Roof flashing plays a significant role in house construction. And a home or building with poor roof flashing installation is prone to several problems, particularly roof and wall leakages.

Problems areas like the roof cricket, the dormer, and even the chimney make properly installing a roof flashing quite a feat. The same problem applies when placing a roof flashing against siding too. This is why—although DIY enthusiasts are free to try at their own risk—it is better to let professionals take the task to ensure that the roofing tool is appropriately applied. 

That being said, if you are a seasoned DIY enthusiast and you are looking for ways to repair a damaged roof flashing, this article could help you. 

But first, what is roof flashing?

    The Importance of Roof Flashing

    Known as a roofing material for redirecting rainwater away from the vulnerable parts of the roof and house, roof flashing is a thin and galvanized steel installed around roof features like skylights, vents, and chimneys. 

    What is step flashing on a roof? It essentially prevents water leakage from the roof and wall that, if not immediately fixed, could lead to much bigger problems like wood rot and deck collapse. With it installed, rainwater should flow down the flashing and towards and towards the shingles.

    Roof flashing can be installed in different ways as well. Depending on the area, professional home builders may use any of the following:

    • Step flashing
    • Continuous flashing
    • Counterflashing
    • Base flashing
    • Skylight flashing
    • Kickout flashing
    • Valley flashing
    Get a Free Roofing Estimate
    Get Connected with Professional Roofers in Your Area

    A Step-by-step Guide to Roof Flashing Installation 

    While it is not wrong to let seasoned DIY enthusiasts attempt to install the fixing, roof flashing installation and repair is a rigorous and time-consuming job that is best to leave with professionals. The process requires thorough knowledge and careful execution to make sure that everything is properly done. 

    On the other hand, if you are familiar with roof flashing and want to give it a try fixing it, then here’s a brief guide to help you.

    Roof Flashing Techniques

    Roof flashing is not a one-method solution. And there is no way you can install (or less, fix) a roof flashing without learning these techniques and what they are for. It is because, for every roof feature, there is a corresponding and correct technique to place it.

    • Counterflashing. Essentially, counterflashing is a method used specifically for chimneys. It is a two-piece flashing placed separately around the bottom of the chimney and into the masonry.

      Professionals used counterflashing for other roof areas too, apart from the chimney.
    • Step flashing. Primarily, step flashing is a technique used when a roof and wall are involved. It prevents wall leakage by directing the water flow away from the wall to the gutter instead. 

      The technique is meticulously installed and typically involves a layer of shingles to ensure that the water flows to where it should go. 
    • Plumbing vent boot flashing. Plumbing vent boot flashing is a piece of flashing formed in the shape of a cylinder. The technique is used to wrap around a vent and act as a barrier from water. Essentially, its height pushes the water away. 

    How to Install Roof Flashing Under Siding?

    When placing flashing against the siding, the technique that you will be most likely to use will be step flashing. To do that, you will need the following equipment:

    • Flashing pieces
    • Coil stack
    • Shears
    • Flashing tape
    • Hand seamer
    • Siding brake (optional)

    Step 1: Cut.

    The first step is to cut a sheet of flashing. There are sheets of flashing available in the market today, which many experts tend to rely on. However, roof flashing is not a one size fits all, and the size you need might not be available for purchase.

    In that case, it’s best to cut your flashing. For roof flashing against the siding, it should be—at least—4 inches or 102mm wide. The size, however, may vary depending on the siding.

    Step 2: Install and insert.

    Next, install a piece of lap siding. Nail it using the recommended method. Once installed, insert a piece of pan flashing vertically under the one end of the siding. 

    When inserting, make sure that the flashing below does not go beyond the lap siding. 

    Step 3: Secure.

    Finally, secure both the flashing and siding by fastening them together into the structure. Once completed, simply repeat the process until the whole house is covered. Do not forget to leave at least 3/16 inches between the pieces of siding. 

    Common Roof Flashing Installation Mistakes

    • Flashing’s slope. Poor flashing installation may result in what is called “flashing slope.” This happens when the flashing gets bent, resulting in a slope that typically goes towards the wall. 

      As such, make sure not to cause any bends in the material. Make sure that the roof flashing is facing away from the wall as well, so as to keep the wall from accumulating too much moisture. 
    • Caulk in the gap between siding and roof flashing. As mentioned, it is important to leave at least a ⅜ of an inch gap between the flashing and the siding. Do not fill the gap with caulk, this would defeat the purpose of the gap. The gap between each siding acts as a window to let the moisture accumulating in the walls escape. 
    • Closing the gap. Another common mistake folks—even professionals—make is to close the gap between each siding. As said, the gap serves as a window to let the accumulated moisture in the wall escape. That way, it prevents the wall from becoming a breeding ground for mold and mildew. 


    Installing a roof flashing—be it against the siding, around the chimney, or other parts of the roof—is a meticulous task that needs to be done properly. If done improperly, issues such as wall and roof leakage can ensue which will gradually damage parts of your house.

    As such, if you are not an expert or an experienced DIY enthusiast, it is best to hire professional roofers to do the job. Otherwise, it will likely cost you more than your allocated budget. If this is your choice , we refer you to read our article titled guide to hiring a professional roofer.

    Get a Free Roofing Estimate
    Get Connected with Professional Roofers in Your Area
    Editorial Contributors
    avatar for Matt Greenfield

    Matt Greenfield

    Matt Greenfield is an experienced writer specializing in home improvement topics. He has a passion for educating and empowering homeowners to make informed decisions about their properties. Matt's writing focuses on a range of topics, including windows, flooring, HVAC, and construction materials. With a background in construction and home renovation, Matt is well-versed in the latest trends and techniques in the industry. His articles offer practical advice and expert insights that help readers tackle their home improvement projects with confidence. Whether you're a DIY enthusiast or a seasoned professional, Matt's writing is sure to provide valuable guidance and inspiration.

    Learn More