You can, but it needs to be treated wood. Preferably ground contact it has a deeper depth of treatment. Plus you won't get the life out of it as you would if it was above ground... it would probably outlast us though?

Anyway, joist it as you normally would, say if you have a 7" step into the porch. You need to scribe the joist to the concrete, then paint the rips with copper naphthenate.

Solid Block it at 7' to 8' just as a normal deck too.. basically typical construction except it's on the concrete just like a house plate. Vent it as well as well as you possibly can, and try to frame it parallel to the water flow.

There are other options as well, this is the way I do mine.

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