FHB Logo Facebook LinkedIn Email Pinterest Twitter X Instagram Tiktok YouTube Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Restoration
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Fine Homebuilding Project Guides

Roofing

Guide Home
  • Tools and Materials
  • Roof Replacement and Reroofing
  • Roof Ventilation
  • Waterproofing
  • Installing Asphalt Shingles
  • Installing Other Types of Roofs
  • Roof Maintenance and Repair
Installing an Asphalt-Shingle Roof

Critical Roof Flashings Keep Water Out

Building expert Mike Guertin explains the what, where, and why of roof flashing.

By Mike Guertin
  • X
  • facebook
  • linkedin
  • pinterest
  • email
  • add to favorites Log in or Sign up to save your favorite articles

August 26, 2021, is the first annual International Flashing Awareness Day—a day for all builders, remodelers, roofers, and siding installers to pause and think about their flashing practices and evaluate whether those practices are sufficient to prevent water from leaking into the homes they work on. Flashing keeps our houses dry and roof flashings are a critical part of a leak-proof roofing installation. We want our houses to stay dry so we and our belongings stored in the home don’t get wet, and we want the roof structure to stay dry so it won’t rot and lead to expensive repairs. In this article, I’ll review several typical roof flashings that block water entry: boot flashing, step flashing, continuous flashing, kickout flashing, skylight flashing, and valley flashing.

Roofs don’t usually leak in the field of the roofing unless there’s damage or very poor installation. Instead, roofs leak at connections between the roof and a wall, one roof plane to an adjacent plane, or at penetrations like vent exhausts and plumbing vent pipes. These locations require flashing to block water from getting into the connections and to redirect water onto the surface of the roofing so it can drain harmlessly off the roof. Flashing can be made from metal, plastic, or self-adhering membrane that meets the requirements of the building code.

The importance of roof flashing is often overlooked by roofing installers today. Part of that is due to legacy flashing practices that have carried over from one generation of roofers to the next—practices that worked years ago are no longer sufficient yet many roofers continue to follow them. It’s the old explanation: “This is the way I learned how to do it and I’ve been doing it this way for [fill in the blank] years without a problem.” Unfortunately, they’ve never gone back to check to see how those flashings failed. And part if it is due to a lack of awareness of current good flashing practices, building code requirements, and manufacturer requirements.

20th-century flashing practices meet 21st-century building science

For a long period of time, homes were built with little or no insulation in the attics. Small water leaks along flashings wet the solid wood sheathing planks and framing beneath, but water didn’t reach and stain the ceiling below so homeowners were unaware of the leaks. The water would do little or no harm to the wood. It would dry due to the heat in summer or the heat escaping from the living space into the attic in winter. And when leaks in flashing were bad enough that water reached the ceiling, homeowners noticed the stains and had repairs made before rot took hold.

Over the past 50 years, several things have changed in the way we build homes that raise the importance of roof flashing. Roof framing and lumber aren’t the same. Roof trusses with 2×4 or 2×6 top chords have replaced 2×8, 2×10, and 2×12 stick-framed roofs. Rot in smaller truss chords will lead to structural failure faster than in larger-depth rafters. And the lumber is from younger, rapidly growing trees that will decay faster than old-growth lumber of a generation ago. Roof sheathing has shifted from old-growth solid wood planks to plywood and then OSB; with each change, the potential for decay increased. Insulation and building airtightness requirements— especially in cold climates—have increased. Heat loss into the attic in winter is no longer enough to dry damp wood due to small flashing leaks, so wood stays wetter for longer and will begin to rot. And once wood-decaying fungus takes hold it can consume the wood quickly.

The insulation itself can hide roof leaks that would have stained ceilings in an older home. Spray foam applied directly to the underside of the roof sheathing is becoming more common. Water that leaks at flashing and saturates the roof sheathing and framing can no longer dry into the attic. The wood stays wet for a long time and rot can take hold. Even thick fibrous insulation can hold more water and prevent small and moderate flashing leaks from staining a ceiling. This results in the roof sheathing and framing repeatedly getting wet without water reaching the ceiling and alerting the owners that there’s a roof leak occurring.

There’s no such thing as overkill when it comes to roof flashing

Longer-lifespan roofing means less-frequent replacement, which means critical flashing won’t get checked and replaced as regularly. So it makes sense to buy, fabricate, and properly install good flashing that will last whether installing a new roof or replacement roof.

Here’s my list of critical roof flashing and resources to guide selection and installation. But first, consider backing up all flashing with a second line of defense—a strategic use of self-adhering, self-sealing underlayment applied at all junctions and roof edges that collects water that may get behind flashing and redirects the water to the surface of the roofing. I call the process “stormproofing a roof,” but it’s effective in ordinary weather too.

Boot flashings often rely on rubber or plastic seals around pipes to block water. The rubber or plastic deteriorates with the extremes of heat, cold, and sunlight on a roof, so a choosing a better boot flashing that will last longer is wise. And before installing any boot flashing, preflash the pipe to the shingles using flexible flashing tape for backup protection.

Vertical-wall, outside-corner, and step flashings are applied in sequence around the perimeter of a dormer to block water from getting into the roof.

Step flashing is installed along sidewalls where the end of asphalt, wood, and synthetic shingle courses meet a wall. Size and thickness matter in order to meet requirements of building codes and roofing manufacturers. Some building-materials dealers stock step flashings that don’t meet code requirements, so here’s what you need to know to ensure your step flashings do.

Continuous sidewall flashing is an alternative to installing step flashing. Rather than individual pieces of flashing installed at the end of each shingle course, an L- shaped continuous flashing is installed either below or on top of the shingle courses. The continuous flashing method is permitted in the building code and is common in some parts of North America. And some types of interlocking and panel roofing systems utilize continuous flashing along sidewalls because they don’t have horizontal courses.

Kickout roof flashing at eave edge of the intersection of a roof plane and a wall
A kickout flashing turns the water flowing down a roof plane alongside a wall away from the wall. It replaces the first piece of step flashing along an eave edge.

Whether you install step flashing or continuous flashing along sidewalls, the code requires a kickout flashing (diverter flashing or kickout diverter) at the bottom of a roof eave that terminates along a wall—think the eave of a one-story part of a house meeting the wall of a two-story portion. A kickout flashing is the first piece of step flashing along the eave edge that directs water collected by the step flashing or continuous flashing away from the wall so it doesn’t leak into the wall. Kickout flashings have been required by building codes for 15+ years yet are often overlooked.

This kickout flashing video demonstrates one approach to installing the layers of preflashing tape, housewrap, and the kickout flashing.

Skylights present their own flashing challenges. Fortunately, most skylight manufacturers provide specially designed base, head, and step flashing to install around a skylight curb. But when working on an existing skylight that doesn’t have good flashing or the flashing can’t be reused, you can follow this site-made skylight flashing system.

Flashings around a masonry chimney are installed by the mason when laying up the brick or stone. Over time, the flashing will deteriorate or become damaged and need to be replaced. Here’s a simple retrofit counterflashing approach.  

Skylight curb on a roof being wrapped with self-adhering membrane as backup protection to the primary roof flashing.
Self-adhering membrane wraps a skylight curb to provide backup protection to water leaks.

A 'W'-shaped copper sheet is placed in the center of a roof valley.
An open metal valley is a specialized type of flashing. Roofing materials will overlap and be sealed to the edges of the metal valley.

Valleys collect rainwater from two roof planes and channel it off the roof. The water volume can overwhelm the joints and overlaps where roofing materials meet in a valley so the building code requires valley liner or flashing beneath the primary roofing material. Valley liner can be self-adhering membrane, roll roofing, or metal. And the valley itself can be open metal that is treated much like a sidewall continuous flashing where the roofing materials on each roof plane overlap and drain onto it. This article focuses on asphalt shingles but the valley liner and open metal valley details can be used with many roofing systems.

 


 

RELATED ARTICLES:

  • 4 Ways to Shingle a Valley (Valley Flashing)
  • Stormproof Your Roof
  • Time-Tested Approach to Chimney Flashing
  • Direct Water into Gutters with a Kickout Flashing Diverter
  • Kickout Flashing: How to  Flash Troublesome Roof to Wall Intersections
  • Retrofitting a Curbed Skylight
  • Looking Out for Future Roofers (Step Flashing)

 

Fine Homebuilding Recommended Products

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

Fall Protection

With a 50-ft safety line and a hinged, reusable anchor, this complete safety harness kit allows you to easily protect yourself while working comfortably and efficiently.
Buy at Amazon

Peel & Stick Underlayment

Often required by code, this durable, slip resistant, self-sealing, and self-adhered roofing underlayment is easy to install and ensures maximum protection along roof eaves, rakes, and valleys.
Buy at Amazon

Shingle Ripper

This tool makes tearing off shingles easier and faster, providing powerful leverage that allows you to work comfortably. Its beveled teeth and wide blade make it easy to remove nails and push shingles off the roof.
Buy at Amazon
Previous: How to Install Drip Edge When Replacing a Roof Next: Replacing PVC With an EPDM Membrane on a Flat Roof

Guide

Roofing

Chapter

Roof Replacement and Reroofing

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Roofing

Roofing

A complete resource that includes the best materials and the right techniques to build a sturdy, leak-free, attractive roof on any house

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Roofing Project Guide.

Start Free Trial

Tools and Materials
  • Roof Coverings
  • Roofing Materials
  • Power Tools
  • Handy Helpers
  • Staging and Ladders
  • Safety Equipment
Roof Replacement and Reroofing
  • Preparing the Old Roof
  • Sheathing
  • Installing the New Roof
Roof Ventilation
  • Venting Basics
  • Installation
Waterproofing
  • Underlayment
  • Drip Edge
  • Flashing Adjoining Walls
  • Dormer and Skylight Flashing
  • Chimney Flashing
  • Vent Flashing
  • Gutters
Installing Asphalt Shingles
  • Calculating Shingles
  • Layout
  • Installation
  • Valleys
Installing Other Types of Roofs
  • Cedar Shingles
  • Metal
  • Low-Slope Roofs
  • Slate and Tile
Roof Maintenance and Repair
  • Assessing the Problem
  • Repairing Asphalt-Shingle Roofs
  • Other Roofing Repairs

Fine Home Building

Newsletter Sign-up

  • Fine Homebuilding

    Home building tips, offers, and expert advice in your inbox.

  • Green Building Advisor

    Building science and energy efficiency advice, plus special offers, in your inbox.

  • Old House Journal

    Repair, renovation, and restoration tips, plus special offers, in your inbox.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters

Follow

  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X
  • Fine Homebuilding

    Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.

    Start Free Trial Now
    • Facebook
    • Instagram
    • X
    • LinkedIn
  • GBA Prime

    Get instant access to the latest developments in green building, research, and reports from the field.

    Start Free Trial Now
    • Facebook
    • YouTube
  • Old House Journal

    Learn how to restore, repair, update, and decorate your home.

    Subscribe Now
    • Facebook
    • Instagram
    • X

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences
  • About
  • Contact
  • Advertise
  • Careers
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2025 Active Interest Media. All rights reserved.

Fine Homebuilding receives a commission for items purchased through links on this site, including Amazon Associates and other affiliate advertising programs.

  • Home Group
  • Antique Trader
  • Arts & Crafts Homes
  • Bank Note Reporter
  • Cabin Life
  • Cuisine at Home
  • Fine Gardening
  • Fine Woodworking
  • Green Building Advisor
  • Garden Gate
  • Horticulture
  • Keep Craft Alive
  • Log Home Living
  • Military Trader/Vehicles
  • Numismatic News
  • Numismaster
  • Old Cars Weekly
  • Old House Journal
  • Period Homes
  • Popular Woodworking
  • Script
  • ShopNotes
  • Sports Collectors Digest
  • Threads
  • Timber Home Living
  • Traditional Building
  • Woodsmith
  • World Coin News
  • Writer's Digest
Active Interest Media logo
X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Project Guides
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Online Learning

  • Courses
  • Project Guides
  • Reader Projects
  • Podcast

More

  • FHB Ambassadors
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • X
  • YouTube
  • instagram
  • facebook
  • pinterest
  • Tiktok

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

You have 1 free article remaining.

Get complete site access, including thousands of videos, how-to tips, tool reviews, and design features.

Start your FREE trial

Already a member? Log in

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in

Privacy Policy Update

We use cookies, pixels, script and other tracking technologies to analyze and improve our service, to improve and personalize content, and for advertising to you. We also share information about your use of our site with third-party social media, advertising and analytics partners. You can view our Privacy Policy here and our Terms of Use here.

Cookies

Analytics

These cookies help us track site metrics to improve our sites and provide a better user experience.

Advertising/Social Media

These cookies are used to serve advertisements aligned with your interests.

Essential

These cookies are required to provide basic functions like page navigation and access to secure areas of the website.

Delete My Data

Delete all cookies and associated data