Glazing Packers vs. Setting Blocks vs. Shims — Professional Comparison Guide

Professional glazing installers and project managers often use the terms "packers," "setting blocks," and "shims" interchangeably — but they shouldn't. These are three distinct support methods with different purposes, materials, and placement strategies. Understanding the difference isn't pedantic; it's essential for compliant, durable installations that meet FENSA standards and Building Regulations.

This guide clarifies what each method does, when to use them, and how to choose the right combination for your installation scenario. Whether you're specifying materials for a high-wind coastal project, installing an aluminium frame with a deep rebate, or maintaining heritage timber sashes, the terminology and techniques matter.

Let's start with a quick answer: setting blocks are large load-bearing supports (typically at the sill); packers are thin spacers distributed around the frame perimeter; shims are temporary wedges for fine alignment. Most professional installations use all three methods — not one or the other.

Read on to understand when and how to use each, the scenarios that demand specific combinations, and why FENSA inspectors care about your terminology.

What Are Glazing Packers?

Glazing packers are thin, rigid plastic spacers that support the window or door frame within the opening, maintaining a uniform gap and proper alignment for sealant application. They are the foundational support method in modern glazing work.

Dimensions and thickness: Standard packers range from 1mm to 6mm thick, with 28mm width and 100mm length being the industry norm. The thickness you choose depends on the frame type, rebate depth, and structural requirements. For example, a shallow UPVC rebate might need 3–4mm packers, while an aluminium frame with a deeper rebate may require 5–6mm.

Placement: Packers are positioned inside the frame rebate, distributed around the full perimeter of the frame (top, sides, and bottom). Typical spacing is 300–400mm centres, with additional packers at corners and load points. They sit between the frame back and the reveal (opening edge), holding the frame at the correct depth and preventing it from tilting inward or outward.

Purpose: Packers serve multiple functions: they maintain uniform frame-to-opening gap, support the frame weight before final fixing, align the frame perpendicular to the opening face, and provide a solid base for polyurethane or silicone sealant. Without adequate packers, frames can sag, twist, or allow water penetration.

Materials: Professional packers are made from polypropylene — a rigid, waterproof plastic that resists compression, moisture, and UV degradation. This is critical; timber or soft materials will compress over time, allowing the frame to settle. Read our comprehensive guide to glazing packers for more detail on material selection and best practices.

Shop our full range of glazing packers in stock.

What Are Setting Blocks?

Setting blocks are larger support blocks, typically placed at the sill (bottom) of a frame, that bear the primary load of the glazing unit and frame assembly. Unlike packers, setting blocks are structural — they are the critical failure point if undersized or absent.

Dimensions and thickness: Setting blocks are typically 10–15mm thick and 50–75mm wide, with lengths ranging from 100–150mm depending on frame width. They are noticeably larger and firmer than packers, designed to handle concentrated loads without compression.

Placement: Setting blocks are installed at sill level, positioned under the frame at the bottom corners and at intermediate points (every 600–800mm for wider frames). They sit between the bottom of the frame and the sill structure (masonry, timber, or concrete), directly supporting the weight of the glazing unit and frame assembly. In some cases, installers place a second setting block internally (inside the frame rebate) to prevent the frame from dropping under load.

Purpose: The primary role is load-bearing. A double-glazed unit with a frame can weigh 80–150kg per square metre. Setting blocks distribute this weight safely to the sill structure, preventing frame sag, seal failure, and glass contact with the frame edges (which can cause thermal stress and breakage). Without adequate setting blocks, the frame will eventually settle, compressing packers and distorting the seal.

Materials: Setting blocks are made from plastic (polypropylene or neoprene-infused polymers), rubber, or synthetic elastomers. Many manufacturers offer "firmer" formulations than standard packers — some include reinforcing fibres — to resist compression under sustained load. Some traditional installations used timber or slate; modern practice strongly favours engineered polymers for consistency and durability.

Note: Setting blocks are not a substitute for packers. They work alongside packers. The setting block handles vertical load; packers handle alignment and sealant support across the full frame perimeter. Most professional installations use both.

What Are Shims?

Shims are thin, tapered wedges used for temporary fine-adjustment of frame alignment during installation. Unlike packers and setting blocks, shims are intentionally variable in thickness — they are wedge-shaped, allowing precise vertical or horizontal adjustment.

Dimensions and thickness: Shims are typically tapered, ranging from 0mm (at the thin end) to 3mm (at the thick end). They are narrower than packers (often 15–25mm wide) and are used sparingly, at specific adjustment points rather than distributed around the full perimeter.

Placement: Shims are inserted between the frame and the reveal, then adjusted (tapped in or out) to level and plumb the frame. Once the frame is correctly aligned and fixed, shims are often left in place or replaced with permanent packers. In some modern installations, shims are used only during the trial fit, then removed and replaced with rigid packers at final fixing.

Purpose: Shims provide precise levelling and plumbing. Apertures in buildings are often non-plumb, sloped, or out of square; shims allow the installer to adjust the frame to true vertical and horizontal planes before final fixing. This ensures the glazing units sit square to the frame, the glazing rebates are perpendicular to the sill, and the frame won't rock or bind during use.

Materials: Traditional shims are made from wood (typically pine or cedar). Modern alternatives include plastic (polypropylene) or composite materials. FENSA-compliant installations increasingly use plastic shims, as wood can compress, swell with moisture, or cause thermal bridging issues over time.

When to use shims vs. packers: Use shims during the trial fit and alignment phase. Once the frame is level, plumb, and square, replace temporary shims with permanent packers (or ensure packers are installed at those points). Do not rely on shims alone for permanent support; they are a temporary adjustment tool.

Side-by-Side Comparison Table

The table below compares the key characteristics of packers, setting blocks, and shims at a glance. This helps you select the right method (or combination) for your project:

Feature	Glazing Packers	Setting Blocks	Shims
Thickness Range	1–6mm (uniform)	10–15mm (uniform)	0–3mm (tapered)
Width	28mm (standard)	50–75mm	15–25mm
Typical Qty per Frame	12–16 (perimeter @ 300–400mm centres)	3–6 (sill level)	2–4 (temporary)
Primary Placement	Full perimeter (inside rebate)	Sill corners & mid-span	Temporary, at adjustment points
Primary Purpose	Frame alignment & sealant support	Load-bearing (prevent frame sag)	Fine alignment & levelling
Expected Lifespan	Life of frame (30+ years)	Life of frame (30+ years)	Installation phase only
Typical Cost (relative)	Low (£0.02–0.05 each)	Mid (£0.10–0.25 each)	Low (£0.05–0.15 each)

When to Use Each — Installation Scenarios

In practice, installers rarely choose one method in isolation. The scenarios below show how to combine packers, setting blocks, and shims based on frame type, aperture condition, and environmental factors.

Scenario 1: Standard UPVC double-glazing (residential, most common)

This is the bread-and-butter installation for most FENSA-registered glaziers. The frame sits in a typical masonry or timber opening, the sill is reasonably level, and wind loading is moderate. Recommended approach: 3–4mm packers distributed around the full perimeter, setting blocks at sill corners and mid-span (every 600–800mm on wider frames), and shims used during trial fit for levelling, then removed or replaced with packers. The combination ensures load-bearing support at the critical sill, frame alignment around the perimeter, and no residual misalignment. This meets FENSA and Building Regulations compliance and is the standard method taught in industry training.

Scenario 2: Aluminium frame with deep rebate (commercial, high-performance)

Aluminium frames are heavier than UPVC and have deeper, more complex rebates. Wind loading is often higher (pressure differentials across large glazed areas). Recommended approach: 5–6mm thick packers (to fill the deeper rebate), setting blocks at sill with a secondary internal setting block to prevent frame drop under sustained load, and precision shims for alignment. The thicker packers provide better structural support; the dual setting blocks reduce sag risk on large spans. This scenario is common in curtain walling and commercial shopfronts.

Scenario 3: Timber sash window (heritage, traditional)

Timber sash windows have shallow rebates, and heritage conservation standards often restrict material choice. Modern packer materials can look out of place; traditional timber shims blend visually. Recommended approach: minimal packers (1–2mm, often plastic imitation wood colour) or selective shim use for levelling, with no setting blocks (the sash weight mechanism supports the sash itself). This approach prioritises heritage authenticity while meeting modern performance standards. FENSA acceptance varies by region; confirm with your inspector before specifying.

Scenario 4: High-wind area (coastal, exposed)

Coastal properties experience sustained wind loading and salt spray. Frame racking (lateral movement) is a primary failure risk. Recommended approach: thicker packers (5–6mm) distributed more densely (250mm centres instead of 400mm) to resist racking, substantial setting blocks to prevent vertical movement, and careful frame levelling with shims. Consider reinforced packer materials (neoprene-infused or fibre-reinforced) for extra resistance to compression. This scenario demands more support points and firmer materials than standard installations.

Scenario 5: Sloped or non-plumb aperture

Older buildings and renovations often have sloped sills or out-of-square openings. Forcing a frame into a sloped aperture without compensation causes stress and seal failure. Recommended approach: use shims liberally during trial fit to bring the frame true (level and plumb, independent of the aperture slope), then install packers at the corrected position. This may mean thicker packers on one side of the frame and thinner on the other, or a combination of packers and selective shims. The key is that the frame itself is level and plumb, even if the aperture isn't. See our installation guide for step-by-step details on handling non-plumb apertures.

Why Terminology Matters (FENSA & Building Regs)

FENSA Building Regulation Approved Documentation and Building Regulations Technical Guidance both specify that frames must be "adequately supported using packers, shims, or equivalent methods." The terminology isn't accidental — it reflects a legal requirement.

FENSA Compliance: FENSA inspectors will ask to see evidence of adequate support. Many installers document this in warranty paperwork, photographs, or installation records. If you've specified "shims only" when the frame actually required setting blocks for load-bearing, you've created a compliance gap. Conversely, if you've listed "setting blocks" when you've actually used only packers, you've overstated your support method. Inspectors can identify the discrepancy during inspection or from photographic evidence, and they may flag the installation as non-compliant or request remedial work.

Building Regulations Thermal Movement: Modern Building Regulations emphasise that frame fixing must allow for thermal expansion and contraction. Rigid fixing (screwing directly through the frame into the aperture edge) can cause seal failure and glazing stress. Packers and shims provide compliant support by holding the frame within the opening without restricting movement. Setting blocks must be made from materials that compress slightly (plastic, rubber) rather than rigid materials, to allow the frame to move without constraint. Specifying the right material type is part of regulatory compliance.

Installer's Responsibility: As the registered installer, you are accountable for method choice and documentation. FENSA warranties typically require detailed records of the support method used. Vague descriptions ("standard packers") are less defensible than specific ones ("3mm PP packers @ 400mm centres, setting blocks at sill corners and mid-span, plastic shims at adjustment points"). If a dispute arises years later (e.g., frame sag, seal failure), detailed documentation can protect you and the customer.

Common Mistake: Many installers use the correct methods but describe them incorrectly in paperwork. For example, setting blocks might be placed at the sill, but the warranty says "packers only." Or shims might be left in place permanently (becoming packers) but listed as "temporary shims." These discrepancies can trigger re-inspection or denial of warranty claims if failure occurs. Accuracy matters.

FAQ

Q: Can I use just packers without setting blocks?

A: No, not in most installations. Packers alone will support the frame horizontally and maintain alignment, but they are not designed to bear the full vertical load of a glazed unit (80–150kg+). Without setting blocks, the frame will gradually sag under load, compressing the packers and distorting the seal. Setting blocks are the primary load-bearing method; packers are supplementary. The exception is very small frames (e.g., small fan lights) in low-stress positions, where packers alone may suffice — but FENSA expects documented justification.

Q: Are shims required by FENSA?

A: FENSA does not mandate shims specifically; it requires "adequate support." Many installations use packers and setting blocks alone, with no shims, if the aperture is already plumb and level. However, shims (or an equivalent levelling method) are essential whenever the aperture is out of plumb, sloped, or out of square. Skipping shims in a non-plumb opening will result in a crooked frame, which FENSA inspectors will reject. Use shims where the aperture requires it.

Q: What's the lifespan of packers vs. setting blocks?

A: Both should last the life of the frame (30+ years) if made from polypropylene or engineered polymers. Packers and setting blocks don't degrade significantly unless exposed to extreme UV (unshielded external use — uncommon for interior supports) or chemical attack. However, inferior materials (soft plastic, wood) can compress over time. The critical factor is material quality at installation. Use certified, branded products from reputable suppliers. Learn more about packer sizing and material selection.

Q: Can I use wood shims?

A: Traditional installations often used timber shims. However, modern FENSA best practice favours plastic or composite shims. Timber absorbs moisture, swells and shrinks, can compress under load, and may cause thermal bridging in modern high-performance frames. If you use timber shims, ensure they are treated (not raw) and replace them with plastic packers at final fixing. Leaving timber shims in place long-term is not recommended and may be flagged during FENSA inspection.

Q: Do I need to replace packers during the frame lifespan?

A: No, if installed correctly with quality materials. Packers are not consumable items. They sit inside the frame rebate, protected from UV and weather. The only reason to replace them is if the frame is being re-glazed (e.g., existing frame, new unit) or if the original installation used poor-quality packers that have compressed. In normal operation, packers remain intact for the frame's lifetime.

Conclusion

Understanding the difference between packers, setting blocks, and shims is foundational to professional, compliant glazing installation. Packers support frame alignment and sealant; setting blocks bear the glazing load; shims enable precise levelling during the installation phase. Most installations use all three methods in combination, selected and proportioned based on frame type, aperture condition, and environmental factors.

Accurate terminology in documentation protects you, your customers, and your FENSA warranty. Take the time to specify the method clearly: packer thickness, setting block placement, shim use. This simple discipline elevates your professionalism and reduces the risk of disputes or compliance challenges.

Ready to stock your van? Browse our full range of glazing packers — all sizes in stock, next-day delivery available.