How Sash Windows Work
The hidden mechanics inside London's most recognisable windows — and why a heavy sash stays open with nothing holding it up.
Sash windows are one of the most recognisable features of traditional London architecture. Their design combines simplicity, functionality and durability, making them a long-lasting solution for ventilation and light control.
If you're considering repairs or upgrades, visit our timber sash window services page or speak with our London sash window fitters.
The Basic Structure
A sash window consists of two movable panels, known as sashes, which slide vertically within a frame. These sashes are held in place using a counterbalance system. In a traditional box sash window the sides of that frame are hollow — and that hidden space is where the counterbalance lives.
The Secret
A heavy sash stays open because it is perfectly balanced. Weights hidden inside the frame are matched to the weight of the sash they carry, so the sash neither falls nor rises — it stays exactly where you leave it.
The Weight and Pulley System
Each sash is connected to a hidden weight via a cord or chain. The weight balances the sash, allowing it to be raised or lowered smoothly. This system eliminates the need for hinges and allows precise positioning.
Inside each side of the box frame hang weights, made of cast iron or lead. A cotton cord is fixed to the side of the sash, runs up and over a brass pulley, and drops down to its weight. As the sash goes up, its weight comes down — and the other way round.
Four Cords. Four Pulleys. Four Weights.
Cotton Cords
A cotton cord is fixed to each side of each sash, so every sash hangs on two. This is the part that wears, and the part that snaps.
Brass Pulleys
Each cord runs up and over a brass pulley at the top of the frame, then drops back down inside the hollow box to its weight.
Matched Weights
Cast iron or lead, hung inside the frame. Each pair of weights is matched to the sash it carries, so the sash is perfectly balanced.
Two Tracks and Controlled Airflow
A thin parting bead — a strip of timber set into the frame — keeps the two sashes on their own tracks. That is what lets the top sash slide down while the bottom sash slides up, without the two ever rubbing against each other. We explain it in more detail in our guide to the sash window parting bead.
Ventilation Benefits
One of the key advantages of sash windows is their ability to provide controlled airflow. Opening both the top and bottom sashes allows warm air to escape while cooler air enters, improving ventilation.
To better understand different designs, see our guide on identifying sash window types.
When a Sash Cord Snaps
Over time, components such as cords, pulleys and weights can wear out. This can cause sticking, rattling or imbalance. These issues are usually resolved through professional refurbishment.
Snap a cord and the sash drops, because the weight on that side is no longer holding it up. The fix is simpler than most people expect: we open the pocket at the bottom of the frame, lift out the weight and fit a new cord.
No New Window Needed
A snapped cord is a repair, not a replacement. See our sash cord replacement service, or our guide to a stuck sash window if yours is sticking, rattling or won't stay up.
Modern Improvements
Today's timber sash windows, particularly those made with Accoya wood, retain the traditional mechanism while offering improved durability and insulation. It is a design over three hundred years old, and still working today. Some modern sash windows use spring balances in place of weights and cords, but the weight-and-cord system is the original design — and the one we keep and restore on period homes.
If you are deciding between repair and replacement, our guide on refurbishing sash windows can help.
Understanding how sash windows work helps homeowners make informed decisions about maintenance, upgrades and replacements — ensuring long-term performance and reliability.