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.
GreatForge t1_je3jjx5 wrote
Reply to Do I need an expansion joint here? by blackhatrob
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.