Sash windows in London, drawn from the originals.

The box sash is London's native window, and a new one should be built the way the Victorians built them, as a counterbalanced machine. We install new timber sash windows in engineered redwood or Accoya, mortice-and-tenon joinery, with traditional cords and cast weights or discreet spiral balances where the box must slim down. Every window arrives factory-primed and finished in your colour, with brush-pile draught seals concealed in the beads, so it glides in winter and doesn't rattle when the 134 goes past.

Engineered timber matters: laminated sections won't twist or cup like single sawn boards, which is why we guarantee the joinery for decades, not seasons. Expect roughly 4-6 weeks from survey to fitting, one to two windows fitted per day, openings never left unglazed overnight.

The old objection to double-glazing a sash (the unit is too fat for the rebate and the glazing bars go coarse) is answered by slim heritage units of 12-14mm overall: two panes around a 4-6mm cavity, krypton-filled, with slender warm-edge spacers in place of the chunky aluminium bar. They sit within a traditional sash section, keep putty-line sightlines and authentic glazing bar proportions, and still roughly halve the heat loss of single glazing.

We specify them honestly: slim units cost more per pane and give up a little performance against a full 28mm unit, so we reserve them for the elevations that need the look, and use standard units where nobody will ever squint at the spacer bar. Heritage glass options (drawn-effect, low-iron) complete the deception.

A replacement sash betrays itself in the details, so the details are where we spend the time. At survey we trace your existing mouldings: ovolo or lamb's-tongue glazing bars, staff and parting beads, meeting rail depths and sash horn patterns, and reproduce them so one new window sits indistinguishable in a terrace of originals. Where the box itself survives, we can hang new double-glazed sashes in the existing box; where it doesn't, complete new boxes are scribed to the reveal.

Cills get the same care: full timber cills in hardwood with a proper drip groove, spliced or replaced as condition demands, never capped over rot. It's the difference between a window that was replaced and a window that looks like it was always there.