Am I missing anything vital before the concrete truck shows up Friday morning?

If the ground is soft from the rain, those 2x4 forms will definitely bow or kick out once the concrete hits them. Double up on the stakes, expecially near the corners and mid-spans. You shoud also drive some kickers—diagonal stakes—off the main stakes and into firmer ground a couple feet back. It’s alot easier to add a few extra pieces of wood now than to try and sledgehammer a bulging form back into place while the mud is setting up. That 12x20 area holds a lot of weight and pressure.

For the expansion joint against the house, don't just let it float. It will move or rise as soon as you start screeding. Use a few small masonry nails or even just some dabs of construction adhesive to tack that foam to the brick. You want the top of it about a quarter-inch below the finished surface of the slab so you can caulk it later with some self-leveling sealant. If it's loose, it will just get buried or pushed sideways by the chute. The chair sinking issue is a constant battle on crushed stone. If you're using rebar, you can zip-tie the bars to the chairs to keep them from tipping, but if the gravel is loose, they’ll still sink into the subbase. Some guys use small squares of scrap plywood or plastic sand plates under the chairs to spread the load. If you're using that heavy wire mesh—the stuff that looks like **chainmail**—it’s even harder to keep centered. I ussually just hook it with a rake and pull it up into the middle of the pour as we go, but that is a messy way to work.

Also, make sure you've got a solid **vapor barrier** or at least a heavy-duty fabric under that rock if you're worried about moisture wicking up into the slab later. It helps with the cure by preventing the ground from sucking the water out too fast. When the truck arrives, don't let the driver add too much water just to make it flow better. It weakens the final product. Have a couple of extra hands ready with shovels because 12x20 is a decent amount of work for a small crew once the concrete starts to stiffen.

That adhesive trick for the foam makes way more sense than what I was planning. I'm still sweating the screeding part though. For a 12x20 area, is a 12-foot board even manageable with just two people? It seems like alot of weight to pull all at once. shold I put a temporary board down the middle to break it into two 6x20 strips instead, or will that leave a wierd seam in the finished patio?

A 12-foot screed is a monster for two guys. You’ll be fighting the weight the whole time. Instead of a permanent bulkhead that leaves a seam, set up a temporary screed pipe or a piece of conduit down the center. Prop it up on some dobies or concrete mounds so it's level, screed your two 6-foot sections, then just pull the pipe out and fill the groove as you work backward. definitly rent a bull float too. You can't reach the middle of a 12x20 with hand tools without crawling on the wet mix. That long handle is a lifesaver for getting the cream to the top before you start edging.

Finish that thougt—renting a bull float is non-negotiable for a 12x20. You won't get it smooth enough with just a hand float. Also, order an extra 10% on your yardage. Being half a yard short at the end of the pour is a total disaster and a massive headache to fix. Keep a hose ready to wash your tools the second they leave the mud; if that stuff dries on your screed, you're in for a bad time. Oh, and keep some cold Gatorade for the driver—it goes a long way if you need him to hang out an extra few minutes.

One more thing to watch is your edging tool timing. You shold run it around the perimeter as soon as that bleed water starts to vanish. If you wait untill the slab is firm, you'll be fighting rocks near the forms and end up with a ragged edge that chips off later. Also, keep an eye on the wind. If it's a breezy day, the top can 'crust' while the bottom is still soup, wich causes surface checking. Having some plastic sheeting ready to throw over it can save the finish if the evaporation gets ahead of you.