Concrete Slab for Shed Help

[goofiefoot]

I currently have a 10'x12' barn-style shed in my backyard that needs to be replaced. It's built on skids, and the previous owners had it installed many years ago on a 10'x10' concrete foundation, so the rear 2' hangs off the end, and is supported by blocks. Functionally, it's fine, but it's old and leaky and I'm wanting to build something.

I want to build a new 10'x12' shed, but I'd like the 12' wall as the front, not the side (hope that makes sense). I'd love to build it on the slab, with the concrete as the floor, but I need 2' more concrete on one side.

The current slab looks to be level and in really decent shape. If I wanted to add 2' more of concrete, and have a solid, 10'x12' floor to build my shed, how would that work? How would the existing slab be modified to accommodate the extra material, and how much height would need to be added to ensure a solid floor across it all? I'd probably hire it out, so what should I expect from a contractor on this project?

Thanks for any insight and advice you might have!

 

[Badasssapper67]

I believe you need to drill holes into the end of the slab horizontally and install rebar and epoxy it in. That rebar would be used to hold the new slab in place.
If it were me, before I did all that I would rent a cement saw and cut the end of the old slab off to make sure it's straight and also to have a clean end. But that's just me I cant say it would make a difference for sure.
Also use some wire mesh to hold it together. Make sure the ground under the new slab isn't going to sink under the weight of the slab.
Im not a pro at concrete but since no one has posted yet thought I'd give you a start.

 

[goofiefoot]

Drilling the old slab for rebar makes sense.

Since I want the floor to be seamless, all 120 sqft would need a new top layer of concrete. Would the existing slab top need to be scored, drilled, or broken up to ensure good adhesion of new concrete, or just an extra layer of rebar, forms, and pour over? How thick would an overlay ideally need to be to maintain integrity in the long run?

 

[Cyberbear]

To answer your questions, it helps to know how much weight will be placed on the newly remodeled slab? Ordinarily, I'd opt for an additional minimum of 2" on top of the old slab if the existing one is clean, or recently power washed. Placing some wire mesh over the old slab before the new concrete isn't a bad idea, couldn't hurt. Dowling is also a good idea.Also, if the existing does not have some type of false footing around the perimeter, you may want to consider that as well. I've seen too many such modifications fail due to not enough planning.

 

[KDXSR5]

Pouring a new 10x12 slab 4 inch thick is approx. 1.5 cu yd. What about just breaking up the current slab and pouring a new one? That will avoid the potential for an overlay failure.

A 10x10 at 2 inch overlay plus a 10x2 4 inch pour is almost 1 yard and will require the same amount of finishing as a new 10x12 slab.

If you can break up and haul the existing slab yourself, it may make more sense to just pour a new one. If you want to do it cheap, forget the overlay, and just add on a 10x2 section.

 

[Falcon67]

A shed won't put very much weight on that - I'd just pin and pour. If you want it up from the ground a bit more, then form it all in, rough the surface, put on some boding agent and pour 2" over the top of everything.

 

[goofiefoot]

Thanks! This is helpful. Basically, if I use the existing slab, plan for a 2-4" rise (which would put the floor almost to level it is now on the skids. If I tear out and build a new slab, I can make it whatever height I want.

In the end, I'm trying to get as much height from the final shed itself as the city will allow (I'll double-check, but it believe it's 12-15 ft), so knowing the base height is important.

 

[K'ledgeBldr]

Since more than likely you'd be dealing with Vertisol (Black Gumbo) soils- and not knowing the actual physical aspects of the existing slab, I'd tear it out and start fresh with a slab that will withstand the expansive soils.

Nothing like having a "2pc" slab bending and swaying everytime it rains- or doesn't rain for an extended amount of time.

 

[goofiefoot]

Since more than likely you'd be dealing with Vertisol (Black Gumbo) soils- and not knowing the actual physical aspects of the existing slab, I'd tear it out and start fresh with a slab that will withstand the expansive soils.

Nothing like having a "2pc" slab bending and swaying everytime it rains- or doesn't rain for an extended amount of time.

There's not a ton of soil, period. We have a few inches of dirt, then rock. I'll be bringing in a pro to take a look, but our area hasn't had much issue with movement and expansion.

Certainly something to keep in mind, though.

 

[bb_max]

Just tear out the old and pour new.
Your existing slab is 10x10x(guess) 4" thick. That's just over 1yd of concrete that you would save. Not worth $100 of savings for the trouble involved.

Tear out all the old and pour a new 10'x12'x4" slab. That's 1.5yds.
Around here you get an added charge for a short load of concrete that is less than 3yds.

So order 3.25, and pour yourself a nice approach, fix sidewalks, etc.

 

[Falcon67]

We've got expansive here, maybe not quite like Travis County LOL. What you said - I'd expect to hit rock pretty quick going into the ground there. It's a lot like my old grandparents place near Lake Whitney. Septic tank holes there might require the help of big firecrakers.

You could just dig a 12" x 12" footer as part of the add on and that should keep it in place.

 

[kbs2244]

Taxes may come into play here.
Often concrete floor means "permeant" and will be taxed as such.

A phone call may avoid future problems.

 

[goofiefoot]

Just tear out the old and pour new.
Your existing slab is 10x10x(guess) 4" thick. That's just over 1yd of concrete that you would save. Not worth $100 of savings for the trouble involved.

Tear out all the old and pour a new 10'x12'x4" slab. That's 1.5yds.
Around here you get an added charge for a short load of concrete that is less than 3yds.

So order 3.25, and pour yourself a nice approach, fix sidewalks, etc.

The current slab is about 12" thick. It's been there a while, so it's not all exposed, but the front is easily 10" to ground.

Taxes may come into play here.
Often concrete floor means "permeant" and will be taxed as such.

A phone call may avoid future problems.

Wow, very good point. I will need to look into this. My understanding is that if the structure is 120 sqft or less, it's outside taxation, or even codes, as long as it's within the lot easements.

 

[bb_max]

The current slab is about 12" thick. It's been there a while, so it's not all exposed, but the front is easily 10" to ground.
QUOTE]

I would seriously doubt its 12" thick all the way through. Probably a 12" fitting/thickened edge to keep rodents from burrowing (at least that's what we do here).

Maybe it is that thick. Still seems like it would be a lot cleaner to remove and put what you want.

Another thought, do you have room around this one (property wise)? If so, use the old slab as your approach in front of the door, and put the new shed adjacent to it on a new slab.

 

[Bib Overalls]

The current slab is about 12" thick. It's been there a while, so it's not all exposed, but the front is easily 10" to ground.

Dig out both sides for a 1'x1'x10'. Epoxie 1/4" rebar pins about 1' apart on both sides. Form up. Use 4 3/8" rebars the long way in both footings. Place concrete and finish. Should take 2 1/4 yards. Put sill bolts (4 on each side should do it) while the concrete is still plastic. Let it sit overnight and in the morning get out your carpentry tools.

 

[goofiefoot]

Dig out both sides for a 1'x1'x10'. Epoxie 1/4" rebar pins about 1' apart on both sides. Form up. Use 4 3/8" rebars the long way in both footings. Place concrete and finish. Should take 2 1/4 yards. Put sill bolts (4 on each side should do it) while the concrete is still plastic. Let it sit overnight and in the morning get out your carpentry tools.

I like that idea, but I'm right up against the easement on one side, so I'd have to put the 2' on the other side.

 

[tthornto]

I wouldn't worry about the floor being seamless, just form up and pour 2 more feet to the same level as the old slab. Build the shed then install cabinets, workbench or shelving over the new portion of the slab to hide the seam.

 

[James-W]

I agree with the "tear it out and pour a new slab" crowd. We aren't talking about a fortune in concrete here, and by doing it this way he can have exactly what he wants instead of trying to cobble something together that may, or may not, work out well for him.