Sash windows tell you whether a terrace has been looked after or left to sulk. Stand on Tredegar Square in Bow and you can read the street off the glazing bars alone — the houses that see a decorator every few years, and the ones running the same cracked gloss they’ve had since the eighties.
I clean a lot of them. Georgian six-over-six and Victorian two-over-two. The occasional horned sash a previous owner dropped in during some ill-advised refurb. They all collect London the same way. Soot, brake dust, pollen off the plane trees, and the greasy film that lands on anything within fifty feet of a bus route. The frames cop the worst of it, because the glazing bars hold muck in every corner and nobody thinks to wipe them.
What follows is how I do the frames and bars without stripping paint or drowning the sash pockets. Slower than the glass. Worth every minute.
Why do sash windows in a London terrace get so filthy in the first place?
Position, mostly. A terrace sits flush to the pavement, so the lower sashes are a metre from passing traffic and take a constant dusting of whatever the road throws up. On Roman Road, that’s diesel and brake particulate from the 8 and the 388 grinding past all day. The upper sashes get less traffic but more weather — driving rain that carries rooftop grime down the face of the building and parks it on the meeting rail.
Then there’s the box itself. A traditional sash runs in a hollow frame with weights and cords, and that cavity breathes. Air moves through the gaps around the sashes, and it drags dust in with it. The corners of the glazing bars — the little rebate where the timber meets the glass — act like a gutter. Muck washes down, catches on the putty line, and bakes there.
Single glazing makes it worse. Cold glass sweats in a heated room, condensation pools on the bottom rail, and that damp film glues the dust down into a paste. That paste is the tell. If you run a finger along a bottom rail and it comes away with a grey smear rather than dry dust, the window’s been sweating and nobody’s touched it in a while.
What’s the black grime actually made of?
People assume it’s soot, and part of it is. The rest is iron. Brake dust is mostly iron oxide and it’s magnetic and it stains — which is why the black bloom on a windowsill near a junction goes slightly rust-brown when it’s wet. Add pollen in May and the oily haze that comes off warm tarmac in July. The neighbours still burning smokeless throw in a bit of chimney carbon. Together you’ve got a coating that plain water shifts about ten per cent of.
The iron content is why I don’t bother with a quick wipe on these. It smears. You need to lift it, not spread it.
What kit do you actually need, and what can you leave in the van?
Less than you’d think. Two buckets — one wash, one rinse, because if you rinse from the same bucket you’re painting grime back on. A handful of microfibre cloths, the flat-weave kind, not the fluffy ones that shed and catch on old timber splinters. A soft detailing brush, the sort you’d use on a car dashboard, for the corners. Wooden cocktail sticks. A pot of neutral pH cleaner — I use a dilute sugar-soap substitute at about half the strength the tub tells you, because full strength on old gloss will dull it.
That’s the list. Everything else is showing off.
The case against a pressure washer here
Someone always suggests it. Don’t. A pressure washer on a sixty-year-old sash will drive water straight past the putty and into the box, where it sits against the cords and the weights and starts them rotting from the inside. I’ve pulled sashes apart in Bow where the cord snapped six months after a well-meaning window firm blasted the frames — the water never left the box, the timber went soft, and the whole balance failed.
The water-fed pole brigade have a lot to answer for on period terraces. Deionised water and a stiff brush is fine on uPVC and fine on the glass. On painted Victorian timber it’s clumsy — too much water, no control in the corners, and it leaves the glazing bars streaked because the brush can’t get into the rebate. I’ll take a bucket and a cloth over a pole on a heritage frame every time, and I’ll argue it with anyone.
How do you clean the frames and glazing bars without wrecking the paint?
Start dry. Before any water touches the frame, take the soft brush and work all the loose grime out of the corners and off the bars — top rail, meeting rails, the vertical stiles, then the bottom. Loose dust plus water makes mud, and mud in the corners is a nightmare to shift, so get it dry first. Brush it downward and let it fall onto a dust sheet on the sill.
Now the wash. Wring the cloth out properly — damp, not dripping. You want enough moisture to lift the grime and not a drop more running toward the putty line. Work top to bottom, one section at a time, folding the cloth to a clean face every couple of passes. The moment a cloth face looks grey, turn it. A dirty cloth on old gloss is sandpaper.
The glazing bars are the slow bit. Each bar has two faces meeting the glass at a shallow angle, and that angle is where the paste sits. Wrap the cloth over a fingertip, press it into the rebate, and draw it along the whole length of the bar in one stroke. One stroke, then reposition — don’t scrub back and forth, because scrubbing worries at any hairline crack in the paint and lifts it. On a six-over-six you’ve got a lot of bars and it’s tedious. There’s no shortcut I’ve found in fifteen years, and I’ve looked.
Rinse each section with the second cloth and clean water, same top-to-bottom order, same damp-not-dripping discipline. Then dry it. A dry microfibre over the timber and the bars stops water spotting and stops any residual moisture creeping toward the box. That’s the tell of a rushed job — water spots dried onto the paint and a faint tide-line along the bottom rail where someone left it wet.
Dealing with flaking paint on a pre-1970s sash
This is where you stop and think. Any painted timber from before about 1970 may have lead in the older layers, and a flaking sash on a Victorian terrace almost certainly does under the newer coats. You do not brush hard at flaking lead paint, you do not sand it, and you do not create dust. If the paint is stable — no loose flakes, no chalky powder coming off on the cloth — clean it gently and leave it. If it’s actively shedding, that’s a decorator’s job with the right controls, not a cleaning job, and I tell the client so.
I’ll wipe a flaking frame with a barely-damp cloth to knock the surface grime back, working slowly so nothing goes airborne, then bag the cloth rather than shaking it out. Overcautious, maybe. It’s lead. I’d rather be dull about it.
How do you get into the corners of the glazing bars?
The corners are where every rushed job shows. Where the glazing bar meets the frame you get a tight junction that a cloth-covered finger can’t quite reach, and that’s where a black triangle of grime survives even after the rest looks spotless. A black triangle in the corner is the tell every time.
This is what the cocktail sticks are for. Wrap a corner of a damp microfibre round the blunt end of a wooden stick — never a metal tool, which scratches glass and gouges soft old paint — and use it to push into the junction and drag the grime out. Slow, fiddly, and it’s the difference between a frame that looks clean from the pavement and one that looks clean from eighteen inches, which is where the client stands.
Warm soapy water sits on stubborn spots better than a cold wipe. For a baked-on deposit that won’t budge — south-facing bottom rail, full sun, years of neglect — lay a warm damp cloth over it for a minute or two and let it soften before you touch it. Patience does more than pressure on old timber. That’s a rule I’d carve over the door.
What about the putty line?
The putty is fragile and you treat it as such. Old linseed putty goes hard and brittle and it’ll crack if you lean on it, so clean along the line, not into it, with a light touch. If a section is already cracked or missing, note it and move on — a cleaner poking at failing putty just speeds the glass loosening. Wipe the grime off the surface and leave the repair to someone with a putty knife and a reason to be there.
How often does a London terrace really need this?
Twice a year does most houses. Once in late spring after the pollen drops, once in autumn before the wet sets in and the condensation season starts. A ground-floor front on a bus route wants more — quarterly, sometimes — because the brake dust just keeps coming.
Roadworks change the maths. When they had the gas main up on Grove Road the whole terrace opposite was grey inside a fortnight, frames included, and those clients went from twice a year to monthly until the works finished. If there’s scaffolding or a dig within a hundred metres, expect the sashes to load up fast.
Does the back of the house need the same schedule?
No. The back of the house is always cleaner than the front, so don’t quote the same interval for both. A garden-facing sash in a quiet Bow terrace can go a full year and look fine. The front of the same house, forty feet away on the road side, will be filthy by August.
I keep a note of which elevation faces what, and I book accordingly.