Glass Thickness Explained: When 4mm Is Not Enough

Most domestic sealed units are built on 4mm glass, and for average window sizes that is exactly right. But thickness is a structural decision, not a habit — and three things should push you up a step.

1. Pane size and deflection

Glass behaves like a plate: double the span and deflection under wind load rises fast. As a working rule, once the shorter edge of a pane passes about 1100mm, or the area passes roughly 1.2m², it is time to consider 6mm. Large picture windows and sliding-door panels routinely need 6mm or 8mm simply to stay acceptably stiff.

2. Exposure and wind load

The same pane that is fine in a sheltered street may be undersized on a hilltop or coastal plot. BS 6262 gives the calculation route for wind loading on vertical glazing; when in doubt, we will size it for you from the postcode and elevation.

3. Sound

Thicker glass shifts a pane’s resonant behaviour, and mixing thicknesses — say 6mm outer with 4mm inner — breaks up coincidence dips so the unit performs better across traffic-noise frequencies. If acoustics drove the enquiry, asymmetry beats simple thickness.

Weight matters too

Stepping from 4mm to 6mm adds roughly 5 kg/m² per pane. On big units that changes how many people you need on site and what the hinges or gaskets must carry — check any build-up in seconds with the glass weight calculator before you order.