Box Sash windows, especially those made in the early part of the last century, have usually lasted remarkably well although over painting or lack of maintenance may well have taken their toll. A common fault is to paint the sashes on their running surfaces so they either jam or are very stiff to move.

In the early days the sash run grooves and the sides of the sashes were left unpainted so the sashes would run smoothly. In most cases a layer of candle wax was applied to the running surfaces. If the running surfaces are painted, for protection, there is a tendency for the painted surfaces to bind together unless a large enough gap exists. Refitting staff beads too close to the sash is a common mistake.

When rot has set in, usually due to lack of maintenance, it may well be confined to the sashes, particularly the bottom one, which means to bring the window as a whole back up to full working condition is not a major exercise, nor need it mean total replacement, just the sash or sashes affected.

Historically Box Sash windows were not weather-stripped, they relied on the Fitch catch to clamp the two sashes to the parting bead to effect a seal. While this prevented rattling it did little to seal any gaps top and bottom. Now it is usual to fit sealing strips to the top and bottom sash rails as well as to the mid-rail. Similarly, the parting bead can be weather-stripped to seal the vertical faces. All this can be done to existing windows at not very great cost.

Because of the constraints imposed by the weights, the sashes themselves have been traditionally thicker than those of a casement window. In most cases this means double glazed replacement sashes can be fitted in the same run groove. The weights will have to be uprated which means a change to lead from iron, but the overall cost will be considerably less than replacing the window. On the average size window the maximum width of sealed unit that can be accommodated will be 14mm (or 4-6-4). This will not meet 'Part L' regulations, but if filled with a special gas it will comply. Thus if a Trickle vent opening is incorporated in one of the sashes, usually in the bottom rail where it is invisible from the exterior, the parting beads are replaced with ones incorporating seals, a pair of replacement sashes can bring an old Box Sash window up to current 'Part L' standards.