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?

Thanks