How to Fit Glazing Packers — Step-by-Step Installation Guide for Professionals

Welcome to the professional glazing packer installation guide. This guide is designed for FENSA-registered window and door installers, glaziers, and trade professionals who need to master the proper fitting of glazing packers. Whether you're installing new frames, replacing units, or troubleshooting installations, correct packer placement is fundamental to creating lasting, weathertight installations. Proper packing ensures warranty compliance, prevents frame movement, eliminates rattles, and protects your customers' investment in quality windows and doors. This step-by-step resource covers everything you need to know—from pre-installation checks through to final sealant application and FENSA documentation. Let's get started.

Pre-Installation Preparation

Before you open a single packer box, thorough preparation is essential. Rushing this stage is one of the most common reasons for callback work and warranty disputes. Begin by measuring the aperture precisely: check the rebate depth (the recessed area where the frame sits), the width at three points (top, middle, bottom), and the height at three points (left, centre, right). Record these measurements in your installation notes. Next, inspect the frame condition. Look for any damage to the rebate edges, contamination, or moisture damage. If the frame is damaged, document it and advise your customer before proceeding.

If you're replacing an existing frame, carefully remove old packers and all debris from the aperture. Use a soft brush or compressed air to clean out dust, mortar residue, and old sealant. A clean aperture is critical—packers won't sit properly if dust or debris is present underneath. Now calculate the quantity of packers you'll need. As a rule of thumb, use the 400mm spacing rule (explained in the next section). For a standard 900mm × 1200mm frame, you'll typically need 12–16 packers depending on mullion divisions.

Finally, conduct a safety checklist before you begin. Ensure you have appropriate PPE: safety glasses (in case of frame splinters), work gloves, and closed-toe boots. If you're working at height, ensure proper scaffolding or ladder setup. Have your phone or radio on hand to call for assistance if needed. Take a moment to review the frame installation method (whether this is a cavity installation, solid wall, or timber frame) so you understand the fixing strategy before placing the first packer.

Packer Placement Strategy — The 400mm Rule

The industry standard for glazing packer spacing is 400–450mm apart around the entire perimeter of the frame. This spacing prevents frame deflection, manages thermal movement, and ensures even load distribution across the aperture face. The 400mm rule is recognised across FENSA, building regulations, and all major window manufacturers' installation guidance.

Start by marking packer positions on the frame before you install it. Use a pencil and a tape measure. Begin at one bottom corner and mark every 400–450mm along the bottom. Always place one packer at each corner (minimum). Then work up the left jamb, across the top, and down the right jamb, maintaining the same spacing. If your frame has mullions (vertical or horizontal divisions), place an additional packer on both sides of each mullion to manage the load independently.

For a typical 900mm wide × 1200mm tall frame with a central vertical mullion, a practical layout might look like this: bottom sill (3 packers at 0, 450, 900mm), left jamb (3 packers at 0, 600, 1200mm), right jamb (3 packers at 0, 600, 1200mm), top head (3 packers at 0, 450, 900mm), and mullion divisions (2–3 packers on each side). This gives approximately 14–16 packers total. Spacing that exceeds 450mm significantly increases the risk of frame racking, rattles, and long-term movement. Never space packers more than 500mm apart.

Step-by-Step: Installing Glazing Packers

Step 1: Position the frame into the aperture. With your packer positions marked and the aperture clean, carefully lift the frame into position. If you're working with a large frame (over 1.5m × 1.5m), always use at least two people or a mechanical hoist. Position the frame so the marked packer positions align with where packers will actually sit—usually at the rebate edge. The frame should rest lightly on temporary props or adjustable feet; do not fully tighten any fasteners yet.

Step 2: Prepare packer placement points. Using a small wooden spacer or straight edge, check that your marked packer positions are clear and accessible. If you're working with masonry or rough aperture edges, use a packing chisel to gently tidy up the surface immediately behind where each packer will sit. The packer must have solid, flat contact with both the frame rebate and the aperture face.

Step 3: Install packers at the bottom (sill level)—the most critical stage. This is where most installations fail if done carelessly. Begin at one bottom corner. Slide the first packer into position behind the frame sill, ensuring it contacts the rebate firmly. Use hand pressure only at this stage—do not hammer. Check that the packer is pushed all the way into the rebate and that it sits flush with no gaps. The packer should be tight enough that it doesn't fall out, but loose enough to slide by hand. Install all bottom packers (typically 3–4 for a standard frame) from left to right, checking each one sits evenly. Use a straight edge across all three to verify they're at the same height—this prevents the frame from tilting.

Step 4: Install packers at jamb level. Once the sill is packed, work up both sides simultaneously to maintain frame squareness. Install one packer at roughly mid-height on each jamb, then work towards the top. Again, use hand pressure only. Slide each packer in fully, check it sits flush, and do not over-tighten. After installing 2–3 packers on each side, check that the frame face is vertical using a spirit level. The frame should not rock or lean at this stage.

Step 5: Install packers at the head (top). Once both jambs are settled, complete the perimeter by installing top packers. Follow the same method: slide in, check flush contact, verify no gaps. Do not hammer or force any packer. If a packer is difficult to insert, check that the frame is square and that the aperture edge is clear. Forcing a packer is often a sign that the frame position is incorrect.

Step 6: Final alignment check. With all packers installed, spend 2–3 minutes verifying the entire frame. Use a spirit level on the sill, jambs, and head to confirm the frame is square and level. Check that the frame does not rock. Open and close any doors or casements gently—they should operate smoothly without binding. If you notice any movement, loosen a few packers, re-align the frame, and re-pack. Never proceed to fixing or sealing if the frame is not perfectly square.

Packer Fixing Methods — When and How to Secure

Once the frame is perfectly positioned and all packers are in place, you need to decide whether to fix (permanently fasten) the packers or leave them friction-fitted. For most installations in masonry or concrete, friction fitting is sufficient—the weight of the frame and the fixed frame fasteners (through-frame fixings or angle brackets) hold everything in place. Friction-fitted packers provide flexibility for thermal movement and are the preferred method in modern installations.

However, in some scenarios—particularly cavity wall installations or where the frame will be subject to significant wind loads—you may need to fix one or two packers at the sill to prevent any movement during the sealing and early cure phase. If you do fix a packer, use a stainless steel or galvanised screw driven carefully through the packer into the masonry or timber behind. Do not over-tighten or hammer screws, as this can crush the packer and prevent it from functioning properly. Use no more than one fixing per packer, and never fix all packers. The frame must remain free to move slightly as it thermally expands and contracts—this is critical for longevity. Never use silicone or foam to fix packers; these methods trap moisture and cause long-term problems.

Sealant Application Over Packers

Sealant application should happen only after all packers are installed and the frame is fully aligned. The packers remain inside the rebate—they must never be exposed to the exterior. Once the frame is secured and the opening is ready, apply sealant around the perimeter. The sealant depth should typically be 6–8mm for acrylic or polyurethane sealants. This provides flexibility and weatherproofing without being so thick that it creates an unnecessary barrier.

Before sealing, check once more that all packers are seated correctly and that the frame is level and square. Apply sealant in a continuous bead around the perimeter, running along the joint between the frame and the aperture edge. Use a wet finger or sealant tool to smooth the bead and ensure good adhesion. Do not apply sealant directly onto the packer; sealant around the frame perimeter will naturally flow over and seal the packer area. Once sealant cures (typically 24–48 hours depending on product), the installation is complete. The sealant provides the final weathertight seal, while the packers have already done the critical job of positioning and supporting the frame.

Common Installation Mistakes & Solutions

Even experienced installers can run into problems with packer fitting. Here are six of the most common mistakes and how to avoid or fix them:

1. Spacing too wide. Installing packers more than 500mm apart allows the frame to deflect, especially over large openings or where units are heavy. Result: rattles, frame movement, and potential warranty issues. Solution: Always measure and mark positions before installing. Use the 400mm rule strictly.

2. Over-tightening or hammering. Driving packers in with force or a hammer crushes them and prevents them from functioning as a load-spreading medium. It also creates voids behind the packer where the frame can still move. Solution: Always use hand pressure only. If a packer is hard to insert, stop and check the frame position or aperture condition.

3. Wrong packer size. Using a packer that's too thin leaves gaps; too thick creates excessive pressure on the frame and restricts thermal movement. Solution: Choose the right size first based on your rebate depth and aperture dimensions.

4. Uneven placement. Installing packers at different depths so the frame sits unevenly causes racking (frame distortion). Solution: Use a straight edge to verify all packers at the same location (e.g., all bottom packers) are level.

5. Leaving gaps between packer and frame. If the packer doesn't contact the rebate fully, it's not doing its job. Gaps allow the frame to move. Solution: Slide each packer all the way in; it should feel snug but not forced. Check visually that contact is full.

6. Installing packers outside the rebate. Exposed packers look unprofessional, absorb water, and may protrude into the finished reveal. Solution: Packers must sit inside the rebate, hidden once sealant is applied. Mark positions carefully before installing.

FENSA Compliance & Documentation

FENSA regulations require that all window and door installations are carried out by registered installers using recognised installation methods and materials. Proper glazing packer fitting is a fundamental requirement of FENSA-compliant installation. FENSA does not specify exact packer spacing in its rules, but industry standards (BS 6375 and similar) call for the 400–450mm spacing we've covered here.

When you complete a FENSA-registered installation, your documentation must include a record of the packing method, materials used, and that the frame is correctly positioned and aligned. This doesn't require photographs of every packer, but your install notes should state: "Frame packed at 400mm intervals, verified square and level before sealing." If a future issue arises, this documentation proves that proper installation procedures were followed. Warranty disputes often hinge on whether installation was carried out to recognised standards, so keep thorough notes. If you need to explain your packing method to an inspector or customer, a simple sketch or photograph of the packer layout is helpful.

Troubleshooting: What If Things Go Wrong?

Frame still loose or rattles after packing. If the frame moves or rattles after all packers are installed and sealant has cured, the most common causes are: (1) spacing is too wide, (2) one or more packers are not making full contact with the frame, or (3) the frame is not secured to the aperture (fixings not tightened). Solution: Carefully remove sealant around the problem area, check each packer visually, re-pack if necessary, re-square the frame, and re-seal. Check that all frame fixings (angles, brackets, or fasteners) are properly tightened.

Gaps appear in sealant after sealing. Small gaps sometimes appear during sealant cure as the material shrinks. This is usually minor and not a problem if the frame is secure. However, large gaps suggest the frame has moved during cure, or the sealant wasn't applied deeply enough. Solution: Once fully cured, apply additional sealant into any gaps. If gaps are large or recurring, re-examine the packer fit and frame security.

Frame tilts or rocks. A frame that tilts after packing usually indicates uneven packer placement or that one packer is not making full contact. Solution: Use a spirit level immediately. If tilting is detected, remove sealant, locate the problem packer, re-seat it, verify level, and re-seal.

Water ingress around packers. In rare cases, water can track into the frame around the packer area, usually if sealant is missing or if the packer is not fully sealed. Solution: Once you identify the leak point, dry the area completely, remove any failed sealant, re-pack if the packer has moved, and reapply sealant. Use a sealant rated for wet conditions (not just acrylic).

Frequently Asked Questions

Can I reuse old packers when replacing a frame? No. Old packers compress over time and lose their load-bearing capacity. Always use new packers when installing new frames. Reusing old packers is a common source of frame movement and warranty problems.

Do packers need replacing during the warranty period? If the original installation was done correctly with proper spacing and sealing, packers do not need replacing. A well-fitted packer should last the life of the window. If a customer complains of a rattle or movement, investigate the original packing method before assuming packers need replacement.

What's the difference between glazing packers and shims? Glazing packers (also called "shims" colloquially) are load-spreading wedges designed to support the weight of a frame and manage thermal movement. Shims is a broader term sometimes used for any thin wedge. For professional window installation, always use purpose-made glazing packers—they are calibrated for load distribution and made from materials that won't compress.

Can I use foam or rubber instead of plastic packers? No. Foam and rubber compress under load, absorb moisture, and can deteriorate. Plastic (typically high-density polyethylene or polypropylene) packers are industry standard because they maintain their shape, resist moisture, and spread load evenly. Always use recognised glazing packers.

How do I remove packers if I need to adjust the frame later? If the installation is not yet sealed, carefully remove sealant if any is present, gently tap the packer out from behind with a soft mallet, adjust the frame, and re-install the packer. If the frame has been sealed for weeks or months, removal is more difficult. Use a packing chisel to carefully prise the packer from the side, avoiding frame damage. Once removed, re-pack and re-seal the area. This is why getting the installation right the first time is so important.

Conclusion

Proper glazing packer installation is the foundation of lasting, professional window and door fitting. By following the 400mm spacing rule, taking time to pre-prepare, using hand pressure only, and verifying squareness at every step, you ensure installations that remain tight, rattle-free, and warranty-compliant for years to come. Your customers notice the difference—smooth operation, no movement, no callbacks. And your reputation is built on getting the details right the first time.

Ready to stock up on glazing packers for your next job? Browse our full range of glazing packers and find the exact size for your frame dimensions. Or, if you're new to our products and want to understand the different types available, check out our ultimate guide to glazing packers.

Questions? Questions? Our team is here to help. Reach out with any installation questions or material requests—we're here to support professional installers.