A window that's out of square will bind, leak air, and be difficult to operate. Proper shimming during installation is the key to a window that opens smoothly, seals tightly, and stays that way for years. Glazelock® composite plastic shims are the ideal choice for window installations — moisture-proof, non-compressible, and Made in the USA.
Why Window Shimming Matters
Rough openings are rarely perfectly square, plumb, or level. Shims fill the gap between the rough framing and the window frame, allowing you to fine-tune the position until the window is perfectly aligned before fastening. Skip this step and you'll end up with a window that's hard to open, drafty, or prone to water infiltration.
What You'll Need
- Glazelock composite plastic shims
- Level (4-foot preferred)
- Square
- Hammer or drill
- Utility knife (to score and snap shims)
- Exterior caulk
Step-by-Step: How to Shim a Window
Step 1: Level the Sill
Start at the bottom. Place shims under the window sill until it's perfectly level. This is your foundation — everything else depends on getting the sill level first. Use Glazelock shim pairs from opposite sides so the combined thickness is even.
Step 2: Check Square
With the sill level, check the window frame for square by measuring diagonally corner to corner. Both diagonal measurements should be equal. If not, adjust shims on one side to bring the frame into square.
Step 3: Plumb the Sides
Place shims along both side jambs — at the top, middle, and bottom of each side. Use your level to confirm both sides are plumb. Adjust shim thickness as needed.
Step 4: Check Operation
Before fastening, open and close the window to confirm it operates smoothly. If it binds or drags, adjust shims until operation is correct.
Step 5: Fasten Through the Shims
Drive screws or nails through the window frame and shims into the rough framing. Glazelock plastic shims won't split when fastened through — unlike wood shims which can crack and shift.
Step 6: Score and Snap Excess
Score Glazelock shims with a utility knife and snap off the excess flush with the window frame. The composite material snaps cleanly without splintering.
Step 7: Insulate and Caulk
Fill gaps around the window with low-expansion foam insulation, then caulk the exterior perimeter for a weathertight seal.
Why Use Glazelock for Window Shimming
- Moisture-proof: Windows are exposed to rain, condensation, and humidity — Glazelock plastic shims won't absorb water, swell, or rot like wood shims
- Non-compressible: Holds its thickness under the clamping force of fasteners, keeping the window square long-term
- Interlocking tabs: Keep shim pairs together during installation so they don't slide apart while you're working
- Clean snap: Scores and snaps cleanly for a flush finish
- Made in the USA
Frequently Asked Questions
How many shims do I need per window?
Typically 8–12 shim pairs per window — shims at the sill, both sides (top, middle, bottom), and the head jamb.
Can I use plastic shims for exterior windows?
Yes — Glazelock composite plastic shims are moisture-proof and ideal for exterior window installations where wood shims would absorb water and cause the frame to shift over time.
What's the difference between shimming a door and a window?
The process is similar, but windows require extra attention to squareness since an out-of-square window will bind and may not seal properly. Always check diagonal measurements when shimming windows.
