The worst handover I have walked into was a block of eleven flats at Barking Riverside, finished glazing, scaffold down, sales suite booked for the following Monday. The render subcontractor had worked the top two floors in a crosswind in July and every pane on the north elevation had a fine spatter of cement slurry across it that nobody had touched for five weeks.

We got the splash off. The marks stayed.

That is the thing about a builders clean that developers and main contractors keep discovering the hard way – two of the three residues on this list are a labour problem, and the third one is a countdown that started the day the splash landed.

Why Does Cement Splash Have a Deadline the Other Residues Do Not?

Wet cement carries free lime and it runs at pH 12 to 13. Glass tolerates acid perfectly well and it has almost no defence against alkali at that strength. Hydroxide pulls sodium out of the surface layer, the silica network opens up, and the glass corrodes underneath the splash while the splash is still sitting there holding moisture against it.

In summer on a south elevation you have days. Through a wet November you might get three or four weeks. Past that, whatever you remove mechanically will leave a ghost behind it in exactly the shape of the original droplet.

I tell site managers this and about half of them treat it as a sales line until the first pane comes back etched. The other half put a temporary wash-down into the programme, which costs them a man and a hose reel twice a week and saves a five-figure glass order at practical completion.

What the ghost looks like at thirty degrees

Straight-on in flat light the glass reads clean. Drop your eye to a shallow angle with a dark background behind the pane – a stairwell, a hoarding, the inside of an unfitted flat – and the ghosting appears as a dull constellation with soft edges. That is the test I run at handover on any elevation the wet trades worked above.

At Barking I ran it in front of the contracts manager with a torch at eight in the morning and he could see it from four metres. Eighteen units went back to the fabricator. I have the photographs from the day I first attended, which is the only reason the argument went the way it did.

The other trap is the removal product. Some cement and mortar removers sold into UK construction are built on ammonium bifluoride, which is a fluoride chemistry and will etch glass on contact whether or not there is any cement left to remove. Read the safety data sheet before it goes anywhere near a facade. Sulphamic and phosphoric acid formulations are the ones to use, dwell rather than scrub, and flood off with far more water than feels necessary because acid left in a glazing gasket will bleed back out and stain the frame instead.

Where Does Silicone Smear Come From and Can It Be Taken Off?

The glazier runs a perimeter bead, tools it, and wipes the surplus off the glass with a dry rag. That wipe is the whole problem. It spreads a film of uncured silicone across the face of the pane in a broad arc, the film cures overnight into a continuous layer maybe a micron thick, and the pane looks perfect until the first shower or the first low winter sun through it.

Cured silicone cannot be dissolved. Aromatic solvents swell it, which is a different thing – they make it soft and rubbery enough to come away with a blade, and that is the whole method. Xylene-based digesters, ten minutes of dwell, then a new blade at a low angle with plenty of lubrication.

Reckon on twenty to thirty minutes a pane on a bad frontage. There is no shortcut and there is no product that removes it by wiping. A commercial cleaner on a Wembley Park scheme once put four people on a frontage with microfibre and a general-purpose degreaser, produced a beautiful result in the wet, and watched the whole elevation go milky as it dried.

The gasket problem nobody mentions

Those solvents attack EPDM gaskets, they soften new powder coating, and they will craze certain acrylic and polycarbonate panels beside the glass. On a commercial frontage at Kew Bridge I masked every gasket line on a 40-metre elevation before we started, which added most of a day and looked absurd, and I would do it again. The alternative is explaining to a facade contractor why his weather seals have gone chalky.

Uncured silicone, caught within a few days, comes off with a solvent wipe and no blade at all. So the real answer to silicone smear is to be on site while the glaziers are, which is almost never how the programme is written.

Why Is Protective Film Harder to Remove Than the Muck It Protected Against?

Peelable protective film is polyethylene with an acrylic pressure-sensitive adhesive, and its working life is short. Manufacturers commonly quote six months for the film itself, and a south-facing London elevation with the scaffold sheeting off will burn through that in a fraction of the time. UV degrades the adhesive, the polymer chains break, and what was designed to peel in one piece starts coming away in strips the width of your thumb with the glue staying behind.

Once it has baked on, you are looking at a heat gun, a limonene-based adhesive remover, a blade and roughly an hour per large pane. On a mixed-use scheme in Sutton I put two people on a ground-floor retail frontage for a day and a half for that reason alone. The film had been on since the previous autumn.

The tension in the programme

Everyone wants the film left on as long as possible so the glass is protected through the wet trades, and everyone wants it off before it fails. Those two things pull against each other and the second one loses every time, because taking film off early creates work for somebody and leaving it on creates work for nobody until it is too late.

My position on it is straightforward. Get the film off the moment the wet trades above that elevation are finished, then protect with correx or hoarding if the frontage still needs it. Correx costs a fraction of what a film-residue clean costs in labour.

Who Pays When the Glass Comes Up Scratched?

This is the fight, and I will state my side of it plainly. Toughened glass leaves the furnace with fused particles in its surface – fabricating debris, picked up off the rollers in the toughening line. Run a blade over that surface and the particles lift and drag, and you get fine curved scratches that look exactly like careless scraping. The fabricators’ standing position is that scraping toughened glass carries risk, which reads as a warning and functions as a liability transfer.

I think that position is indefensible. Glass shipped with abrasive contamination in its outer face is defective glass, and telling the cleaner not to scrape is a way of billing the consequence to the last trade on site. Blades are the only viable tool for cement and cured silicone. A builders clean without scraping is a wipe-down.

The photograph set that ends the argument

So I survey and photograph every elevation before a blade comes out of the van – date-stamped, close and wide, pane by pane, and a note against any unit that already shows damage. Then I do a small controlled scrape in a corner of the least visible pane on each elevation and photograph that too. If the corner drags, the whole elevation gets flagged to the main contractor in writing before we proceed.

A dispute on a scheme in Bromley cost a cleaning firm I know about eleven thousand pounds because they had no record of the glass condition on arrival. I have the photographs from every builders clean I have run since 2018.

What Does a Builders Clean Cost When It Is Done in the Right Order?

Two-stage pricing, always. A rough clean while trades are still working, then a final sparkle after the last wet trade has left the elevation, and the second visit priced separately because nobody can predict what the intervening six weeks will deposit.

Single-visit builders cleans priced off a drawing are how contractors end up with three cleaners in a row on the same job. The first one prices the drawing, arrives, finds cured silicone and dead film, and either eats the loss or walks.

What the second visit is really for

The final visit is a defects clean as much as a glass clean, and it is when everything else on this list surfaces. Film failures show up. Silicone smear shows up the first time the sun comes round to that elevation. Cement ghosting shows up in a torch test. A contractor who has priced one visit has no budget left for any of it and will push it back onto the sales team, who will discover it the week the show flat opens.

I ask for the second visit to be programmed after the last scaffold lift comes down and before any soft landscaping starts, because irrigation commissioning on a podium will put mineral spotting back on the ground-floor glass inside a fortnight.

Rates in London for a residential new-build sit somewhere between £2.50 and £6 a square metre of glass for a straightforward final clean, and the top of that range does not cover a frontage with baked film and five-week-old mortar spatter. That work is priced by the hour or it is priced wrong.

The eleven flats at Barking got their new units in the end. The sales suite opened a fortnight late, the render subcontractor’s insurer paid for the glass, and the developer’s programme for the next phase specified sheeting to the working elevation and a fortnightly wash-down during the wet trades. I have the photographs from that job in a folder I still open when a contracts manager tells me his site is different.