How do you use plastic shims for window installation? Insert shims in pairs from opposite sides at hinge points, latch points, and corners. Adjust until the frame reads plumb and level on all sides. Glazelock interlocking shims snap together to prevent slipping — unlike wood shims, they won't compress, rot, or shift after installation. Made in USA since 1997.
Tom's Quick Answer: Shim at structural points only (hinges, latches, corners), always use pairs from opposite sides, and use plastic — wood shims compress over time and your frame will shift.
Tom's Real-World Advice
I've installed dozens of windows and doors over the years. The shimming step is where most DIYers make mistakes — either using too few shims, placing them in the wrong spots, or using wood shims that compress and shift over time. Plastic shims changed everything for me. They don't compress, they don't rot, and Glazelock's interlocking design means they stay exactly where you put them.
I pulled out wood shims from a 10-year-old window installation last spring that had compressed so much the frame had shifted nearly a quarter inch. The seal was gone, there was moisture damage in the framing, and the whole window had to come out. That never happens with Glazelock.
What You'll Need
- Glazelock shims (U-shaped horseshoe shims for most window and door work)
- Level (4-foot for doors, 2-foot for windows)
- Tape measure
- Drill and screws
- Utility knife or snap-off tool
Shim Selection Guide
| Application | Recommended Glazelock Shim |
|---|---|
| Standard window installation | U-Shaped 3" x 1-1/2" (1/2" slot) |
| Heavy door installation | U-Shaped 3" x 2-5/16" (3/4" slot) |
| Large commercial windows | Interlocking U-Shim 4" x 3" (7/8" slot) |
| Tight spaces / small frames | Compact U-Shaped 2" x 1-1/2" |
| Custom thickness needed | Snap-Off Stackable 1-7/8" |
| Rough framing / subfloor | Wedgelock or Tapered Wedge |
Step-by-Step: Shimming a Window
- Rough opening check. Measure your rough opening — it should be 1/2" wider and 1/2" taller than the window unit to allow for shimming and adjustment.
- Set the sill. Place the window in the opening and check the sill for level. Insert Glazelock U-shims under the sill jamb at each corner until level reads true.
- Shim the sides. Working from the bottom up, insert shims at hinge points (for casements) or at 12" intervals along the side jambs. Insert from opposite sides for maximum stability.
- Check plumb. Use your level on the side jambs. Adjust shims until both sides read plumb.
- Check square. Measure diagonally corner to corner. Both measurements should match within 1/8".
- Secure and fasten. Once plumb, level, and square, drive fasteners through the frame and shims into the rough framing. Do not overtighten — this can rack the frame.
- Trim or snap off excess. Glazelock snap-off shims break cleanly at score lines. Standard shims can be scored and snapped with a utility knife.
-
Insulate. Fill gaps around the frame with low-expansion foam. Do not use high-expansion foam — it can bow the frame.
Pro Tips from Tom
- Always shim at structural points — hinge locations, latch points, and corners — not just anywhere
- Use interlocking shims (Glazelock's snap-together design) to prevent shims from walking out over time
- Color-code your shims by thickness for faster installation on multi-window projects
- Never use a single shim alone — always use pairs from opposite sides for balanced support
- Plastic shims won't compress, rot, or absorb moisture — critical for exterior applications
- Low-expansion foam only — high-expansion foam bows frames and voids most window warranties
Step-by-Step: Shimming a Door
- Set the hinge side first. The hinge side is your reference — get it plumb and the rest follows. Insert shims behind each hinge location from both sides.
- Check plumb on hinge side. Adjust shims until the hinge jamb reads perfectly plumb top to bottom.
- Set the latch side. Insert shims at the latch strike location and at the top and bottom of the latch jamb. Adjust until the gap between door and jamb is consistent — typically 1/8".
- Check the head jamb. Shim the top jamb to maintain consistent reveal across the top of the door.
- Test the swing. Hang the door and test the swing before fastening permanently. The door should swing freely and latch without force.
- Fasten and trim. Drive fasteners through jamb and shims into framing. Snap or score off excess shim material flush with the jamb face.
FAQs
How do you shim a window correctly?
Insert shims in pairs from opposite sides at hinge points, latch points, and corners. Adjust until the frame reads plumb and level. Use Glazelock interlocking shims to prevent slipping. Fasten through the frame and shims into rough framing, then trim or snap off excess.
What is the difference between U-shaped and horseshoe shims?
They are the same thing. Horseshoe and U-shaped both describe the open-slot design that slides around a fastener or bolt without removing it. Glazelock uses both terms interchangeably.
How many shims do I need per window?
A standard double-hung window typically requires 6–8 shim locations. A standard door requires 8–12 shim locations. Buy in bulk packs for multi-window projects.
Can I use plastic shims for exterior window installation?
Yes. Glazelock plastic shims are moisture-resistant and UV-stable — making them superior to wood shims for exterior applications. Wood shims absorb moisture, rot, and compress over time, causing frames to shift.
Are Glazelock shims Made in USA?
Yes. Glazelock shims have been manufactured in the United States since 1997.
More Glazelock Resources: Glazelock Shims Hub | Shop Glazelock Shims | Learn From Tom
