By foundation, do you mean just a concrete slab? I would try to tie the two together by drilling holes horizontally into the existing slab and inserting some rebar into the holes. How much effort to spend depends on the soils below and what you're going to do with the shed. If the soil is pretty solid and you're just going to store some yard tools, you might just butt them together and call it a day.

Yep, it's a concrete slab. The existing seems to be ~4inches, and I was hoping to have about the same thickness for the new one.

I'm hoping to use the shed as a bit of a workshop/storage space for some light duty woodworking. The soil is potentially questionable, though, as I'm in a place with some marine clay. That said, I put a retaining wall behind the shed it that seems to be holding >3 years later, so the soil isn't moving a ton. But to be on the safe side I figured I'd dig out a gravel bed for the new slab. It sounds like I'll also try to tie them together with a couple of pieces of rebar too. I'm also open to any other advice.

Thanks you Seabornman

Hi watertoiceXI,

If you live in a town that has a government you may need a building permit. Sometimes the town government has an engineer or equivalent who can be helpful with your plans. I received some good help from the city engineer on a retaining wall project that I did. He changed my plan to change 1/4-inch rebar to 1/2-inch rebar and had me use more rebar than I included in my original pencil sketch. Thanks to their advice my retaining wall was much better and more stable. And I had a building permit for it. Their charge was quite reasonable. They did a couple of "inspections" during the project. The wall was near the property line, but all on our lot by a foot. So I didn't have any trouble with our neighbors. It's a good idea to have your town government on your side.

ZM

If you don't want the two slabs to ever separate leaving a large crack, do as SeaB said, but glue the rebar into the old slab with an adhesive.

https://www.menards.com/main/building-materials/concrete-cement-masonry/bagged-concrete-cement-mortar/sakrete-reg-high-strength-anchoring-epoxy/121219/p-1561444091005-c-5648.htm?tid=3490101238887901515&ipos=8

zen_man: I would have been happy to have a permit and go through that process to get feedback from the engineer, but apparently the shed is small enough that I don't need a permit.

formulaross: Thanks, I'll see if I can find that epoxy. That and rebar sounds like what I'll be going with.

Thank you all for your help.

I would ensure you have adequate drainage 1st , then dig out the clay and spread fresh gravel that is properly tamped . The rebar and adhesive is a must , good luck !

In my area increasing the size of the concrete slab would require a permit because the concrete doesn't allow water to permeate into the ground and they are very concerned the ratio of permeable to impermeable ara. However, other cities have different requirements.

If you want the slabs to stay aligned, drilling and epoxying the rebar for the new slab into the side of the existing slab is a common solution. You can still leave space for expansion joints, but they may not be needed.

Bruce

Joining two Slabs that will remain tight in our Dallas-Fort Worth Area is A True Challenge. We had an Addition to our Home and I have poured lots on Driveway, Sidewalks and Trim with "Plenty Rebar Steel Includes. Over the Years most of the Lighter Work has held w/o Rebar supported Joints Changing.

The Small Addition as Den/Patio and Storage has Moved and Moves. From my experience from our situation. Our Den/Patio and Storage Needed More Beams and Piers Supporting The Slabs.

The Soil and Bedrock and Etc, Can Add To Your Challenges...