How to Install Roof Flashing where 2 Roof Valleys Meet

Lead Image
  • 3-8 hours
  • Intermediate
  • 20-150
What You'll Need
Flashing material (aluminum, lead or copper, etc.)
Roofing nails (same material as flashing)
Sealant material
Granule paint

You have to install roof flashing correctly to ensure a leak-free roof. Every little crevice is an entry point for water, and one of the most vulnerable points in any roof is the valleys. These valleys are the meeting points between 2 roofs, which are highly dangerous areas as they are not only prone to water seepage, but their structure also makes it difficult for them to be effectively waterproofed. The following instructions should give you a fair idea about how to proceed with your own roof valley flashing.

Step 1 - Make Sure All Your Homework is Done

Make sure that all of the procedures preceding roof flashing are done so that you can flash the roof valley properly. These procedures include underlayment, laying of a waterproof membrane and rolling out of builder’s felt. Flashing cannot be done if these processes are not completed first.

Step 2 – Consider Your Available Flashing Options

There are several options available when it comes to flashing, you can use copper or lead-coated copper flashing, aluminum flashing, galvanized or stainless steel flashing, etc. For utility’s sake, we are using the aluminum flashing as it is relatively cheaper and easier to work with. You must, however, be sure to only use aluminum nails with the aluminum flashing and to use sealants and caulking instead of soldering to waterproof the flashing.

Step 3 – Calculate Needed Flashing

Calculate the amount of flashing that you will need, this can be done easily by measuring the length of the valleys that you will flash. Multiply the length by 1 1/2 for a steep roof and by 1 1/4 for a non-steep roof.

Step 4 – Purchase the Flashing

Purchase valley flashings; these are "V" or "W" shaped flashings prepared commercially. They have a small ridge in the center to assist water runoff, of the calculated length of your desired metal, in this case, aluminum. Also purchase the relevant nails, which in this case are made of aluminum.

Step 5 – Begin Installation

Install valley battens on both ends of the roof valley, then attach first flashing 3/4 inches over the roof flange, and another 3/4 inches over the fascia if any is present. Remember to caulk the underside of the flashing with a sealant; now nail this first flashing at both its far ends.

Step 6 – Install the Flashing

Now start laying down the flashings over the valley, ensure that each valley flashing is coated with caulking material on the underside. Also, note that each flashing is to overlap its succeeding flashing, with the overlap being greater for a greater slope. Maintain a minimum overlap of at least 4 inches. Nail all the flashings as described before.

Step 7 – Paint and Seal

Caulk any openings left and made by the flashing process with a suitable sealant, then dust the flashing once it is dry and paint over with granule paint, available at hardware stores, of your desire.

Now ask a family member to stand inside the house, under the roof valley, and then standing on the roof, pour a bucket of water onto your handiwork.