You can calculate how much overhang you need by the following a simple formula: Y=1/2X (in most cases). For example, if the measurement from the base of your door to the bottom of your overhang is ten feet, the overhang should be a minimum of five feet deep.Other Variables
The Exposure coefficient may need to be modified based on the climate and the direction your door faces. In some situations, the exposure can be too extreme for a door - regardless the overhang.
Without the correct overhangs, doors with a southern, southwestern, southeastern western exposure will require frequent maintenance. They're also susceptible to performance problems, such as rapid finish deterioration, color fading, wood splitting, warping, molding shrinkage, wood joint separation and water penetration between the moldings, panels, and glass. Improperly protected doors with a high gloss and dark stain or paint colors are also susceptible to large temperature differences (e.g., the door's surface is 30 degrees higher than the ambient temperature) and temperature jumps (e.g., hot in the day, and cold at night).
Doors used in combination with storm door can cause warping, splitting or cracking due to buildup between the two doors.