How to Flash Around Doors & Windows
Modern doors and windows come as assembled units from the factory and are ready to be dropped into their pre-sized openings. The joint between the siding and the door of window jamb is a weak point that can allow water to seep behind the siding and cause damage to the building's sheathing and to the jamb itself if it is a wooden jamb. The vapor barrier alone is not enough to shed and channel water away from the underlying wood structure to prevent this damage, and so flashing is used to cover this weak point. Flashing can be of the rubber sticky-back type or the old fashioned sheet metal type. Does this Spark an idea?
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Instructions
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Determine the length of the flashing necessary to cover the vulnerable areas. Measure wooden or plain jambs from the edge of the hole. Measure aluminum or vinyl jambs from the edge of the jamb itself and not the edge of the nailer lip that laps onto the sheathing. The lip will be covered by the flashing. Measure the width of the flashing material and double the number, then add the new number to your outer measurements. This is the actual flashing lengths. Cut two pieces of flashing for the horizontal runs and two pieces for the verticals with the snips for metal or a razor knife for rubber flashing. Disregard the bottom piece for flashing a door opening.
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Place the bottom flashing in position and slide it up tight to the bottom of the sill pan or the jamb itself. Lap the flashing over the nailer lip and balance the overhang on either side. Lift one side of the flashing away from the building while holding the other side in position, and remove the backing paper from the flashing if using stickyback. Ensure that the flashing laps over both the lip and the vapor barrier, and press it firmly into place. Lift the other side of the flashing and repeat the process. Metal non-adhesive type flashing may be nailed as close to the edge as practicable to hold it in place.
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Lay the vertical pieces into place. Dress them flush at the bottom with the bottom edge of the lower flashing and butting them tightly to the edge of the jamb, overlapping the bottom flashing and the lip shingle-style so the sides will shed water onto the bottom. Fasten with the adhesive backing or roofing nails near the edge. Place the header (top) piece into place butted against the upper jamb and overlapping the lip and the vertical pieces. This will create a water-shed from top to bottom, and prevent water from seeping under the nailer lip. The water will drip down the flashing, onto the vapor barrier below the opening, and then outside the wall below the bottom course of siding.
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Tips & Warnings
Do not nail the flashing anywhere but near the outer edge. Nails placed in the field of the flashing will make weak points that will eventually leak. The flashing should be wide enough (8 inches minimum) so that any fastener holes will be well covered by the siding material. Be careful when handling sheet-metal flashing. The corners on factory edges can be quite sharp, and any cuts you make with the snips will likely be sharp enough to injure as well.
