Repair or replace old sash windows
Applies to: Assumes counterbalanced double-hung timber sashes in a masonry opening, painted, with cord-and-weight or spiral balances.
Last worked on 2015-01-22
The question is usually asked as a performance question and answered as a condition question. Four checks settle it in most cases.
Check one: the sill and the bottom rail
These are where water sits, so they fail first and they fail from the underside. Press with a blunt point along the underside of the bottom rail and along the sill nose. Sound timber resists; failed timber gives with no spring back.
Localised softness in these two members is repairable and routine. Softness that has travelled up into the stiles beyond the joint is a different job, because the joint itself is what holds the sash square.
Check two: the joints
Sash corners are usually mortise and tenon, pinned. A sash that has dropped out of square at a corner has a failed joint, a failed pin, or rot in the tenon. Look for a diagonal crack in the paint across the corner, and for a sash that binds on one side of the frame only.
One loose corner is a repair. All four loose corners on both sashes, in a window that has also lost its sill, is the point at which repair stops being the economical answer.
Check three: the frame in the opening
The frame is the part you cannot easily replace without disturbing the masonry, and it is also the part most likely to be sound, because it is protected and rarely painted shut. A sound frame with two failed sashes is the clearest case for repair there is: the expensive, disruptive component is already there.
Check four: what you are actually trying to fix
If the complaint is draughts, replacement is an expensive route to something draught-sealing achieves. If the complaint is rattling, the fix is at the meeting rail and the parting bead. If the complaint is condensation on the glass, secondary glazing or an internal panel changes it more reliably than a new frame does, because the cold surface is the glass rather than the timber.
If the complaint is that the sashes will not move, that is nearly always paint or a broken cord, and both are an afternoon.
The decision, in order
- Establish whether the frame is sound. If it is, the case for replacement is already weak.
- Establish whether failure is confined to the sill and bottom rails. If it is, this is standard repair work.
- Count failed joints across both sashes rather than judging by appearance.
- Name the actual complaint - draught, rattle, condensation, or seized - and check whether replacement is the shortest route to it.
- Only if the frame is failed and the majority of the joints have gone does replacement become the straightforward answer.
The common outcome of working through this honestly is a repair to two members, new cords, draught seals and a re-glaze - which addresses the complaint that prompted the question in the first place.