How to Install Roof Flashing

A red tile roof with a chimney and 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, flashing is something you'll need to install. It's not a difficult job, provided you're not afraid of heights that is, but it is a necessary one. Before we get into the details, however, let's examine what roof flashing is and where it fits into your roof so that you'll know exactly what and where you need to install it.

What Is Roof Flashing?

Roof flashing is a material used as a protectant against water leaks or seepage and is typically 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 for roof 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 of the roof for any problems with the existing flashing. Oftentimes the nails have worked their way loose and the flashing has pulled away from the roof meaning it needs to be reattached. If it's in bad enough shape, replacement may be necessary.

Pre-made Roof Flashing

Much of the roof flashing that you need comes already shaped or formed for you and will vary slightly 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 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 cylindrically shaped and will have a large flange at the base where you will lap it into the shingles as you lay your roof.

Valley Flashing:

Wherever you have a valley in your roof, which the junction is where two roof planes meet at the bottom of a slope, you'll apply your valley flashing. This flashing comes shaped like 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 type of flashing will prevent water from seeping in and leaking underneath the finished roof. Vinyl flashing can be purchased that will guide water run-off to the gutters.

Installing Roof Flashing

Installing your roof flashing is going to be slightly different depending on the specific type that you're installing.

Step Flashing for Chimneys

Once the shingles have been installed, lay the base flashing around the base of the chimney, approximately 4" past each end. 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 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 plastic roofing cement.

Vent Pipe

To install 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 exposed on top of the shingles. Lifting the shingles around the vent if you've already laid them or they're already part of an existing roof, slide the upper portion of your vent flashing beneath them. Cut some 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 must overlap the material, not the other way around.

Fasten the base of the vent into place with galvanized roofing nails. The shingles should lay on top of the upper portion of the vent flashing while the bottom portion will be on top of the shingles and showing. Apply silicone along the shingles to help secure them more securely to the flashing any other means you employ for attaching your shingles such as tar, and / or roofing nails.

Valley Flashing

For the valley areas of the roof, you will be using valley flashing. Before you apply the flashing you'll need to lay down a waterproof membrane. Especially if you live in an area that gets a lot of snow and ice. Install the underlayment as directed by the manufacturer then set the valley flashing on top of it. You can choose from "V" or "W" shaped types of flashing. When you lay the 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 securely in place approximately every 12-inches 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 correctly installed the valley flashing you can install the drip edge which is corner-shaped and will be attached with nails along the edge of the roof. The drip edge should be nailed beneath the builder's felt, but over the fascia board. Cover the nail heads with roofing cement and stick the felt down. On any overhangs, you might encounter, install your drip edge over the felt and the fascia to protect the felt from being damaged by wind and rain.

Cutting Your Own Flashing

If you're installing the new flashing in an area that requires covering a larger surface, or you would like to make your own flashing and bend it to fit around objects on your roof, you can purchase the flashing material 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 is available in a variety of 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 narrow as six inches which can be cut further down from there if needed.

Final Tips For 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.