My house (Florida) is built on a slab. I'm looking to add a 9' long by 3' wide slab on the side of the garage where a door exits to the side yard. To be clear, this would be touching the slab of the house and extend 3' into the yard along 9 feet of the existing slab. I was looking to pour concrete right up to the foundation since this slab is rather small, but I'm wondering if placing an expansion joint against the foundation is warranted or necessary.

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Spinaccio t1_je29db5 wrote on March 28, 2023 at 9:52 PM

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

blackhatrob OP t1_je2de0f wrote on March 28, 2023 at 10:19 PM

This is great information, thank you!

GreatForge t1_je3jjx5 wrote on March 29, 2023 at 3:41 AM

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.

blackhatrob OP t1_je4dl1n wrote on March 29, 2023 at 10:00 AM

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.

Hopeful--Spring t1_je3rr9u wrote on March 29, 2023 at 5:03 AM

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.

blackhatrob OP t1_je4drkl wrote on March 29, 2023 at 10:03 AM

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.