The call comes in a particular tone. Somewhere between apologetic and furious, because the customer has already had the house cleaned once, has paid the builder for a sparkle clean on top of that, and is now running a finger along the top of a door in a freshly painted hallway in Crouch End and finding a grey line on it. Nobody has been in the house for eighteen days. The windows have been shut. There is no obvious source, and that is the part that makes people doubt their own eyes.
The dust is not new. It has been in the building the whole time, sitting in places nobody cleaned, and the house has been quietly handing it back a gram at a time.
Why is dust still landing three weeks after the last trade left?
Because plaster dust behaves nothing like household dust, and everyone treats the two the same.
Ordinary house dust is heavy. Skin, fibre, soil tracked in from the pavement, the odd bit of pollen. It falls, it stays where it falls, and a vacuum takes it away. Gypsum dust from skimmed plaster and jointing compound is milled far finer by the sanding process, much of it under ten microns, and particles that small stay airborne for hours after they are disturbed. Walk across a room and the air movement from your own legs lifts what is on the floor. It comes down again half an hour later, on a different surface, in a different room.
So the house does not get dusty once. It redistributes, give or take, for as long as there is a reservoir feeding the process.
What gypsum dust does that ordinary dust does not
It is alkaline and slightly soluble, which is why wiping it with a damp cloth so often makes things worse rather than better. Water turns the powder to a thin slurry, the cloth spreads it, and it dries as a chalky film that bonds to paint and glass. Anyone who has watched a new tenant attack a window with a wet j-cloth in a flat off Fishponds Road in Tooting has seen the result: a pane that looked dusty at nine and looks fogged at ten.
The other property that matters is static. Fine gypsum carries a charge and clings to plastic, to freshly emulsioned walls, to the inside of light fittings, to the back of skirting where it meets the floor. It does not sit there loosely. It has to be lifted with something that grabs it rather than pushes it, which in practice means dry microfibre first and moisture second.
Where is it hiding in a house that has already been cleaned twice?
This is the whole question, and the answer is always the same handful of places.
Ceiling voids come first. If the refurb involved lifting first-floor boards for pipework or cabling, or if downlighters went into a lath and plaster ceiling, there is now a route between the void and the room. The void is full of a century of soot, coal dust and construction debris, and it has been mixed with fresh plaster dust by the works. Every time the heating cycles and the joists move, or the front door slams, air pushes out of that void through the downlighter cut-outs and the loft hatch. A terrace on Tressillian Road in Brockley kept dropping black-grey dust onto a white kitchen island for a month, and the source was six spotlight apertures that had never been sealed with fire hoods.
Loft hatches are next, and they are the single most reliable culprit in a London house. Most refurbs involve someone going up there. The insulation gets walked on, boards get moved, and nobody cleans a loft afterwards because it is not a room. The hatch itself is usually a sheet of ply in a rebate with a two-millimetre gap all the way round. Warm air rises through that gap continuously, and it takes particulate with it in one direction and drops it in the other whenever pressure equalises.
Then the cavities behind and under things. The gap between the back of a fitted wardrobe and the wall. The floor void under a bath panel. The space beneath kitchen kickboards, where sanding dust runs like water and settles in a band along the whole run. Stud walls with service cut-outs. The boxed-in soil pipe in the corner of the bathroom, which almost always has an inspection panel that has been screwed back on over a pile of debris.
Soft furnishings hold a surprising amount. Curtains that stayed up during the works are saturated with it and release a puff every time they are drawn. Same with a carpet that was covered with sheeting rather than lifted, because sheeting protects the surface pile and does nothing for the underlay, and foot traffic pumps the underlay like a bellows.
Last one, and the one that catches everybody: the extract system. Bathroom fan ducting, cooker hood ducting, and any trickle vents in the new windows. A fan that ran during the works pulled dust-laden air through a flexible duct, and that duct is now lined. Switch it on eleven weeks later and it will spit a fine grey cloud into the room for the first ten seconds. Nobody looks up while a fan is running.
The negative pressure problem
Houses breathe. A London terrace with a gas boiler, an extractor fan and a chimney is under slight negative pressure whenever anything is running, which means it draws air in through every gap it has. Those gaps lead to the voids listed above. Running the bathroom fan for twenty minutes after a shower pulls loft air down through the hatch and into the landing, give or take the exact route, and delivers it to the bedrooms.
Seal the hatch and cap the downlighters and you will do more for the problem than a third clean will.
Which surfaces show it first?
Glass, every time. A window that has been wiped rather than washed carries a film that only shows in low sun, and low sun in a London terrace means about forty minutes in the afternoon, which is why customers spot it on a Sunday and ring on a Monday.
Radiators are the second. Fine dust falls into the fins through the top grille and stays there until the heating goes on, at which point convection lifts it and paints a grey shadow up the wall above each unit. That shadow is not a paint fault and it will keep coming back until the radiator is brushed out from the top with a long-handled tool, which takes about four minutes per radiator and gets missed on every builders’ clean I have ever inspected.
The socket-and-switch test
Take the faceplate off one socket in the room in question. If there is grey powder banked behind it, the room was never properly cleaned and the walls are still holding a supply, give or take what the vacuum took. If it is clean behind there, the dust is arriving from somewhere else in the house and no amount of work in that room will fix it.
Was the post-build clean done wrong, or done too early?
Usually too early, and here is the part customers do not want to hear: the impatience is generally theirs. The builder says he is finished on the Friday, the clean is booked for the Saturday, and the family moves back in on the Sunday because the rental has ended. Fine. But plaster continues to dry for weeks, joinery is still being trimmed and touched up, and the last painter has not been back to do the second coat on the architraves. Every one of those things generates more dust after the clean.
A post-construction clean is two visits. It always was. The first is a builders’ clean: bulk removal, debris out, gross dust off every surface, floors done wet and repeatedly. The second is a sparkle clean, and it should happen at least a week later, after the house has been walked through and the snagging is done.
Booking one visit and expecting the result of two is the most common way people end up paying for three.
What a builder’s “clean included” is worth
Very little, in my experience, and it is priced accordingly. The line item exists so the quote looks complete. The person who does it is usually the youngest lad on site with a shop vac and a mop bucket, working at four in the afternoon on a Friday, and what he produces is a site tidy rather than a clean. It gets the rubble out, which has value. It does not touch the top of a door frame or the inside of a socket faceplate.
Refuse to pay for it separately if it is itemised, or accept it as a rubbish clearance and budget for the real clean on top.
When is it not builders’ dust at all?
More often than the trade admits, and the alternatives are worth ruling out before anyone books a third visit.
Sanded and lacquered floorboards shed a fine pale dust for weeks as the finish cures and traffic burnishes it. A first-floor flat on Empress Avenue in Ilford had what everyone assumed was plaster on the skirtings, and it was oak dust from a floor sanded five weeks earlier and finished with two coats instead of three.
Soot is the other one. Open up a blocked chimney breast, or fit a new stove without a liner, and the house has a source that will keep producing for as long as it stays open. Grey-black rather than grey-white, and it smells faintly of the fire that stopped being lit in 1974.
Condensation dust is another. New airtight windows in an old house change the moisture balance, and what looks like a dusty film on a north wall in November is the start of something else.
What to do instead of booking a third clean
Seal the loft hatch with self-adhesive foam strip and fit hoods over the downlighters. Run the bathroom fan on a long overrun for a week with the door shut and a window ajar. Take the curtains down and get them cleaned or replaced, because nothing else will fix them. Vacuum with a machine that has a sealed HEPA path rather than the one from the site, which is venting a percentage of what it collects straight back into the room.
Then wait a fortnight and see what lands. If the answer is nothing much, the problem was sequence rather than cleaning.
A customer in Chiswick rang back after the hatch was strip-sealed to say the landing had stayed clean for the first time since March. The bill for the foam was four pounds twenty.