Why both springs get replaced together
On a dual-spring door, both springs were fitted on the same day and have done exactly the same number of cycles. When one fails, the other is not healthy — it is simply a few weeks behind.
Fitting one new spring against one aged spring also unbalances the door, because the two no longer pull with the same force. The door lifts skew, one cable takes more load than the other, and the rollers grind against the track on the loaded side. Replacing the pair costs marginally more once and avoids a second call-out, a second labour charge and a door that never quite ran right in between.
What happens when a spring breaks
The door becomes its raw weight — commonly 80 to 140kg on a double sectional. An automated opener will strain, trip its overload and stop, or lift the door 100mm and give up. If the opener is strong enough to keep trying, it damages itself, the trolley, and eventually the top section of the door.
Manually, the door is now genuinely dangerous. A door part-way open on a broken spring is held by nothing but friction and the second spring, if there is one. It can come down fast and it does not stop for whatever is underneath it.
The right response is to stop. Leave the door closed if it is closed. If it is open, do not walk under it, do not put a car under it, and call for an emergency garage door call-out.
Signs a spring is about to go
The failure is instant but the warning is not. A spring on its way out shows up first in how the door behaves, and it is worth acting at that stage rather than the next one.
- The door feels heavier by hand than it used to, or will not stay at waist height
- It opens more slowly, or the motor sounds like it is straining on the way up
- You can see a gap in the coil, or the spring has separated into two visible sections
- The coils look rusty, stretched, or unevenly spaced along the length
- The door judders or jerks upwards instead of moving in one smooth run
- It slams the last stretch on the way down instead of settling