Dave,

But I'm wondering whether it might be cheaper and easier just to put
in a couple inches of qwikrete, smooth it to a close fit to the bottom
of the door with a slope that makes water coming down the door to
drain away from the garage.

I suppose it might work if you can figure out a way to form it up and
finish it smooth for the weatherstripping to contact.

I've never done much with concrete, and I'm wondering if I'm not
seeing some kind of problem inherent in making a concrete garage sill.

The concrete may crack when it dries if it's less than a couple inches
thick.

You may get water leaking on either side of the concrete sill. If you have
wood framing on either side, that could lead to rot. That's basically the
same situation as the metal sills up against the wood door frames.

I would think rough concrete would tend to wear out the weather stripping
faster.

A bag of concrete is cheap enough if you want to give it a try. Worst case
you have to tear it out and use another approach.

Good luck,

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com