As a general rule, when in doubt I always will. You’re pouring new concrete to older, more cured concrete that is considerably harder, so they will expand and contract at different rates which will very likely cause a crack to open between them. If you’re tech-minded, here’s what the American Concrete Institure has to say: https://www.concrete.org/Portals/0/Files/PDF/302.1R-15_Chapter5.pdf

This is great information, thank you!

Just to be technical, what you are asking about is an isolation joint, not an expansion joint. It never hurts to add one in this scenario, the main reason being differential settlement of the two slabs causing cracking if they can’t slide past each other. You can use impregnated asphalt isolation joint or a couple layers of 30 lb felt paper. Make sure you compact the soil and put a base of gravel under the slab prior to pouring. This helps prevent settlement cracking.

I don’t know if this is applicable to your case, but I would recommend looking up the local building codes. Expansion joints usually have to be done when a certain amount of feet of concrete has been poured, so you might need one even for such a small slab.

Also, make sure you’re not pouring directly against the foundation. If it’s poured too close, it can cause a lot of problems. You should create a gap between the slab and the foundation, then fill it with some type of filler material.

Thanks for this

Was planning to contact the soil, add 2-3” of gravel, then compact again before pouring. I’m thinking the felt paper may be the best material for me as the hose slab has some decent chunks of concrete that came through the underside of their forms that I’ll be pouring over. The paper would let me wrap around that as opposed to a fiber board.

It’s not large enough to be necessary by code here, and I’ve not seen professional companies use anything when I’ve had larger patios poured in the past either so I was scratching my head.