How to Install Roof Flashing

What You'll Need
Flashing material
Tin snips
Metal break tool
Galvanized roofing nails
Roofing cement glue
Silicone caulking
Preformed flashing

Whether you're building a new home and doing the roof yourself, or you're making a repair to an existing roof, roof flashing is something you'll need to install. It's not a difficult job (if you're not afraid of heights, that is) but it is a necessary one. Before we get to the details, though, let's examine a bit on just what roof flashing is and where it goes on your roof so you'll know where exactly you need to install it.

What Is Roof Flashing?

Roof flashing is a material used as a protectant against water leaks or seepage. It's usually made of galvanized metal, but it can also be in the form of aluminum, sheet metal, copper, or even plastic. Galvanized metal or aluminum is considered the best to use as flashing. Aluminum is easiest to install due to how well it bends and is preferred by most DIYers.

Where Do You Install Roof Flashing?

As a protectant, flashing is placed anywhere on the roof's surface that requires extra protection. These areas include anywhere there are intersections or joints such as edges around chimneys, vents, skylights, roof valleys, and soil stacks. If you're having a leak issue in your home, you should first check these areas on your roof for any problems with the existing flashing. At times the nails work loose and the flashing has pulled away from the roof and needs to be reattached, or if it's in bad shape, replacement may be necessary.

Pre-made Roof Flashing

Much of the roof flashing that you need is already shaped or formed for you and will simply vary based on where it's being installed. Purchasing it this way can make the job much easier, and each variety is described below.

Step Flashing: Step flashing is a flashing that is made to protect the joints between the roof and the chimney, and sometimes other objects, such as skylights. The step flashing is bent and shaped in such a way that it fits to each course of shingles and goes up the wall of the chimney, making it appear to "step" up.

Vent Pipe Flashing: This flashing fits over any pipes or flues sticking up from your roof. It's shaped as a cylinder and will have a large flange at the base where you will lap it into the shingles as you add your roof.

Valley Flashing: Wherever you have a valley in your roof, which is where two roof planes meet, you'll apply your valley flashing. This flashing comes shaped as a "V" or a "W." Valley flashing is attached on top of the roofing felt, before shingles are added.

Drip Edge Flashing: Along roof eaves you'll want to install drip edge flashing. This flashing will prevent water from leaking under the finished roof. This flashing can be purchased in vinyl and will guide run-off to the gutters.

Installing Roof Flashing

Installing your roof flashing is going to be slightly different depending on the flashing is being installed.

Step Flashing for Chimneys

Around the chimney base and after shingles have been installed, lay the base flashing approximately 4" past each end of the chimney. Now, notch the first step flashing so that it will bend to go around the chimney front and side as well as at the base. Under the base part of the flashing, use plastic roofing cement and then finish attaching it to the roof with galvanized roofing nails.

The top piece that bends around the chimney is left unattached. You continue this process around the length of the chimney, cutting each step flashing to fit. Counter flashing will then be wrapped around the entire chimney and attached with steel masonry fasteners (depending on your chimney flashing kit directions). Be sure to leave a 1-inch gap between your shingles and the bottom of the side flashing. Seal the top of the counter flashing and any gaps with the plastic roofing cement.

Vent Pipe

To install your vent pipe flashing, you will be placing the upper portion below the shingles while allowing the bottom portion that extends below the pipe to lay on top of the shingles and be exposed. Lifting the shingles around the vent (if you've already laid them or they're already in existence), slide the upper portion of your vent flashing beneath them. Cut a membrane material to seal between the vent flashing and the roof and cut an opening for the vent. Apply a thin bead of silicone around the opening of the vent pipe area and lay the membrane in place. Next, place your vent flashing on top of the membrane. The metal flashing should overlap the material, and not the other way around.

Nail the base of the vent into place with your galvanized roofing nails. The shingles should lay on top of the upper portion of the vent flashing and the bottom portion will be on top of the shingles and showing. Apply some silicone along the shingles to help secure them better to the flashing as well as however else you're attaching your shingles (such as tar and / or nails).

Valley Flashing

For the valley areas of your roof, you'll be using valley flashing. Before you apply your flashing you'll need to lay a waterproof membrane, especially if you live in an area that gets a lot of snow and ice. Lay the underlayment as directed by the manufacturer and set your valley flashing on top. You can choose from "V"or "W"- shaped valley flashing. When you lay your flashing down, make sure it is flush with the drip edges at the bottom of the valley. Trim as needed with your tin snips. Nail the flashing in place every foot or so along the outside edges -- never in the middle. You can seal any seams with roofing cement and nail heads as well for extra protection.

Drip Edges

After you've added your valley flashing you can install your drip edges. The drip edges are corner-shaped and will be attached by nail along the edge of the roof. The drip edge should be nailed under the builder's felt, but over the fascia board. Cover the nail heads with roofing cement and stick the felt down. At any overhangs, install your drip edge over the felt and fascia to protect the felt from the wind and rain.

Cutting Your Own Flashing

If you're installing your roof flashing in an area that requires covering a larger surface or you would like to make your own flashing and bend it to go around objects on your roof, you can purchase the flashing in bulk. When purchasing it this way, estimate 15 percent over what you think you'll need due to cutting, bending, and waste.

Roof flashing usually comes in rolls and comes in multiple different widths and lengths. If you know you're covering a large area, such as where the roof shingles meet a side wall, you can purchase a width of 14-20 inches to be sure it's well covered, or for smaller areas of your roof they sell width sizes as small as six inches which can be cut further down from there if needed.

Final tips to remember when installing roof flashing yourself: The purpose of roof flashing is to direct any water flow away from any intersections, so when you install it keep this in mind to ensure that it can indeed do its job. Also, with each of these types of flashing materials, the top edge of the flashing is installed in such a way that it should never come in contact with the wood panel or sheathing.