Plastic shims vs. wood shims — which should you use for window and door installation? Plastic shims every time for exterior applications. Wood shims absorb moisture, rot, and compress 5–15% over time — causing frames to shift and seals to fail years after installation. Glazelock plastic shims maintain their exact thickness indefinitely, interlock to prevent slipping, and are Made in USA since 1997.
Tom's Pick: Glazelock. I've pulled out wood shims from 10-year-old window installations that had compressed so much the frame had shifted a quarter inch. Never had that problem with plastic.
Tom's Real-World Advice
Wood shims have been the default for generations — they're cheap, they're at every hardware store, and every contractor has used them. But cheap upfront doesn't mean cheap long-term. I've seen wood shims fail in as little as five years on exterior installations. Moisture gets in, the wood swells and then compresses, and suddenly your plumb window isn't plumb anymore.
Glazelock plastic shims cost a little more per shim. But they last the life of the window — which is 20, 30, sometimes 40 years. Do the math. The shim cost is irrelevant compared to the cost of a failed window installation.
Head-to-Head Comparison
| Feature | Glazelock Plastic Shims | Wood Shims |
|---|---|---|
| Moisture resistance | ✅ Excellent | ❌ Absorbs moisture, rots |
| Compression over time | ✅ None | ❌ Compresses 5–15% |
| Precision thickness | ✅ Consistent | ❌ Varies by piece |
| Interlocking capability | ✅ Yes (Glazelock) | ❌ No |
| Snap-off customization | ✅ Yes | ✅ Yes (score and snap) |
| Reusable | ✅ Yes | ❌ Usually not |
| Made in USA | ✅ Yes (Glazelock) | Varies |
| Long-term performance | ✅ Superior | ❌ Degrades |
| Cost per shim | Slightly higher | Lower upfront |
Why Wood Shims Fail
Wood shims fail for three reasons:
- Moisture absorption. Exterior window and door installations expose shims to seasonal moisture. Wood absorbs water, swells, then dries and compresses — a cycle that degrades the shim over time.
- Compression under load. Wood fibers compress under the sustained load of a window or door frame. Even in interior applications, wood shims can compress 5–15% over years, causing frames to shift out of plumb.
- No interlocking. Wood shims rely on friction alone to stay in place. They can walk out over time, especially in high-vibration environments like doors that are slammed regularly.
Why Glazelock Plastic Shims Win
- Zero compression. High-impact polystyrene maintains its exact thickness under load indefinitely — no settling, no shifting.
- Moisture-proof. Plastic doesn't absorb water, swell, rot, or degrade in exterior conditions.
- Interlocking design. Glazelock's snap-together interlocking shims connect to each other, preventing movement after installation.
- Consistent dimensions. Every Glazelock shim is the same thickness — no sorting through a bundle to find matching pairs like you do with wood.
- Reusable. Remove, reposition, and reinstall without degradation.
- Made in USA since 1997. Invented by a professional window installer who needed a better solution.
When Wood Shims Are Acceptable
Wood shims are acceptable for interior, non-structural applications where moisture is not a concern — shimming interior cabinet bases, leveling furniture, or temporary blocking during construction. For any exterior window or door installation, or any application where long-term stability matters, plastic is the professional standard.
The Real Cost Comparison
A bundle of wood shims costs $3–5. A pack of Glazelock plastic shims costs more per shim. But consider the full cost of a failed installation — labor to remove and reinstall a window, moisture damage repair, new shims, new insulation, and potentially new trim. The shim cost is a rounding error compared to the cost of doing it twice.
Professional window installers who switch to Glazelock don't go back. The consistency, the interlocking stability, and the zero-compression performance make every installation faster and more reliable.
FAQs
Are plastic shims better than wood shims?
Yes, for most applications. Plastic shims don't compress, rot, or absorb moisture. Wood shims can compress 5–15% under load over time, especially in exterior applications where moisture is present. For any exterior window or door installation, plastic shims are the professional standard.
Do wood shims compress over time?
Yes. Wood shims can compress 5–15% under sustained load, especially in exterior applications where moisture cycles cause swelling and drying. This causes frames to shift out of plumb and level, leading to sticking doors, failed seals, and drafts.
Can plastic shims be reused?
Yes. Glazelock plastic shims are fully reusable. Remove, clean, and reinstall without degradation. Unlike wood shims, they won't split, crack, or compress when removed and reinstalled.
Are Glazelock plastic shims Made in USA?
Yes. Glazelock shims have been manufactured in the United States since 1997 — invented by a professional window installer who needed a better solution than wood.
Why do professional installers prefer plastic shims?
Plastic shims maintain consistent thickness indefinitely, don't absorb moisture or rot, and Glazelock's interlocking design prevents shims from slipping after installation. These properties make plastic the professional standard for exterior window and door installation.
More Glazelock Resources: Glazelock Shims Hub | Shop Glazelock Shims | How to Use Plastic Shims: Tom's Step-by-Step Guide | Learn From Tom
