The Brief
A band of Victorian glass that had to survive the job
The windows on this Islington terrace had reached the end of a long life. Paint was lifting from the cills, the lower sashes had swollen against their frames, and draughts came through the meeting rails on a windy night. What made the job more interesting than a straight replacement was what sat at the top of the ground-floor bay: a band of original stained glass margin lights, the coloured squares that Victorian builders used to frame a bay and catch the afternoon light.
That glass was not going anywhere. The homeowners were clear from the first visit that whatever we did, the coloured lights had to survive it — and survive intact, not as a reproduction. Everything else about the project followed from that single constraint.
Paint failure across the cills and sashes, and the same bay rebuilt in Accoya — original stained glass back where it started, above the top sash.
The Careful Part
Working around the original stained glass
Removing century-old coloured glass from a sash and putting it back into a new one is a job where the risk sits entirely at the start. The glass is thin, often slightly uneven, and held in putty that has gone hard as stone. It does not bend, and it does not forgive being twisted out of a rebate in a hurry.
Each panel was taken out by hand, the old putty softened and picked back rather than chiselled, and every piece catalogued by its position in the bay so that it went back exactly where it came from. The coloured squares in a Victorian margin light are rarely a perfect repeating pattern — the sequence varies, and putting one back a pane out from where it started is the sort of mistake that is invisible on paper and obvious forever once the window is in.
The new sashes were then built around the glass rather than the other way round. Each rebate was cut to the panel that was going into it, which is slower than working to a drawing but the only way to get old glass sitting properly in new timber without packing it out.
Inside the bay — the retained margin lights, and the slim sightlines that 12mm heritage glazing allows.
Materials
The timber and the glazing
Every window on the property, front and back, was remade in premium grade Accoya®. Accoya is a modified softwood: the timber is treated through its full thickness so it barely takes up moisture, which is what causes a traditional sash to swell in winter and stick in its frame — exactly the fault the old windows had developed. The dimensional stability also means the paint film sits on a substrate that is not constantly moving underneath it, so the finish lasts substantially longer than it would on standard softwood.
The clear panes were replaced with 12mm slimline heritage double glazing. The number matters on a period property. A standard modern double-glazed unit is around 24–28mm thick, which forces a much heavier sash section and a wider sightline — the visible band of timber around the glass. Put that on a Victorian bay and the proportions go wrong immediately; the window reads as modern from across the street even when everything else is right.
At 12mm, the unit fits within a traditional sash section. From the pavement the glazing bars and meeting rails hold the same slim proportions as the originals, while the room behind them gets the thermal and acoustic performance of a sealed unit. The stained glass margin lights were retained as original single-glazed panels, as they should be — encapsulating or replacing them would have cost the glass its character.
The new sashes were built around the glass, rather than the glass being made to fit the sashes.
On Site
The finished windows, inside and out
The Detail
Like for like — with one change
The new windows replicate the originals in every dimension that matters: section sizes, glazing bar profiles, horn detailing, meeting rail heights and the proportions of the bay. Held up against the before photographs, the joinery is a match.
The one deliberate departure was colour. The old frames were white — as most of the street still is — and the homeowners wanted a dark olive-bronze instead. It is a significant visual change and it was their decision, made with the elevation in front of them. It suits the red brick better than the white did, and because the profile underneath is correct, the window still reads as a period sash rather than a modern insertion wearing a period colour.
The Result
Draught-free, double glazed, and still Victorian
Front and back, every window on the property now opens and closes properly, seals against draughts, and carries slimline double glazing without looking like it. The stained glass came through the job intact and sits where it has always sat, now with a new sash around it that should outlast the one it replaced by some margin.
Seen from inside, the margin lights do the thing they were installed to do in the first place: throw colour across the room when the light is right. That was the whole point of keeping them.
Been told your stained glass can't be kept?
In most cases it can — it is a question of how the sashes are built, not whether the glass is salvageable. Send us a photograph of the window and we will tell you honestly what is possible.