Hi All, Curious if its possible to install an exterior door directly to concrete without using a PT buck. My walls are 12" thick ICF, with drywall on the inside and stucco exterior. My doors are fibreglass with a composite frame. Manitoba, Canada, so weather fluctuates from -35C to +35C over the year.

I would make sure there isn't any inward slope and apply lots of silicon....

Can you elaborate? It appears you are worried about water infiltration (fair point; though my builders poorly installed doors that sit on treated 3/4” plywood already leak under the sill as he re/installed them 3x without ever removing the old silicone…so this will be a replacement).

The new doors will still have an ADA sill but an upgraded one that includes a back dam under the sill and a specialty lip designed to better resist driving radons than a standard ADA sill.