I just had homedepot ( whoever they contract out to) to install an exterior door...I think they scewed up the sill size....The jam in my door is 5 1/4inch...the sill is 13 7/16 (or something close)...ANyways, the sill sticks out about an inch and a half past the brick mold and is unsupported underneath..When you step on that portion of the sill, you can feel the back lift up.....I assume this is not normal but I haven't really paid attention to door sills before....I would think the sill should be relatively flush with the brick mold....Am I crazy????
Can it be supported with a 2x trimboard under the sill?? That would be correct. Sounds like that door may have been a special order for going to a deck with no stepdown. Nothing should be flapping in the breeze.
They installed a 6.5" sill extension instead of a 5.25" extension. It will look bad if you support it and will create a hazard if you do not support it. Have the HD correct it, they can order the correct extension.