Is rebar overkill for a 20x5 storage shed slab in Toronto?

Rebar is total overkill for a slab this thin and for those specific loads. With a 3.5-inch pour, you barely have enugh room to get the proper concrete coverage over 10mm rebar anyway. If the rebar sits too low, it does nothing; if it's too high, it'll cause the surface to spall or pop during those Toronto winters. Stick with the **6x6 wire mesh**. It's much easier to pull up into the center of the wet concrete while you're pouring, wich is where it acutally needs to live to provide any benefit. Since your base is already compacted gravel from the driveway project, you’ve already done the hardest part. Concrete doesn't relly get its strength from the metal inside; the metal is just there to hold the pieces together once it inevitably cracks. And it will crack, especially in Ontario. The real trick for a 20-foot long run is to put in **control joints**. I'd cut a joint every 5 or 6 feet. That way, when the ground heaves in February, the slab cracks exactly where you want it to instead of snaking across the middle of the floor. Save the $150. For a snowblower and some tires, that mesh is more than enough reinforcement. Just make sure you aren't laying the mesh flat on the gravel and walking all over it. Use some small chairs or even just chunks of brick to keep it suspended in the middle of the pour. If it ends up at the bottom of the slab, it’s basially decorative reinforcement at that point. One thing to think about is that 5-foot width. By the time you get some shelving or tool racks on the walls, a 5-foot wide shed gets real tight, real fast. If you haven't framed it yet, you might want to consider going a bit wider if you have the space, maybe an 8x12 or 6x16. But for the concrete itself? Mesh is the way to go.

That part about the 5ft width being tight is making me second guess my layout before I pour. I'm also wondering about those fiber additives you can get mixed right into the concrete. A guy at the hardware store said it can replace the mesh entirely for small slabs like this. Since I'm worried about the Toronto frost, whould the fiber be enough on its own or shoud I still definitely do the wire mesh? Trying to make this as easy as posible for a first-timer.

Fiber is a decent additive for surface strength, but I wouldn’t trust it to replace mesh entirely for a 20-foot stretch in our climate. It helps with tiny spiderweb cracks, but it won’t stop the slab from pulling apart when the frost heaves.

If you realy want peace of mind, throw the fiber in as a 'belt and suspenders' approach, but don't skip the mesh. Also, make sure your gravel base extends at least 6 inches past the slab edge—it helps keep the edges from undermining when the snow melts and things get sloppy in the spring.

Don't forget about the slope. If you're parking a snowblower in there, all that slush is gonna melt and pool right in the middle if you pour it perfectly level. I'd give it at least a 1-inch drop toward the door so the mess drains out.

Also, since you're in Toronto, the salt from the driveway is gonna hitch a ride on those tires and eat the concrete over time. defintely hit it with a good siloxane sealer once it cures. It’ll stop the surface from pitting when that salty slush sits on it all winter.

Since you're storing tires and tools, don't skip a vapor barrier. Throw a 6-mil poly sheet down over that gravel before you pour. In Toronto, the ground stays damp forever, and without plastic, moisture wicks right up trough the concrete. You'll end up with rusty tool chests and moldy tire sidewalls in a few seasons. It also helps the slab cure better since the gravel won't suck the moisture out of the mix while it’s setting. It’s a cheap addition that makes a huge difference for a storage space where you want things to stay dry.