How to Add Flashing to a Roof

Dormer which requires flashing when roofed.

Roof flashing is made from metal, rubber or other materials that are impervious to the effects of water. The flashing is installed everywhere that two different planes intersect, such as at the ridge and valleys of the roof and where the siding intersects the roofing in multiple-story buildings. When installed correctly, the flashing will catch water that tries to leak through these weak points and shed it onto the top of the roofing material, where it can harmlessly drain away. Age and seasonal movement of the roofing materials can eventually break seams in the flashing and open up nail holes that will cause water to leak into the roof. Periodic inspection and maintenance is required to preserve the waterproof integrity of the roof.

Things You'll Need

• Hammer
• Flat bar
• Flashing material (aluminum sheeting)
• Polyurethane caulk
• Caulk gun
• Tin snips
• Copper nails
• Roofing nails
• Siding nails

Instructions

1. Expose the areas to be flashed. Carefully remove any shingles or siding from the area with a hammer or flat bar. Try not to damage any removed materials unless you have new material on hand with which to replace it.

2. Cut lengths of flashing material to cover the desired area. Start laying the flashing down at the downhill side of the run. Overlap any joints in the flashing by at least three inches and apply caulking between the flashing pieces to protect against water being blown uphill and under the lap. Lap the joints in a watershed position, or lap each successive piece going uphill on top of the piece below it so that water cannot get into the joint without running uphill.

3. Form the flashing material to lay flat against the structure and conform it to any bends or angles in the structure. Lap the flashing under the vapor barrier (plastic or felt) on the upper edge and lap it on top of the vapor barrier on the bottom edge. This will form a watershed that will prevent water from entering at these points.

4. Nail the flashing in place with the copper nails. Nail only at the upper edge of the flashing in horizontal applications and only at the outer edges in an angled application. This will help to minimize the chance of a nail hole leaking water through the flashing.

5. Replace the removed shingles or siding. Lap over the flashing with the shingles or siding at the top of the flashing. Allow the flashing to lap over the shingles at the bottom to act as a watershed for water running down the vertical surface and onto the roof.

Tips & Warnings

• Use caution when handling the flashing. The edges of the flashing are rather sharp and can cause serious injury. Do not drop any scraps of flashing from the roof without first verifying that there is nobody below that could be hit.

References

• Extreme How-To: Roof Flashing Done RIght
• University of Florida Cooperative Extension Service: Roof Repairs, Flashing and Sealing