Support Your Window While Keeping Drainage Paths Clear
By Kevin Cody | Founder & CEO, FLI Products / Mary Joan Isorena, Communications
Published: October 6, 2026
Why use horseshoe shims under a window sill? Shims help support and position a window, but they must be placed without interfering with drainage paths or weep openings. The right horseshoe shim provides controlled spacing while allowing installers to work around compatible installation features.
For window glazing and installation applications that call for this type of spacer, GlazeLock® Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide combine an open U-shaped profile with stackable thickness control and snap-off length adjustment.
GlazeLock is a partner brand, sold by FLI Products.
For more GlazeLock shim guides and product resources, visit the GlazeLock Hub.
Key Takeaways
- Shims provide support and help establish the position required for a level, plumb and square window installation.
- Required weep holes, channels and other drainage provisions must remain unobstructed.
- A horseshoe shim’s open U-shaped profile does not create or replace a window’s drainage system.
- Follow the specific window manufacturer’s support, shim and fastening locations.
- GlazeLock Snap-Off Stackable Horseshoe Shims are 1-7/8" wide with a 5/8" slot and are made from high-impact plastic.
- Stacking adjusts thickness; snap-off sections adjust length.
- Applications include window glazing, glass installation, door installation, cabinet alignment and general construction spacing and leveling.
Why Are Shims Used Under a Window?
A window opening and the window unit do not always meet at exactly the position required for installation. Shims provide controlled support or spacing where the window manufacturer’s instructions call for it.
During installation, position the window correctly and verify that the unit is level, plumb and square before final fastening. Published installation instructions use shims as part of this positioning process. Servnix installation instructions
The sill deserves particular attention because it may do more than support the bottom of the window. Depending on the design, the assembly may use weep holes, channels or other provisions to direct water toward the exterior. Installation materials must not interfere with those intended drainage paths. Certain JELD-WEN installation instructions direct installers to keep weep holes and channels clear. JELD-WEN instructions hosted on Manualzz
Support the window where required while preserving the drainage provisions specified for that particular system.
Why Use a Horseshoe Shim for Window Installation?
A horseshoe shim differs from a solid rectangular plate or tapered wedge because it has an open U-shaped slot. That geometry can be useful when a flat spacing or support piece needs to fit around compatible hardware or other installation features.
GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide have:
- 1-7/8" width;
- 5/8" slot;
- a horseshoe/U-shaped profile;
- snap-off sections;
- stackable construction;
- high-impact plastic construction; and
- assorted colors.
They are Made in USA and have been manufactured in the USA since 1997. Applications include window glazing and glass installation.
Shop GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide
How Does Window Drainage Affect Shim Placement?
A shim provides support or spacing; it is not the window’s drainage system. Window assemblies may incorporate drainage paths, weep holes or channels that allow water to move toward the exterior. Those features must remain functional after installation.
Instructions for certain JELD-WEN windows require clear weep holes and channels. Some replacement-window installations also require openings or notches to correspond with existing weep locations. Installation instructions hosted on Manuals+
AAMA guidance for certain replacement-window installations similarly calls for openings aligned with existing weep holes so moisture can escape and warns against blocking existing weep openings. AAMA replacement-window guidance hosted on Twin Springs Blog
There is no universal horseshoe-shim location that guarantees drainage. Correct support locations and drainage requirements depend on the window and installation system.
How to Place Horseshoe Shims Without Blocking Window Drainage
The exact shim layout must come from the window manufacturer’s instructions. Use the following checks when horseshoe shims are part of the specified installation.
1. Check the Window Manufacturer’s Instructions First
Identify the requirements for the exact window before deciding where to place a shim.
- sill support;
- shim locations;
- fastening locations;
- weep holes;
- drainage channels;
- sill adapters or pans;
- required gaps or clearances; and
- sealant placement.
Do not substitute a generic layout for the window manufacturer’s details. Different systems can require different sill preparation and support arrangements.
2. Identify Weep Holes and Drainage Paths
Locate visible drainage features and any additional drainage requirements shown in the instructions. These may include holes, channels, notches, gaps or other pathways.
For some replacement-window systems, instructions require drainage openings to remain unobstructed. One Pella replacement-window configuration requires sill filler notches to correspond with existing weep holes and warns against blocking them. Consider drainage before final shim placement.
3. Identify the Required Support Locations
Determine where the window needs support. Do not simply push shims into every visible gap.
Manufacturer instructions may specify positions based on construction, fastener locations, mullions or other structural points. One JELD-WEN guide directs shimming at specified locations and checking the installed window for square, level and plumb. Follow the requirements for the actual window.
4. Choose a Shim That Fits the Required Space
Once the support point is identified, determine the shape and dimensions needed. A horseshoe shim provides an open slot rather than a completely solid rectangular profile.
These GlazeLock shims are 1-7/8" wide with a 5/8" slot. Their open slot can fit around compatible pipes, bolts and irregular surfaces.
5. Build the Required Thickness
The stackable design allows multiple shims to be layered for controlled thickness adjustment. Build the spacing required by the installation rather than treating every gap as though it needs the same thickness.
The objective is the required support and positioning, not simply adding more shims.
6. Adjust the Length When Needed
Snap-off sections allow the length to be adjusted. Excess material can be removed after installation for a cleaner finish. This can help when shim material extends beyond the desired installation area.
7. Keep Required Drainage Openings Clear
Before final fastening, inspect the sill again. No shim or other material should cover a weep hole, channel or opening that must remain clear.
Sealants and other materials must also stay clear of required drainage paths.
8. Check the Window Before Finishing
Verify the window according to its installation instructions. Checks can include:
- level;
- plumb;
- square;
- frame alignment;
- sash or panel operation; and
- required drainage openings.
If adding or adjusting a shim changes the position, recheck the unit before completing installation.
Does the Open Horseshoe Shape Create Window Drainage?
No. The U-shaped profile is a feature of the shim. It does not create, replace or guarantee the window’s drainage system.
The window’s water-management design determines where water travels and exits. A horseshoe shim supplies a flat spacing surface with an open slot, but it still needs to be positioned according to support and drainage requirements.
The shim provides spacing, support or alignment. The window drainage system manages water according to the window’s design. A good installation allows both to perform their intended functions.
Why Stackable Shims Can Help With Window Alignment
Not every gap requires the same adjustment. Multiple compatible GlazeLock shims can be combined for thickness control when setting or aligning a window.
Stackability controls thickness, not placement. Where a shim belongs still depends on the window’s requirements.
Why the Snap-Off Design Matters
A shim may provide the required support but extend farther than needed. Sections designed to break off allow the length to be adjusted and excess material to be removed for a more flush finish.
Stacking controls thickness. Snapping controls length. Combined with the horseshoe profile, these features provide adjustable spacing for compatible glazing, window and construction applications.
GlazeLock Snap-Off Stackable Horseshoe Shims: Key Specifications
GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide are designed for professional glazing, window installation and spacing applications.
| Specification | GlazeLock Snap-Off Stackable Horseshoe Shim |
|---|---|
| Width | 1-7/8" |
| Slot | 5/8" |
| Shape | Horseshoe / U-shaped |
| Design | Snap-off sections + stackable |
| Material | High-impact plastic |
| Colors | Assorted |
| Origin | Made in USA since 1997 |
| Primary applications | Window glazing, glass installation, door installation, cabinet alignment, general construction spacing and leveling |
Multiple length and package configurations are available. Check the product page for the option required for your installation.
Shop GlazeLock Snap-Off Stackable Horseshoe Shims
Common Window-Shimming Mistakes to Avoid
Blocking a Weep Hole or Drainage Channel
Do not cover a drainage opening that the manufacturer requires to remain clear. Weep holes and channels allow water drainage.
Assuming the Horseshoe Slot Automatically Provides Drainage
The slot is part of the shim’s geometry, not a built-in window drainage system. It does not replace the window’s required drainage locations.
Using a Universal Shim Pattern for Every Window
A position appropriate for one installation is not automatically correct for another. Follow the instructions supplied for the particular window.
Adding Shims Without Rechecking Alignment
Changing thickness changes the window’s position. After adjustment, recheck the required alignment and operation.
Blocking Drainage With Other Installation Materials
Sealant, foam, fillers and other materials must also stay clear of areas intended to remain open.
Horseshoe Shims vs. Solid Shims Under a Window
A horseshoe shim provides a flat support profile with an open U-shaped slot. A flat plate shim provides a solid rectangular surface. A tapered wedge changes thickness along its length.
The appropriate design depends on the physical requirements of the installation.
The broader GlazeLock range includes horseshoe/U-shaped, interlocking, snap-off stackable, flat plate, tapered plastic and natural pine configurations.
For a comparison, read GlazeLock Shims Explained: Types, Sizes & How to Choose the Right Shim.
The Bottom Line: Support the Window Without Blocking Its Drainage
When installation requirements call for this type of shim, a horseshoe profile provides controlled support and spacing while its U-shaped geometry accommodates compatible features.
The opening itself is not the window’s drainage system. Identify required support points and drainage provisions, select the appropriate size and thickness, and keep required drainage paths clear.
GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide provide a 5/8" slot, stackable thickness control, snap-off length adjustment and high-impact plastic construction. They are designed for applications including window glazing and glass installation and are Made in USA.
Shop GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide
GlazeLock is a partner brand, sold by FLI Products.
Frequently Asked Questions
Why are horseshoe shims used under windows?
Horseshoe shims can provide controlled support and spacing where their U-shaped profile suits the installation. Correct locations depend on the window manufacturer’s support, fastening and drainage requirements.
Do horseshoe shims help with window drainage?
The open profile can leave space around compatible installation features, but it does not create or guarantee drainage. Required weep holes, channels and other drainage paths must remain unobstructed.
Where should horseshoe shims be placed under a window sill?
Place shims at the support locations required by the specific window manufacturer’s instructions. Do not obstruct a weep hole, drainage channel or other required drainage path.
Should you block window weep holes with shims?
No. Keep required weep holes and drainage channels clear.
What size are GlazeLock Snap-Off Stackable Horseshoe Shims?
These shims are 1-7/8 inches wide with a 5/8-inch slot. Multiple length and package configurations are available.
Can GlazeLock Snap-Off Stackable Horseshoe Shims be stacked?
Yes. Multiple shims can be combined for thickness control.
Why do GlazeLock Snap-Off Shims break into sections?
The sections allow length adjustment. Excess material can be removed after installation for a more flush finish.
What are these GlazeLock shims made from?
The 1-7/8-inch-wide shims are made from high-impact plastic.
Are these GlazeLock shims made for window installation?
Yes. Applications include window glazing and glass installation, door installation and leveling, cabinet alignment, and general construction spacing and leveling.
Are GlazeLock Snap-Off Stackable Horseshoe Shims Made in USA?
Yes. These shims are Made in USA and have been manufactured in the USA since 1997.
About Kevin Cody
Kevin Cody | Founder & CEO, FLI Products
Kevin Cody is the Founder and CEO of FLI Products and a product development, merchandising and retail industry veteran with more than 20 years of experience. His background includes merchandising leadership at The Home Depot as well as developing, sourcing, manufacturing, marketing and bringing consumer products to market through major retailers and online marketplaces.
Kevin works directly with manufacturers, inventors, retailers and consumers to develop practical products that solve everyday problems. His experience spans product design and development, retail merchandising, manufacturing, e-commerce, marketplace strategy and consumer product education.
Through the FLI Products Learning Center, Kevin shares practical, experience-based information designed to help consumers better understand products, solve problems and make informed purchasing decisions.
About FLI Products
FLI Products develops and markets practical consumer products designed to solve everyday problems at home, outdoors and on the job. Working directly with manufacturers, inventors and retailers, FLI Products focuses on thoughtful product design, practical functionality and useful solutions for consumers. Its products are available through FLIProducts.com and select major retailers and online marketplaces.
Sources & Resources
GlazeLock Snap-Off Stackable Horseshoe Shims — 1-7/8" Wide — product specifications, design, applications and Made in USA information.
GlazeLock Shims Explained: Types, Sizes & How to Choose the Right Shim — shim types, profiles and selection.
AAMA 2410-03: Installation of Windows With an Exterior Flush Fin Over an Existing Window Frame — replacement-window guidance hosted on Twin Springs Blog.
JELD-WEN Window Installation Instructions — support, shimming and drainage guidance hosted on Manualzz.
Pella Replacement Window Installation Instructions — sill spacers and weep-hole guidance hosted on ManualsLib.
For more GlazeLock shim guides and product resources, visit the GlazeLock Hub.
