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I would like to add 16'x5' to my existing 16'x11' slab.
Should I split the new slab in half 8ftx5ft with expansion joint between and do I also need expansion joint on the 16' side??
One expansion board on the 16' side should be OK.
It will depend on the pattern of your existing deck and the stability of your ground. You will have a natural expansion joint at the separation between the old and new slab, so no worries there. I would recommend splitting the new slab at 8', though I'd do it with a seaming tool rather than a piece of wood or other expansion joint material.