested, add a foundation around the perimeter of the existing slab, or build a post and beam type of structure. Whatever you do, carry the weight of the new structure properly and you will save yourself a lot of potential headaches down the road.
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