Sash Window Journal.
Sash window restoration, with period detail.

How Do Sash Windows Work

The counterbalance mechanism uses hidden weights to make heavy glazed sashes feel weightless and stay positioned wherever you leave them.

Quick answer: Each sash connects via cords to cast iron weights hidden in pockets within the window frame. The weights (typically 2-4 kg each) exactly match the sash weight. When you lift the sash, the weights drop, creating perfect balance. This allows the sash to remain stationary at any position without latches or friction holds.

Last reviewed: 15 June 2026 | 3 min read

The counterbalance principle

A traditional sash window is perfectly balanced. Each sash weighs perhaps 8-12 kg (depending on size and glass thickness). Two weights totaling the same weight hang in concealed pockets on either side of the frame. Sash cords connect the sash to the weights via pulley wheels at the top of the frame.

When you lift the bottom sash, the weights drop. When you lower the sash, the weights rise. The forces exactly cancel out, making the sash feel weightless to operate.

The components

Sash cords: Traditionally waxed flax, now often synthetic. Attach to grooves on sash edges, run over pulleys, connect to weights. Eventually wear through at the pulley and need replacement.

Pulleys: Small wheels (brass or nylon) mortised into the top of the frame. Allow cords to change direction from vertical (weight pocket) to horizontal (along sash edge).

Weights: Cast iron cylinders, typically 75-100mm diameter, 200-300mm long. Sized by the window maker to match each specific sash.

Weight pockets: Concealed compartments in the box frame where weights travel. Access via pocket pieces (removable panels) for repairs.

Why this design works

The genius of the system is simplicity:

Common problems explained

Sash drops: Broken cord or weight disconnected. The sash is no longer counterbalanced.

Sash rises: Weights too heavy, or only one cord broken (imbalanced forces).

Sash sticks: Paint buildup, swollen timber, or misaligned guides. Not a mechanism fault.

Rattling: Gaps at meeting rail or loose sash in frame. Draught-proofing systems eliminate this.

  1. SPAB technical note on sash window repair
  2. Historic England, "Traditional Windows" (2017)

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