When it comes to window installation, one of the biggest challenges faced by contractors is maneuvering bulky and heavy windows into place. The task becomes even more complicated to ensure that the windows are level, flush with the wall and frame, and securely positioned. However, thanks to the advent of plastic window installation shims, this daunting task has become much more manageable. These specially designed shims provide the necessary support and stability for a smooth and efficient window installation process that lasts decades.


Advantages of Plastic Window Shims

Plastic shims for window installation offer several advantages over shims made from other materials. Let's take a closer look at some of these benefits:

  • Fire-resistance

In buildings that must comply with non-combustibility construction codes, such as those with blast-resistant windows, it is crucial that the shims used are non-combustible as well. This requirement rules out the use of wood shims. Plastic shims, on the other hand, are non-combustible and can withstand high temperatures without melting or maintaining a flame. This fire-resistance property ensures that windows remain safe and exits remain accessible during emergencies.

  • Adaptable

Plastic shims for window installation are highly adaptable and can be used in various scenarios. Whether aligning glass storefront panels with the frame or ensuring proper spacing and alignment between panels, plastic shims can be placed vertically and horizontally. Two common plastic shims used in window installation are horseshoe shims and U-shaped shims. These shims are designed to fit around bolts and anchor screws and can withstand significant amounts of compressed force.

The Shimming Process: A Step-by-Step Guide
Now that we understand the advantages of plastic window shims, let's delve into the shimming process itself. Here is a step-by-step guide to ensure a quick and professional window installation using plastic shims:

Leveling the Window Horizontally

Start by placing plastic shims under the sill, approximately 14 to 16 inches apart. This will help level the window horizontally. The plastic shims should be positioned in a way that evenly distributes the weight of the window and ensures a stable foundation.

Positioning and Straightening the Window Vertically
Next, insert shims on both sides of the window near the top and bottom and at 20- to 24-inch intervals. These shims will help position and straighten the window vertically. It's important to ensure that the shims do not compress the frame, which can lead to instability and operational issues.

Allowing for Building Settlement and Shifting
One key aspect of window installation is accounting for a building's natural settling and shifting over time. To accommodate this movement without compromising the window's operation, it's essential to leave the top of the window unshimmed. This allows the window to move with the building, reducing the risk of damage.

Verifying Plumb, Level, and Square
Before securing the window, it's crucial to verify that it is plumb, level, and square. This can be done using a level and a square. Additionally, ensure that all locks, slides, and movable window parts work smoothly.

Securing the Window
Once the window is properly aligned and verified, it's time to secure it in place. This can be done by driving anodized screws through the shims or the nailing flange. The choice of fasteners may vary based on the specific window and frame design.

Finishing Touches
After securing the window, any excess shim material can be scored and broken off to achieve a clean and aesthetically pleasing finish. This step also allows for proper insulation and enhances the overall appearance of the window installation.



Plastic shims have revolutionized the window installation process, making it easier and more efficient for contractors. With their fire resistance, adaptability, and durability, plastic shims provide the necessary support and stability for precise window positioning. By following the step-by-step shimming process, contractors can ensure a professional installation that meets the highest standards. When choosing a plastic shim, Tanner offers a wide range of industry-specific shims and exceptional customer service. So, whether you're working on a small residential project or a large commercial development, plastic shims are the perfect solution for your window installation needs.