Setting blocks (glazing)
Setting blocks are EPDM/synthetic rubber pads in the bottom of a glass frame supporting the glass weight. AS 1288:2021 specifies hardness, position (quarter-points).
Ask Chalkline about this →Setting blocks in glass installation are small synthetic rubber (typically EPDM or silicone-modified) pads placed in the bottom of a glass-pane frame, supporting the entire weight of the glass while allowing differential thermal movement between glass and frame. They are mandated by AS 1288:2021 for almost all framed glass installations. Without setting blocks, the glass weight rests directly on the frame’s metal or timber edge, creating point loads that stress-crack the glass over time and resist thermal movement, causing further cracking under temperature cycling. Setting blocks are tiny but critical (verified 2026-05-16).
What setting blocks do:
| Function | Mechanism |
|---|---|
| Carry glass weight | Distribute load over a controlled contact area; reduce point loading |
| Allow thermal movement | Glass expands at ~9 ppm/°C, aluminium at ~24 ppm/°C; setting blocks slide slightly to accommodate the differential |
| Isolate glass from frame chemistry | Prevent direct contact between glass and frame materials (some sealants, paints, treated timber chemicals can cause glass damage) |
| Compress/relax with seasonal change | Rubber/EPDM absorbs micro-movements without transferring stress |
Specification (AS 1288:2021):
| Property | Requirement |
|---|---|
| Material | EPDM, silicone, neoprene (synthetic rubber); compatible with glass sealant |
| Hardness | Shore A 80-90 (specific durometer rating) |
| Length | 50-100 mm typically; larger units (heavier glass) use longer blocks |
| Width | At least full thickness of the glass + setting tolerance |
| Height | 3-6 mm typically; provides the clearance between glass and frame base |
| Position | Quarter-points of the glass length (25% and 75% of the bottom edge) |
| Quantity | 2 setting blocks per glass pane (typical residential up to 1.5 x 2.5 m); larger panes may need 3-4 blocks |
Why quarter-points?
The glass acts as a simply-supported beam. The quarter-point positions:
| Position | Why |
|---|---|
| Centre of the glass | High bending stress; glass cracks at concentrated point load |
| At the ends | Glass can rotate inward under deflection; ends are weaker; cracks at the corners |
| Quarter-points (this) | Balanced load distribution; minimal bending stress at the support; ends are slightly cantilever and free to thermally expand |
Common defects:
| Defect | Consequence |
|---|---|
| Setting blocks omitted entirely | Glass weight on the frame; stress cracks within months; thermal movement creates corner spalls |
| One setting block instead of two | One end carries full load; unbalanced; visible deflection; failure |
| Wrong material (PVC, polythene) | Hardens with UV and chemical exposure; loses cushion within 3-5 years; reverts to no-setting-block condition |
| Block too soft (Shore A 50 or less) | Compresses fully under glass weight; loses cushion; glass meets frame |
| Block in the wrong position | If at centre or at ends, the glass cracks |
| Setting blocks dirty or wet at install | Reduced adhesion to frame channel; can shift over time |
| Block height too low | Glass meets frame even with blocks present; same as no blocks |
The “stress crack” failure mode:
When setting blocks are missing or compromised:
- Glass weight (300-500 N for a typical residential pane) concentrates on 1-2 contact points with the frame.
- Stress at the contact concentrates to 50-100 N/mm² locally.
- Thermal cycling adds tensile stress at the cooled edge in winter and the heated edge in summer.
- Cumulative stress exceeds glass tensile strength at the contact point.
- Glass starts to crack at the bottom edge, often progressing toward the centre over weeks or months.
- By the time the homeowner notices, the crack is long and the pane needs replacement.
Inspection:
- Visual: lift the glazing bead at the bottom of the frame; verify two setting blocks visible at quarter-points.
- Replacement: if blocks are degraded, replace before re-glazing; the blocks are cheap.
- Pane replacement: any pane replacement should include new setting blocks; reusing degraded blocks defeats the new pane.
Setting blocks for IGUs:
Insulated glass units (IGUs) require block contact only on the structural pane (typically the inner pane in a double-glazed unit), not on the secondary seal at the perimeter. Improper setting blocks can compress the secondary seal and cause early IGU failure (cavity fogging). Manufacturer’s instructions for IGU setting are explicit.
Builder takeaway:
- Setting blocks are non-negotiable for any framed glass installation.
- Specify EPDM or silicone-modified setting blocks per AS 1288.
- Two blocks at quarter-points is the standard.
- If you see cracked glass at the bottom edge, the diagnostic is often “missing or failed setting blocks”.
- During pane replacement, always renew setting blocks.
Also known as: glazing blocks; bottom blocks; glass support blocks; bedding blocks (older term, sometimes); pane bearers.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.