What are they and why are they so important? A drip cap is installed in several areas of the exterior but mainly over windows and doors. A drip cap forces water to the exterior of the home allowing the water to run down the window/door and not get behind it. When water gets behind the window/door it will often damage the sheathing behind the siding and can run into wall cavities causing larger issues. Good drip cap installation involves bending the corners down the horizontal edges of the window. This keeps water from wicking back under the cap at the edge. Some windows may appear to not have this cap installed but the windows themselves (most often vinyl windows) may have a drip cap built into them. Running a bead of caulk above these windows can help prolong the life of the siding over the window or door. If water sits on top of the window it can keep the lower edge of the siding wet which can cause issues over time with deterioration.