Vinyl siding is one of the most used building components available today. Especially in the US, vinyl siding is enormously popular due to its durability, cost-effectiveness, and ease of installation.

Vinyl siding comes in a number of colors, styles, shapes, and prices. As such, it often finds its way onto luxury homes, starter homes, and everything in between.

Today we will focus on some common accessories, what they do, and provide a couple of tips the professionals use to install them. 

What Is a Vinyl Siding Accessory? 

An accessory is essentially any vinyl product intended to solve a specific problem when installing the siding. These components make the siding installation perform better, look better, and last longer. Below is a list of common accessories and their functions:


J-channel is probably the most used vinyl siding accessory. J-channel is used as        a frame for the vinyl, and will usually be found around doors, windows, mounting blocks, or any other opening in the vinyl siding.

J-channel is identified by it’s profile, which as you may have guessed, resembles the letter “J”. J-channel is sold in 8’ to 16’ lengths called “sticks”. These sticks can be cut easily with hand tools and are usually installed with mechanical fasteners like staples or roofing tacks.

Pro Tip. When feasible, professionals will only cut the nail fin section of the J-channel to make a 90-degree turn, such as around a window. This way they bend the J-channel around the window instead of creating four joints that could leak in the future.


F-channel is very similar to J-channel and is installed the same way. F-channel often serves the same purpose as J-channel, but will usually be installed horizontally, such as on a porch ceiling. F-channel is most often used to frame another vinyl product called soffit, which is used around the eaves of a home, often just under the gutter. F-channel has a more narrow profile than J-channel, so it can be used in tight areas where J-channel cannot.

Outside Corner

An outside corner usually comes in 2”, 3”, and 4” wide versions. These accessories are usually installed first, as they will keep the siding straight and plumb. Outside corners can be used anywhere siding can be installed and contains two channels, ninety degrees to each other. Since vinyl siding is usually trimmed to fit, these channels cover the ends of each stick and provide a nice, clean finished look.

Inside Corner

Inside corners are often used in place of J-channel to connect different styles of vinyl siding. Inside corners are actually rounded, which breaks up the straight parallel lines found in a vinyl siding installation. Inside corners are installed vertically, and will often delineate a change in color, vinyl design, or material. These are installed using the same fasteners as other accessories, and will usually come in 9’ to 12’ lengths.

Mounting Block

Mounting blocks are used anywhere a fixture or other feature must be mounted to the wall studs. These blocks vary in size, but will usually be about 6” square and include two sections. 

The first section is mounted to the wall studs, often using roofing tacks. The second section is then snapped onto the first, creating a channel for the siding.

These mounting blocks are designed to be trimmed or modified, depending on the fixture to be mounted. These can include GFCI (Ground Fault Circuit Interrupter) outlets, light fixtures, spigots, or any other component that requires support.

Starter Strip

In contrast to vinyl siding, starter strips are made from metal and are designed to be hidden. In a vinyl siding installation, the first course is always at the bottom of the wall. Before starter strips were introduced, J-channel was often used to hold the first course in place.

However, J-channels can twist, crack and fade over time, leading to necessary repairs. Starter strips avoid this by replicating the locking tab found on a stick of siding. This locking tab snaps onto the siding from underneath, producing a clean, straight edge with nothing visible. This also makes the siding more rigid, and resists movement caused by heat and cold.


Undersill, sometimes referred to as finish trim, is used when a stick of siding has been trimmed in the long direction, removing the locking tab. Undersill gets its name from its common location, which is under a window sill.

Undersill is also very commonly used where a wall meets a ceiling. Used in conjunction with J-channel or F-channel, undersill has a very small profile and often goes unnoticed.

How Do I Install Vinyl Siding Accessories?

Generally speaking, any vinyl siding accessory will use similar installation techniques and materials, as the composition of vinyl dictates that allowances must be made for thermal expansion.

In other words, vinyl will move depending on how cold or warm it is, similar to wood. This is important to know, because vinyl siding effectively protects your home from the elements.

When used correctly, in conjunction with products like aluminum flashing, exterior grade caulk, and coil stock, vinyl siding provides a very low maintenance home exterior. If the vinyl is not installed according to the manufacturer’s instructions, gaps, kinks and cracks are often the result. 

Here we will describe the basics of getting started installing vinyl siding accessories, and offer a few pro tips to make the project easier and longer lasting. Any fastener used to install vinyl products must not be driven up tight to the surface, as would be the case for most materials. 

As a rule, at least ⅛” should remain between the head of the fastener and the surface. Of course, regardless of the installation method, the most important part of any project is safety. Appropriate safety gear should always be used when working with vinyl siding products.

How to Install J-Channel

As with most vinyl siding accessories, J-channel comes with a nail fin. This is just a flat section of the trim that will be hidden behind the siding. It usually contains slots, parallel to the long side of the channel.

These slots mark the location of the fasteners, and serve to prevent the siding from falling, as well as buckling. Therefore, the J-channel should be square and plumb to the structure.

This is critical, because if not, the first course of siding will have to be trimmed along its length, adding unnecessary work to the project. Any J-channel fasteners must be rust resistant, whether using nails, screws, or staples.

Pro Tip. When two pieces of J-channel meet at an angle, the pros will use shears to make a 45-degree cut (also called a miter joint) to both sticks. This makes for not only a more attractive appearance, but also resists water seeping under the J-channel.

How to Install F-Channel

F-channel is installed the same way as J-channel, except that it usually forms the frame of a ceiling, instead of a wall. The pros will use the miter technique here as well, although 90% of F-channel will be hidden from view.

How to Install Outside Corner

An outside corner is critical to the appearance of the project, because it must be square and plumb to the structure, just like J-channel. 

Outside corners have two nail fins, and are installed using the same fasteners as vinyl siding.They are usually installed using nail or screws instead of staples, because if it moves or detaches, it can take the vinyl siding with it.

Extra long galvanized roofing tacks are standard.

How to Install Inside Corner

Inside corners install the same way as outside corners, except only the center section will be visible. Inside corners also have two nail fins, and are commonly mounted using extra-long, galvanized roofing tacks. The channel on inside trim tends to be shallow, so accuracy when trimming the siding goes a long way towards an attractive appearance.

How to Install Mounting Block

Mounting blocks are usually square or rectangular, although other shapes are available. This shape allows for one continuous nail fin around the entire accessory. The same installation techniques are used, but only the back half of the block is actually attached to the structure. After the back half is attached, the siding is installed around it and then the second half of the mounting block snaps onto the first. These two halves compose another channel for the siding to rest in.

How to Install Starter Strip

Just like J-channel and corners, starter strips must be installed square and level to the building. They have the same nail fin and slots as J-channel and F-channel, and can be trimmed or notched just like vinyl.

Although a starter strip isn’t technically vinyl, the pros will still allow for a gap between the fasteners and the strip. This allows the entire system to move freely during thermal expansion and contraction and often prevents buckling.

How to Install Undersill

Undersill installs quickly and easily using nails, screws, or staples. However, with undersill the nail fin is on the bottom of the stick instead of the top. This allows it to accept vinyl siding from below, such as under a window.

The nail fin will contain the same slots as other accessories, and can be trimmed using butt joints, or miter joints. Undersill is often used along with flashing, house wrap, and caulk to provide a watertight seal.

Make Your Vinyl Siding Project Durable and Attractive

Vinyl siding accessories are designed to make installing vinyl siding more attractive, durable, and easy to install. They come in many shapes, styles, and colors and can add interest to an otherwise simple appearance. If you are ready to dive into a vinyl siding project, using the multitude of accessories available may add a level of beauty you weren’t expecting. 

Editorial Contributors
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.

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