Deflection limits for walk-on glass

Deflection is how much a glass pane bends when load is applied. For walk-on roof lights and glass floors, deflection matters as much as strength — a panel that does not break may still flex more than is acceptable for comfort, perimeter seals or drainage on an external terrace.

What is deflection?

When someone walks on a glazed panel, the glass deflects — it bows slightly under the imposed load. Structural glass design sets limits on that movement so the unit feels stiff underfoot and the edge detail is not over-stressed.

Deflection is influenced by:

• Span — longer or wider unsupported dimensions increase bend.
• Glass build — pane count, thickness and lamination stiffness.
• Support layout — continuous four-edge support is stiffer than partial support.
• Load magnitude — heavier imposed loads increase deflection.

Walk-on laminated builds are designed so that, within rated spans, deflection and strength criteria are both satisfied.

Why deflection limits matter

Comfort and perception

Walk-on glazing should feel solid underfoot. Noticeable spring or movement under normal walking can alarm users even if the glass is structurally adequate. Rated walk-on spans account for acceptable stiffness in typical foot-traffic use.

Perimeter seals

Walk-on units are bedded on setting blocks and sealed continuously at the edge. Large deflection cycles under load can stress sealant and gaskets, increasing maintenance and water ingress risk on external terraces.

Drainage and ponding

On roof terraces, water should drain away from the glass edge. If the local fall is marginal, additional deflection under load can encourage ponding at the perimeter — bad for seals and slip risk. Combine correct glass span with adequate roof fall (typically a minimum fall on the surrounding terrace finishes).

Deflection and span: stay within rated limits

Manufacturers publish maximum spans for a given walk-on build — at Vant Glass, configurable sizes run up to 1320 mm × 2820 mm within rated structural limits for the laminated unit supplied.

Exceeding those dimensions without engineering review risks:

• Higher deflection than the design allows.
• Reduced safety margin under concentrated loads.
• Seal and fixing details no longer matching the assumed support conditions.

If your opening is at or near the limit, confirm dimensions in the walk-on calculator before the structure is cast.

Relationship to load calculations

Load capacity asks: does the glass break or laminate fail under force?

Deflection checks ask: does the glass bend too much under that force?

Both are part of walk-on structural design. See walk-on glass load calculations for the load side of the same problem.

Support conditions and stiffness

Deflection is highly sensitive to how the glass is supported:

• Four-edge continuous bearing — standard for walk-on roof lights; stiffest common layout.
• Three-edge or cantilever support — fundamentally different calculation; not covered by standard catalogue spans.
• Point supports — require bespoke structural engineering.

Never assume a span table for one support layout applies to another.

Practical guidance for site

1. Build the opening square and level with continuous edge bearing.
2. Use the manufacturer’s setting block positions — uneven bedding increases local stress and movement.
3. Maintain terrace fall so water runs away from the unit.
4. Do not cut or modify the glass on site — span and deflection assumptions apply to the factory build only.
5. Specify anti-slip glass on wet external terraces to reduce slip risk if water is present.

When to involve the manufacturer

Contact the glass supplier when:

• Your opening exceeds published span limits.
• Support is anything other than standard four-edge bearing.
• You expect heavy concentrated loads on the glazed area.
• Deflection or bounce is a specific client concern on a long narrow panel.

Call 03330 902 592 or configure sizes at custom walk-on roof lights. Units are made in Britain with a 20-year guarantee and free UK mainland delivery.