How Flashing Valleys of Pitched Roofs Saves Headaches and Cost

With the onset of Spring, seasonable rain and floods are a reality many homeowners face. That in mind, simplifying a new home’s exterior architecture and finishes is a popular trend in the current economy.

Reducing number of gables, trimming cladding varieties and not cutting roofs into a hodgepodge of angles and intersecting slopes are all part of the process.

More than just good advice, the process of simplifying also reduces hard costs and boosts curb appeal, while mitigating callbacks.

A cut-up roof, in particular, is a haven for potential surface and latent water damage, that can result in peeling shingles. Or worse, a moisture-induced problem that may require replacing the entire roof. 

A considerable amount of this damage happens at the valleys, where improper or non-existent flashing allows water and melting snow to get caught up along pathsWater and melting snow then work their way under shingles, and wick out along roof decks instead of flowing straight to the gutter.

Latent Defects

An unflashed (or improperly flashed) roof can easily and quickly evolve from mere spot, surface damage on shingles to rot and decay of the roof deck. This can cause leaks throughout the roof, that may be far removed from the real source. Ramifications of leaks can be difficulty with diagnosing the issue, and ultimately higher costs of repair

Flash Valleys Roofs resized 600.jpg

Proper Materials

A 10-foot (max) length of metal-flashing (either galvanized sheet metal or copper) is the foundation of a moisture-resistant roof valley. Make sure the metal flashing spans the valley at least 6-inches to either side of the center. Other essentials include a peel-and-stick waterproof roof membrane, and a bitumen-based tape or mastic. These materials will facilitate proper water runoff, and block moisture from going where it is not wanted.


Overlap the 10-foot sections of metal flashing from the eave up (like you will with the shingles) using at least 4-inch overlap, all the way to the ridge. Also, overlap the membrane to within 3-inches of the center of the metal flashing. Extend the comp shingles at least 12-inches past the center of the valley, and fasten only the far top corner. Fastening the top corner will preserve the integrity of the flashing underneath it. Finally, clip the corner of the intersecting shingle

As with most flashing specs, the entire process requires the right materials and attention to detail.

No secrets, no unreasonable costs. Properly trained and supervised, any roofer worth his salt can and should be able to flash a valley so that water runoff goes where it should.