This shows me chopping the pocket for the door jam, about 1" deep. There will be no movement ! Sills are made of white oak because it does allow moisture seep up its grain. I took Bruce Hoadley's class and have the book, over three decades ago. It is 12" wide and 2" thick, and I 'T-head' nailed it into the sill. With proper care it should last two - three centuries. Storm doors are very helpful, at least for the winter.