Window sash replacement: timber restoration vs slim aluminium in Sydney

How a double-hung sash window works

A traditional double-hung sash window has two panels (sashes) that slide vertically past each other in a timber box frame. Each sash is counterweighted by a cast iron weight hidden in the frame cavity on each side, connected to the sash by a sash cord running over a pulley at the top of the frame. When the cord is intact, the weight balances the sash so it stays at any open position without being held. When the cord breaks — which it eventually does in any older window — the sash drops and won’t stay open.

The outer box frame (the timber frame set into the wall opening) is usually in better condition than the sashes themselves because it’s protected by the surrounding wall. This matters for the repair decision: it’s often possible to replace only the sash within the existing box frame, which is significantly cheaper than replacing the entire assembly.

Common failure modes: broken sash cord (sash drops), rotted timber sash (usually at the bottom rail where moisture accumulates), swollen sash that binds in the frame channels (typically after wet periods), failed glazing putty letting glass rattle or leak, and seized hardware.

Option 1: restore the existing timber sash

The sash is stripped, repaired and refurbished: glazing putty replaced, sash cord replaced with new cord or stainless steel chain (chain lasts longer in Sydney’s humidity), any rot treated with two-part epoxy filler or cut out and dutchman-patched with new timber, and the whole sash repainted. This is the approach that preserves the original window exactly and is the only option acceptable in some Heritage Conservation Areas.

The significant downside is ongoing maintenance. Timber sashes need repainting approximately every 7–10 years in Sydney’s climate, more frequently on sun-exposed elevations. Glazing putty has the same service life and needs periodic replacement. If you’re not prepared for this cycle of maintenance, timber restoration extends the life of the existing window but doesn’t change the underlying maintenance requirement. Cost: $400–$900 per window depending on the extent of rot and the accessibility of the cord cavity.

Option 2: fit a new timber sash

The existing timber sash panel is replaced with a new one made to the same dimensions, while the outer box frame is retained. This makes sense when the sash is too deteriorated to repair economically but the box frame is sound. The new sash can be specified in hardwood for better rot resistance than the original softwood in many older Sydney homes. Appearance matches the original closely. May require a DA in some heritage conservation areas depending on whether the council treats it as a like-for-like replacement or a new element. Cost: $700–$1,400 per sash depending on size and timber species. Ongoing maintenance requirement is the same as the original.

Option 3: replace with slim-profile aluminium

A modern slim-profile aluminium double-hung unit replaces the old timber sash within the existing box frame opening. The aluminium double-hung uses a spring-balance mechanism instead of cord and weights, eliminating the cord replacement maintenance cycle. No ongoing painting is required. The unit is double-glaze compatible from the outset.

Modern slim-profile aluminium systems achieve a sight line (the visible frame face width) as narrow as 40–50mm, which in most non-conservation-area Sydney homes is a sympathetic match to the original timber profile. The aluminium can be powder coated in a timber-matching colour — Paperbark, Gully, or custom — to further integrate with the existing joinery. This is now the dominant choice for sash window replacement in Sydney renovations outside heritage conservation areas. Cost: $700–$1,500 per window supplied and installed.

Cost comparison

Option	Cost per window	Ongoing maintenance	Double glaze possible?	Heritage conservation area?
Timber restoration	$400–$900	Every 7–10 years	Possible but heavy	Usually acceptable
New timber sash	$700–$1,400	Every 7–10 years	Possible but heavy	May need DA
Slim aluminium double-hung	$700–$1,500	None	Yes — specify at order	Usually needs DA if street-facing

Heritage overlay and DA rules in NSW

If your property is in a Heritage Conservation Area (HCA) or is individually heritage-listed on the State Heritage Register or a council’s local heritage schedule, external window changes visible from a public road may require council approval. The general framework in NSW: like-for-like replacement (same material, same configuration, within the same opening) is usually exempt from DA requirements. Changing material — for example timber to aluminium — on a street-facing elevation in a conservation area almost always requires a DA, even if the new product looks similar.

The penalty for undertaking unauthorised heritage work can include a requirement to reinstate the original at your cost. Before ordering any sash replacement on a potentially heritage-affected property, contact your local council’s heritage officer and get the answer in writing. If a DA is required, add 3–6 months to your project timeline for the approval process.