What's inside a box sash: cords, weights and why they still matter

A box sash window is a small machine, and it's been essentially perfect for about three hundred years. Here's what's inside, and why we still build it the original way.

The box. The "box" in box sash is the hollow frame either side of the window. It isn't decorative — it's the housing for the counterweights. That's why a genuine box sash has a substantial, dignified frame where a modern casement is thin: the frame is doing a job.

The sashes. The two sliding panels of glass and timber. In traditional patterns the upper sash sits outside the lower; each runs in its own channel, separated by a slim timber strip called the parting bead, and held by the staff bead you see from inside.

Cords, pulleys and weights. Each sash hangs on cords that run over pulleys at the top of the box and down to cast weights hidden inside. The weights balance the sash almost exactly, which is why a well-made sash window can be moved with two fingers and stays exactly where you leave it — no springs, no friction hinges, no plastic gubbins.

Why weights still beat the alternatives. Modern spiral balances do the same job with a spring in a tube — cheaper, and fine for a while. But springs tire; weights don't. A weighted sash feels different the day it's fitted and keeps feeling different for decades, and when a cord eventually wears (they last decades), any competent joiner can replace it in an hour. It's the most honest kind of engineering: heavy, simple, repairable.

How we build. Strings and weights, traditional mouldings, real bars where the design calls for them — the same construction the mechanism has always wanted, in meranti or pine, double glazed. Our workshop has been building windows this way since 2022, and the method is a couple of centuries older than that. Browse box sash windows