Start by testing the timber
Push a bradawl or a penknife into the bottom rail of the lower sash, the sill, and the base of the box frame outside. If the blade goes in more than a few millimetres with light pressure, that section is decayed. If it resists, the timber is sound however bad the paint looks.
Peeling paint and grey weathered timber alarm people but are usually cosmetic. Decay concentrates where water sits: sills, bottom rails, the joints of the lower sash, and the base of the pulley stile. Everything above tends to be fine.
The case for repair
Original sashes are made from dense, slow-grown timber that is far more durable than most modern softwood. Repairing them keeps that material, keeps the correct glazing bar proportions and, in a listed building or a conservation area with an Article 4 direction, is often the only route that will get consent.
A full sash overhaul typically means splice repairs to decayed sections, new cords and re-balanced weights, new parting and staff beads, brush-pile draught seals routed in, easing and re-hanging so both sashes slide, servicing or replacing fitches and fasteners, and full redecoration. Done properly it should give decades of service.
When replacement makes sense
Replacement is reasonable when the sashes are not original - many London houses already carry 1980s replacements - when decay is extensive across box frames as well as sashes, or when the windows have been altered so heavily that the original pattern is gone.
New timber sashes should match the original in every visible respect: box frame construction, sash horns, glazing bar section, sill profile and the setback of the frame in the reveal. A slim double-glazed unit can be accommodated in a new sash designed for it, though bar sections become slightly heavier.
Draughts, rattles and heat loss
Most complaints about sash windows are about draughts, and most draughts are solvable without replacing anything. Brush-pile seals in the parting bead and staff bead, a seal at the meeting rail, and correctly tensioned cords remove the rattle and the majority of the air leakage.
Beyond that, secondary glazing is the quiet high performer: it improves both thermal and acoustic performance significantly, is usually acceptable in listed buildings because it is reversible, and costs less than replacement. Heavy curtains and well-fitted shutters also make a measurable difference.
Glazing choices
Original crown or cylinder glass has visible distortion that is part of a period facade's character and cannot be replicated by modern float glass. Where original glass survives, it is worth retaining and reusing in repaired sashes.
Slim double-glazed units are available at around 11 to 14 millimetres overall and can fit some sashes, but they add weight - which means re-balancing the weights - and the perimeter seal has a finite life. Vacuum glazing is thinner and better performing but considerably more expensive. Discuss both with the conservation officer if the building is protected.
Consents
Listed buildings require listed building consent for window replacement and often for significant alteration. In conservation areas, an Article 4 direction may remove the permitted development right to change windows on the principal elevation, so replacement then needs planning permission.
Repair, by contrast, is generally uncontroversial and is what conservation officers prefer. Where any doubt exists, a short conversation with the local authority before work starts is far cheaper than an enforcement notice afterwards.
Costs and disruption
Indicative London figures: a full overhaul with cords, seals, splice repairs and redecoration commonly runs several hundred to around two thousand pounds per window depending on size and condition; new purpose-made timber sash windows generally cost considerably more per opening, particularly with double glazing.
Repair work is usually done in situ over one to two days per window with the room protected. Replacement means the opening is out for a period and often involves scaffolding, which is a significant additional cost on upper floors.
Maintaining them afterwards
Sash windows fail because of neglected paint, not because they are old. Inspect the sills and bottom rails every couple of years, deal with any hairline cracks in the paint film before water gets in, and keep the drainage groove under the sill clear.
Lubricate the pulleys occasionally, keep the parting beads intact, and open the windows regularly - sashes that are never used seize, and seized sashes get forced and damaged.
Frequently asked questions
- Are repaired sash windows as warm as replacements?
- With draught seals and secondary glazing, a repaired sash performs far better than most people expect and can approach the whole-window performance of a double-glazed replacement, at lower cost and without losing original fabric.
- Can I fit double glazing into existing sashes?
- Sometimes, using slim units, but the sash must be deep enough and the weights must be re-balanced for the extra mass. Not all original sashes are suitable.
- Do I need permission to replace sash windows?
- In a listed building, yes. In a conservation area with an Article 4 direction, usually yes for principal elevations. Elsewhere, often not, but check with your local planning authority.
Planning a project like this?
SH Joinery surveys every room before quoting, so you get a fixed price against a written specification rather than an estimate. Send us your drawings, photographs or rough sizes and we will come back with an honest view of what your project involves.