Free Sample
The Complete Handbook of Roof Flashings
For Roofers, Contractors, Building Owners, and Serious DIYers
by Alumigogo Books
Chapter 1: Understanding Roof Flashings
A roof flashing is a piece of material—typically metal, membrane, or a hybrid composite—installed at every place on a roof where a penetration breaks through the surface or where two roof planes meet. Chimneys, plumbing vents, HVAC ducts, skylights, parapets, valleys, and roof-to-wall junctions all require flashings. A roof can be perfectly sound everywhere else, but if its flashings fail, the building will leak. Water finds its way in.
This is not a minor detail. Roof flashings are the single largest source of water intrusion claims in residential and commercial buildings. A failed flashing costs thousands of dollars to diagnose because the leak often appears far from its source—water travels through cavities, under insulation, and down structural members before it drips or stains a ceiling. By the time you see the problem inside, the damage has often spread to sheathing, framing, and insulation. Repairing hidden rot and mold is exponentially more expensive than replacing a flashing correctly in the first place.
Understanding what a flashing actually does—and why it fails—is the foundation for making sound decisions about installation, maintenance, and repair.
How Flashings Work: The Physics of Keeping Water Out
A flashing works by intercepting water moving across the roof surface and directing it safely away from the penetration or transition. The principle is straightforward: water flows downslope due to gravity and wind-driven rain pressure. A flashing must be positioned so that water moving toward a vulnerable opening either runs over the top of the