New windows often get attention for their looks and energy savings, but the little things behind the frame matter just as much. One of those small, unseen parts is the shim. This simple spacer helps support the window frame and keeps everything square and level. When it’s used properly, the results are smooth operation and fewer long-term problems. When it’s skipped or done poorly, the result can be leaks, drafts, cracks, or worse.
Proper shimming is one of the most overlooked parts of the job, even though it supports nearly every part of the window’s function. That’s why our team takes this step seriously during every job. If you’ve ever had a window that won’t open right, won’t seal, or lets in noise, poor shimming might be the reason.
Why Shims Are Critical for Long-Term Window Support
Shims are small pieces of plastic or wood that fill the gaps between the rough opening and the window frame. These gaps are never perfect. They shift slightly depending on the material, house age, or even humidity. So, we place shims in specific spots to level and support the window exactly where it needs it.
Over time, gravity and weight will test those points. If the frame is not properly supported, the window can sag. This sagging makes it harder to close and opens gaps for air and water. Eventually, even the frame itself can crack. For this reason, we always test our shims before fastening anything.
Even a small error in alignment can affect the seal or cause stress on one corner. That’s why a windows replacement company in Calgary that focuses on structural support makes such a difference for long-term performance.
Signs That a Window Was Poorly Shimmed
Most of the signs aren’t obvious until after installation is finished. One of the first things you might notice is a window that sticks or tilts slightly when it opens. That usually means one corner sits lower than the others, and the sash isn’t square anymore.
Another common clue is uneven spacing between the window frame and trim. We see this when shims were either missed entirely or used in the wrong spots. Over time, this kind of pressure might lead to small cracks in the drywall around the window or visible gaps where caulking pulls away.
Sometimes, even a newly installed window can feel drafty. That’s not always a problem with the glass. Often, the frame is out of square due to poor shimming, which prevents a full weather seal. That’s why vinyl windows in Calgary only perform well when they’re properly shimmed and squared from the start.
How We Choose the Right Shim Placement
Every window opening is slightly different. That means we never use a fixed shim pattern. Instead, we check each side of the frame using a level and laser measure before we place anything. Then, we support the weight at key pressure points: corners, mullions, and latch areas.
Shims go in before the window gets screwed or nailed into the frame. If we don’t support those zones first, the fasteners can pull the frame out of alignment. After placement, we always double-check our work by opening and closing the window several times. If anything rubs or binds, we adjust the shims.
We also stagger the shims to avoid pressure buildup in one spot. That prevents warping and keeps the frame straight during expansion and contraction. If there’s any bowing or twist in the wall opening, we correct it at this step, not after. That approach protects the investment before sealing anything in.
The Role of Shimming in Preventing Water Intrusion
One of the most serious risks from improper shimming is water damage. Gaps behind the frame may not seem like much at first, but they’re the perfect place for water to settle. During freeze-thaw cycles in Calgary, that moisture expands and contracts until it finds its way inside.
When we shim correctly, we leave space for drainage while still providing firm support. This is especially important at the sill, where weight and water often collect. A tight fit alone isn’t enough. Water needs to escape, or it eventually soaks the wood around it.
We also check the slope under the window before placing shims. If water can’t run outward, it will pool behind the frame. That’s why shimming plays a hidden role in moisture control, especially when paired with a sloped sill pan and flashing tape.
Common Mistakes We Avoid During Shimming
Sometimes we’re called to fix windows that were installed just a few years ago, and the same problems keep showing up. Most of the time, it comes down to rushed or incorrect shimming. The most common mistake we see is placing all shims in one spot. That leads to pressure imbalance.
Others forget to check level and plumb after each shim, which results in a twisted or bowed frame. Even worse, some cut corners by forcing the window into the space without enough support, relying on spray foam to fill the gaps. Foam expands and can shift the frame if there’s no proper support underneath.
To avoid these problems, we check the frame before and after fastening, and we trim excess shim material so that insulation and flashing fit properly. Those steps take time, but they save the customer from leaks, gaps, and stuck windows later.
Why Shimming Vinyl Windows Requires Extra Attention
Vinyl window frames are more flexible than wood or metal. That’s good for energy efficiency and weight, but it also means they can warp if they’re not evenly supported. We pay closer attention to shim spacing with vinyl because a small sag can affect the entire sash alignment.
If the middle of a wide vinyl window isn’t supported, it might bow outward or inward during temperature swings. That leads to air leaks even when the window looks fine from the outside. We always shim under vertical supports inside the frame and secure each one carefully.
Because of that flexibility, we also avoid overtightening screws. Shims help spread the pressure, but we still check every inch of the frame for distortion before finishing. For customers choosing vinyl windows in Calgary homes, that extra care makes a big difference in comfort and performance.
What Happens When There Are No Shims at All
In some cases, a window is installed with no visible shims. This usually means someone tried to save time or skipped inspection. Without support, the window relies entirely on nails or screws to hold its position, which puts uneven stress on the frame.
The result is often movement over time. One season, the window works fine. The next season, it’s stuck, tilted, or has a gap at one corner. These shifts are small at first but grow worse with expansion and settling.
Eventually, the seal fails, or the window begins leaking. We’ve even seen windows fall inward after a few years because nothing was supporting their weight from below. That’s why proper shimming isn’t optional—it’s the foundation of the install. Without it, even the best window can fail early.
FAQs
What is a shim in window installation?
A shim is a small spacer used to adjust and support a window frame so that it stays level and square during and after installation.
How can I tell if my window was shimmed correctly?
The window should open and close smoothly, with even gaps around the edges and no drafts or tilts.
Can bad shimming cause a window to leak?
Yes. Improper shimming can leave gaps where water gets in or allow the frame to sag, breaking the weather seal.
Do vinyl windows need special shimming?
They do. Vinyl frames are flexible and need even support across key points to prevent bowing or warping.
Is foam insulation a replacement for shimming?
No. Foam fills gaps and provides insulation, but it doesn’t support the window’s weight like a shim does.
Need expert help ensuring your window installation is properly supported? Contact us for window replacement services in Calgary.