Plastic Window Shims for Level, Plumb, and Square Installations

Installing windows is precision work. A window has to be supported correctly, aligned with the frame, set level and plumb, and secured without twisting or compressing the unit. When the opening is uneven or the window is heavy, that small adjustment work can quickly slow down the job.

Plastic window shims help contractors make those adjustments with more control. They provide consistent spacing and support during installation, helping keep the window level, square, and properly positioned before final fastening. For residential, commercial, storefront, glazing, and curtainwall work, the right shim can make the difference between a difficult install and a clean, professional result.

Tanner offers a wide selection of plastic and composite construction shims for leveling, alignment, spacing, and support, including horseshoe shims, U shims, wedge shims, stackable shims, flat plate shims, rugged construction shims, keyslot shims, and composite wood shims.

Why Use Plastic Shims for Window Installation?

Plastic shims are commonly used because they are consistent, durable, and easy to work with on the jobsite. Unlike wood, plastic shims resist rot and moisture, which makes them a strong choice for applications where long-term stability matters.

They are also available in multiple sizes, thicknesses, shapes, and bolt-hole styles, allowing installers to choose the right shim for the opening, fastener layout, and load requirements.

Consistent Leveling and Alignment

Window installation depends on small, accurate adjustments. Plastic shims are manufactured in consistent thicknesses, helping contractors fine-tune the position of the window without relying on guesswork.

This is especially useful when installers need to bring a window frame into a level, plumb, and square position before fastening.

Moisture and Rot Resistance

Plastic shims do not rot like wood or rust like metal. That makes them useful around windows, doors, storefront systems, glazing, and other areas where moisture exposure may be a concern.

For long-lasting installations, using a shim material that holds its shape and resists deterioration helps protect the integrity of the finished work.

Job-Specific Shim Styles

Different window and glazing applications call for different shim styles. Horseshoe shims and U shims are designed to fit around bolts, anchors, and screws, making them useful when installers need to adjust spacing without removing the fastener.

Wedge shims and stackable shims are helpful for precise leveling. Flat plate and rugged construction shims provide broader support where heavier loads or larger bearing surfaces are needed.

Common Plastic Shims Used Around Windows

Horseshoe Shims

Horseshoe shims are shaped to fit around bolts, anchors, and fasteners. They are often used in glazing, storefront, commercial door, and window applications where installers need to maintain alignment while working around hardware.

U Shims

U shims are another practical option for fitting around fasteners. Their shape allows for quick placement and adjustment during installation, especially when spacing and alignment need to be corrected without fully backing out the fastener.

Wedge Shims

Wedge shims are useful when a tapered adjustment is needed. They can help fine-tune the window position and are often used where small leveling changes make a big difference.

Stackable Shims

Stackable shims allow installers to build up the exact thickness needed for the opening. This makes them useful when the gap varies or when a precise final position is required.

How to Shim a Window During Installation

1. Level the Window Horizontally

Start by placing shims under the sill to create a stable, level base. Space the shims evenly so the window is supported across the opening rather than concentrated in one spot.

The goal is to distribute the load and prevent the frame from sagging or shifting during fastening.

2. Position the Window Vertically

Place shims along the sides of the window near the top and bottom, then add additional shims where needed based on the size of the unit and manufacturer recommendations.

These shims help straighten the frame and keep the window plumb. Avoid over-shimming or forcing the frame out of shape, since too much pressure can affect window operation.

3. Allow for Building Movement

Buildings settle and shift over time. During installation, avoid creating pressure points that could restrict normal movement or distort the frame.

Follow the window manufacturer’s instructions for shim placement, spacing, and fastening so the unit can perform as intended.

4. Check for Plumb, Level, and Square

Before fastening, confirm that the window is level horizontally, plumb vertically, and square within the opening. Test locks, slides, sashes, and other movable parts to make sure the window operates smoothly.

This step is worth taking slowly. Once the window is fastened, correcting a twisted or compressed frame becomes much harder.

5. Secure the Window

After the window is properly aligned, fasten it according to the manufacturer’s installation instructions. Depending on the window and frame design, fasteners may pass through the frame, shims, or nailing flange.

Use the correct fastener type for the material and application.

6. Trim Excess Shim Material

Once the window is secure, trim or score excess shim material for a clean finish. This helps prepare the opening for insulation, sealing, and final trim work.

Choosing the Right Plastic Window Shim

The best shim depends on the installation. For windows, doors, storefronts, and glazing work, contractors often use horseshoe shims, U shims, wedge shims, and stackable shims because they make it easier to adjust spacing, work around fasteners, and keep frames aligned.

For heavier construction applications, flat plate shims and rugged construction shims may be a better fit because they provide broader support and stronger load-bearing performance.

Tanner carries plastic and composite shims in multiple sizes, thicknesses, shapes, and configurations, helping contractors match the shim to the job instead of forcing one product to fit every application.

Conclusion

Plastic window shims help contractors install windows with better control, cleaner alignment, and more consistent support. They are useful for leveling, spacing, positioning, and keeping window frames plumb and square before final fastening.

Whether the job calls for horseshoe shims, U shims, wedge shims, stackable shims, or heavier-duty construction shims, Tanner offers options built for professional installation work. For contractors handling residential, commercial, glazing, storefront, or curtainwall projects, choosing the right shim can help improve speed, accuracy, and long-term installation quality.