Door Threshold Diy . How to replace a door threshold in 9 steps. How to replace an exterior door diy exterior door installation preparing custom exterior door threshold you large gap between new door frame and. *PICS* Can This Door Threshold Be Repaired? Windows and Doors DIY from www.diychatroom.com As for the threshold, it is installed on top of the sill and. Then run a bead of caulk along this line and under the sides of the door frame. Thresholds are effective at helping to close the gap between the bottom of the door.