Sash Window Repair Calculator

Each traditional sash window needs 4 cords — 2 for the upper sash and 2 for the lower sash. Each cord connects from the sash through a pulley at the top of the frame to a lead or iron counterbalance weight in the box frame. When one cord breaks, it is advisable to replace all four to ensure even operation.

Measure the sash dimensions and glass thickness. Glass weighs approximately 2.5 kg/m² per mm of thickness (so 4mm glass = 10 kg/m²; 6mm = 15 kg/m²) — that is float-glass density of 2,500 kg/m³. Add roughly 3-4 kg for the timber frame of a typical small single-glazed sash; large or double-glazed sashes are heavier, so for an exact result weigh the actual sash. The two counterbalance weights for each sash together equal the sash weight, so each weight is half the sash weight. In practice lead weights run from roughly 1 kg up to 6 kg or more each, sized to suit the sash.

Use pre-stretched waxed cotton sash cord (No.4 or No.6) for traditional windows. No.4 (6mm) suits most domestic sashes; No.6 (8mm) for heavier sashes. Synthetic (nylon) cord is cheaper but stretches and can slip on pulleys. Never use ordinary string or rope — it will stretch, fray, and break quickly.

Install brush-pile or rubber draught strip in channels routed into the frame and meeting rails. A well-fitted continuous seal can substantially reduce air infiltration while still allowing the sashes to slide. The key seal points are the four stiles (in the staff and parting bead channels), the meeting rail where the two sashes overlap, and the top and cill rails. Sealing the meeting rail matters most, as that is usually the biggest single draught source. Professional perimeter systems such as Ventrolla are highly effective.

Staff bead is the inner moulding that holds the lower sash in place — it runs up both sides and across the bottom of the frame. Parting bead sits in a groove between the two sashes, keeping them in separate channels. Both can be replaced to improve the fit and reduce draughts. They are removed to access the sashes for cord replacement.