In the UK a high majority of people have swapped out their old steel or wooden frames for UPVC
Rehau being the supplier of probably the best extrusions
Windows are purchased in a size to accomodate a window sill underneath the frame within the window space
the sill provides a stiff base- its siliconed to the window itself has run off and a drip edge included.
When I look in the US the bottom of the windows seem to be a simple square and you just add a square trim over it and caulk the edge
Pretty sure it rains a lot in some places in the US
Do you have an equivalent system for doors and windows or
do you design in the water damage from the start ? ;)
This is what I'm talking about
(http://store.eurocell.co.uk/img.ashx?i=/images/products/Original_01d714cgvii53.jpg)
http://store.eurocell.co.uk/window-finishing/window-cills/85mm-a-cill-64-6m
Our windows are sliders and have a built in drain channel and weep holes. I haven't looked at other styles for a long time. They have a mounting fin around the perimeter. On the inside ours have to have jamb work done to suit location. The entire RO is first flashed with something like Grace Vycor, the window installed and then the fin flashed. Then final exterior cosmetic surface/trim.