Holes and cut-outs in tempered glass: the rules that prevent breakage

Tempering puts the glass surface into high compression and the interior into tension. That balance is what makes it strong. Cut into it, and you release the balance locally — which is why a hole too close to an edge fails, sometimes days later, on site, with nobody touching it.

The four numbers that matter

• Minimum hole diameter: at least the glass thickness. A 6 mm hole in 10 mm glass is not advisable; use 10 mm or more.
• Minimum distance from hole edge to glass edge: at least 2× the thickness, and never less than 3 mm in absolute terms.
• Minimum distance between two holes: at least 2× the thickness, measured edge to edge.
• Corner radius on rectangular cut-outs: at least 5 mm. Sharp internal corners are crack starters.

A worked example

A 12 mm tempered panel needs a fixing hole 20 mm from the edge, 8 mm diameter. Check: hole diameter 8 mm < thickness 12 mm — fails the first rule. Edge distance 20 mm < 2 × 12 = 24 mm — fails the second. This drawing will break in tempering or shortly after. The fix is either a 24 mm edge distance, a 12 mm hole, or a 10 mm panel.

What we do before cutting

Every drawing is checked against these rules before the cutting list is released. If something does not pass, we send back a marked-up drawing with the specific dimension that fails and two or three ways to resolve it. This happens on maybe one drawing in ten, and it is the cheapest conversation in the whole order.

Cut-outs, notches and slots

The same logic applies with more force. A notch in an edge is a stress concentrator; a slot is two notches facing each other. Both need radiused corners, and both need generous material around them. If a design needs a tight notch, the honest answer is laminated glass instead of tempered — it can be cut after lamination.

Rule of thumb: if you would not be comfortable with the same geometry in a sheet of ice, do not specify it in tempered glass.