I have a rotted door sill (among other things), a consequence of contact
a concrete stoop. I've taken care of the stoop issues, and I need to
replace stuff. The wood (oak) sills I've seen aren't exactly what I'm
looking for, and I'm thinking whether to make my own. My question is,
these undersides of these sills are grooved (lengthwise). Does anyone
know why that is?

The bottom of the sill is grooved to reduce warping.

In recent years I have had to repair several doors with rotted sills. I
chose to replace the entire door frame with a frame made of composite
materials. Then I installed a jamsill pan liner before installing the new
door frame.

I measured my door carefully (to the nearest 1/32"), door size and
thickness, hinge sizes and locations measured from the top of the door,
latch and deadbolt positions, etc. Then I took my measurements to my local
Lowes store and ordered the composite door frame. They're not cheap, about
$400, but unlike the wood frames they won't rot. The jamsill pan liner also
ensures that any moisture that makes it's way past the door frame gets
directed outside the building instead of rotting the subfloor.

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