You notice it first on a laptop screen. Not the screen you use every day, the one on the shelf, tilted, catching the light from the window at about half four in the afternoon. A grey bloom across it that wasn’t there on Sunday. Then it’s the tops of the picture frames, then that faint gritty click under a sock on the second stair, and the thought that arrives with it is always the same one. We had this cleaned. Twice. The builders left in March.
They did. It’s still going. Here’s roughly how long, and why.
Why Does Plaster Dust Come Back After You’ve Already Cleaned It Twice?
Because you never removed most of it. You removed the layer you could see.
Gypsum dust from board sanding is fine, light and irregular, and a good part of it is under ten microns across. Particles that small don’t fall out of the air the way sawdust does. They drift, and they keep drifting for hours after the sander stops, riding whatever air movement the house gives them. A door closing at the bottom of the stairs pushes a column of air up two floors. Somebody opens a sash in the back bedroom and the pressure change pulls the loft hatch seal open a few millimetres.
So while your builders were sanding the joints in the new dormer, dust was settling on the top edge of every internal door in the house, on the curtain headers, down inside the radiator fins in the hall, and into the pile of the stair carpet, which is the single largest dust reservoir in a London terrace and the one nobody touches properly.
It’s already downstairs. It was already downstairs on day three.
The Places It Sits Waiting
The gap behind skirting boards where the board doesn’t quite meet the floor. The eaves cupboards, which are unsealed voids full of settled dust and which everybody opens in October looking for the Christmas things. The tops of wardrobes. Extractor grilles. The underside of the banister rail. And the loft hatch itself, which on most conversions becomes a permanent draught path between a still-dusty void and your landing.
Every one of those releases a fresh dose when something disturbs it. That’s the return. It isn’t new dust. It’s March’s dust arriving on a schedule you can’t see.
What’s the Real Timeline for a Dormer Conversion?
Weeks one and two after the trades leave are the heavy phase. Visible film on horizontal surfaces within twenty-four hours of a proper clean. This is the stage where people panic and ring three companies.
Weeks three to six, it moderates. You’ll get a fine haze on glass and dark surfaces every four or five days rather than daily. Most households read this as the cleaner having done a bad job. They haven’t. Airborne load is dropping and the reservoirs are handing dust back at a slower rate.
Months two to four is the episodic phase, and it’s the one that catches people out. Nothing for nine days, then you move a chest of drawers or the heating comes on properly for the first time in autumn and the convection off a radiator lifts everything that settled in the fins in April. A house in Fortis Green I did in 2022 was fine by June, rang me again in late October in a state, convinced the builders had come back and done something. It was the boiler firing up for the season and eight radiators exhaling six months of gypsum.
Beyond that, you’re into eaves and loft void dust, which will keep giving for a year or more if the conversion left the storage voids unsealed and unpainted. I’ve been in Tooting houses three years on where opening the eaves door still produces a small grey cough.
So: two to three months for the living space to settle, four to six if the stair carpet stayed down through the works, and open-ended for the voids until somebody seals them.
The Three-Clean Rule
One clean does not work and I’d rather say so than sell you one. The pattern that works is a rough clean the day the trades go, a proper deep clean after decorating is finished, and a third light pass six to eight weeks later for what the reservoirs have returned. Anybody quoting you a single visit to make it all go away is quoting on a job that isn’t real.
Is Plaster Dust Dangerous, or Only Irritating?
Mostly the second, with one exception that matters.
HSE groups gypsum with the lower toxicity construction dusts, alongside limestone and marble, and the general workplace exposure limits sit at 10 mg/m³ inhalable and 4 mg/m³ respirable over an eight-hour average. Those are worksite figures for people doing it forty hours a week, and your hallway is not a worksite. For a healthy adult, board dust is an irritant. Dry throat, and a cough that hangs about for a fortnight.
The exception is what’s underneath the finish. Sand and cement backing coats, and the chasing out of brickwork for cable runs and the new steel, produce respirable crystalline silica, and the exposure limit for that is 0.1 mg/m³, a hundred times lower. That dust is in your house too. It settled alongside the gypsum and it’s mixed into every reservoir I listed above.
Which is why the answer to “should I wear a mask while I hoover this” is yes, and it should be FFP3, not the blue paper thing from the pound shop. Children with asthma should be out of the house for the deep clean, and back in an hour after it, not during.
The Bit Nobody Tells Asthmatic Households
Plaster dust is hygroscopic. It draws moisture, and in a damp London winter a settled layer on a windowsill will take on water, then dry, then flake again as something finer than it started. That’s why some people’s symptoms are worse in November than they were in the week of the works.
Which Cleaning Methods Make It Worse Instead of Better?
Sweeping. Dry dusting with anything fluffy. And, sorry, your vacuum.
A domestic bagless cylinder pulls fine gypsum straight through the cyclone, loads the post-motor filter, and then blows a proportion of it back out into the room at head height. You’re not cleaning, you’re relocating and aerosolising. The bin-emptying is worse: you take a canister of concentrated construction dust and tip it into a bag over a kitchen floor you cleaned an hour ago.
It’s already downstairs, and now it’s airborne again.
What works is an M-class vacuum with a genuine HEPA filter and a sealed bag, worked slowly, floor last. Damp microfibre, folded into eight, refolded to a clean face every couple of square metres, rinsed in a separate bucket that never touches the clean water. Top to bottom, always: ceiling line, picture rail, door tops, sills, skirtings, then floors.
Do not put a wet mop on a dust-loaded floor as your first move. Gypsum plus water makes a thin paste that goes into grout lines and tile texture and sets, and then you’re scrubbing rather than wiping. Vacuum it dry first, twice, then wet.
And leave the carpets alone until the airborne phase is over. Hot water extraction on a carpet still receiving fresh fallout drives dust into the backing where nobody can reach it. I’d wait until week six.
Why the Filter Matters More Than the Machine
A vacuum is only as good as the last thing the air passes through on its way out. If the bag isn’t sealed at the collar and the post-motor filter isn’t a rated HEPA element, the suction figure on the box is decoration. Change the filter halfway through a loft-conversion clean rather than at the end of it, because a gypsum-loaded filter drops airflow long before it looks dirty, and a machine that has stopped moving air properly is a machine that has quietly stopped picking anything up. Bags at about two-thirds full, not when they split.
When Should You Book a Professional Clean, and When Should You Do It Yourself?
Here’s the part my accountant dislikes. For weeks one to four, don’t book me.
Hire an M-class vacuum from a tool hire counter for around £30 to £40 a day, buy a pack of forty microfibre cloths, and do a top-down pass yourself twice a week. You will get eighty per cent of the result for a tenth of the cost, because at that stage the job is repetition rather than skill. Paying a company £280 three weeks running to fight fallout that’s still actively arriving is money set on fire, and any firm that lets you book all three without saying so is happy to take it.
Book the professional clean once. After the decorators have finished sanding and applied the final coat, not before. That’s the clean worth paying for, because it’s the one where somebody gets into the reservoirs: door tops, radiator fins pulled and vacuumed with a crevice tool, curtains taken down, carpet lifted at the gripper edge, eaves voids done properly and sealed.
Spend the Money Upstream Instead
The strongest opinion I hold about this trade is that the builder’s “sparkle clean” is theatre. Forty-five minutes with a broom and a mop, priced as a professional service, at exactly the moment when cleaning achieves the least.
Withhold that line from the quote and spend it on containment instead. A zip door at the bottom of the loft stair, taped polythene over the landing doorways, a negative air unit venting out of a dormer window, and the stair carpet lifted and rolled before day one. That costs a few hundred pounds and it prevents the problem, which no amount of cleaning afterwards will. Builders who resist this are telling you something.
I keep a roll of 2-metre polythene and a box of zip doors in the van for exactly this, and I hand them over free to anyone starting a conversion in the boroughs I cover. It saves them two cleans, and it heads off the phone call that always comes in week five. I’d still rather do the one good one.
The dust on that laptop screen in Fortis Green stopped for good in the second week of November.