Published · Clear Vision Glass & Glazing
Almost every week someone tells us a crack has been in a window for months and has not moved, so it must be fine. It is not fine — it is waiting. Annealed glass under a crack is in a fundamentally different state to intact glass, and the question is when it runs, not whether.
Why cracks spread
Glass fails at its flaws. A crack is a very large flaw with an extremely sharp tip, and stress concentrates enormously at that tip. Any additional load — a hot afternoon expanding the pane, a cold night contracting it, a door slammed hard enough to flex the frame, or the frame itself moving as the house responds to seasonal soil movement — pushes that tip a little further.
That is why cracks appear to sit still for weeks and then travel across the whole sheet overnight. Nothing dramatic happened. The accumulated stress simply reached the point where the crack tip advanced, and once it starts moving it tends to keep going.
The three causes we see most
- Impact — a ball, a stone from a mower, a bird, or a door handle into a panel. Usually obvious, with a point of origin you can see.
- Thermal stress — a clean, gently curving crack that starts at the edge of the pane with no impact point. Caused by part of the pane heating far faster than the rest, usually where a dark internal blind, furniture or a sticker traps heat against the glass.
- Frame movement — the frame twists or settles and puts point load onto the glass. Common in reactive clay soils, and it is why a replacement pane sometimes cracks again in the same spot.
Why the early fix is the cheap one
A cracked but intact pane is a scheduled job. We measure, cut the glass, and come back — one visit, standard rate, done at a time that suits you.
The same pane after it lets go is an emergency call-out. Someone has to attend quickly, clear broken glass out of the frame and off the floor, and make the opening safe. If the glass has to be ordered, that is a board-up as well, and then a second visit to glaze it. You have paid for a make-safe you did not need and lost the ability to choose when it happened.
There is also the part that does not show up on an invoice. Glass that fails without warning fails at an unhelpful moment — a windy night, while you are away, or with children in the room.
When a crack means more than a new pane
If a window has cracked twice in the same place, replacing the glass again will not solve it. Repeat cracking in one spot points at either frame movement putting load on the pane, or a thermal stress pattern that will simply repeat with the next sheet of annealed glass.
The fix depends on which it is. Frame movement needs the frame addressed — packed, adjusted or, occasionally, a builder's opinion on what the house is doing. Thermal stress is solved by changing the glass: toughened glass tolerates far higher temperature differentials across a pane and will not repeat the failure.
It is also worth checking whether the pane should have been safety glass in the first place. Glass in or beside a door, below 500mm from the floor, or in a bathroom must be Grade A safety glass. A surprising number of older homes across Pakenham, Berwick and the hills have ordinary annealed glass in those positions because it was replaced cheaply at some point. A crack is a good moment to put that right.
Want this looked at?
We’re glaziers based in Pakenham servicing Cardinia Shire and the City of Casey. Free quotes, and for most jobs a photo is enough to get a price.
