Clean a new glass roof within 14 days of construction dust settling, before cement residue etches the glass. Expect three passes: a dry bulk removal, a mild acid wash for mortar haze, and a rinse. First-clean cost runs 2 to 3 times a routine cycle.
Why is post-construction glass different?
Construction dust is not street dust. It carries cement fines, gypsum, silicone overspray, metal filings and welding spatter, and several of those are alkaline. Left on glass through a few rain cycles, cement residue starts bonding chemically with the surface and leaves a ghost haze that no routine brush will lift. The window to remove it cheaply is short, roughly two weeks in dry weather, less if the roof faces afternoon sun. Facilities teams that wait for the first scheduled quarterly clean often inherit a permanent stain line along the low edge where grit collected.
| Pass | Tool | Purpose | Water use |
|---|---|---|---|
| 1. Dry bulk | Robot, no brush pressure | Lift grit, stop scratches | None |
| 2. Chemistry | Sprayer + soft pad (hand) | Mortar haze, alkaline residue | Low |
| 3. Rinse | Robot, high flow | Carry residue to outlets | High |
| Perimeter | Hand tools | Frame lines, trims | Low |
What is the correct three-pass sequence?
Pass one is dry. Run the robot without water or with the water line off to lift loose grit, because a wet brush drags abrasive sand across the glass and makes fine scratches. Pass two is the chemistry pass: a diluted acidic cleaner at pH 3 to 4 for mortar haze, applied by a separate low-pressure sprayer and agitated with a soft pad, not the robot brush. Pass three is the rinse, robot at high flow, no brush pressure, to carry everything to the outlets. Do not skip pass one to save time. The scratch damage from wet-on-grit is permanent and shows up under raking light.
Where does a robot help and where does it not?
The robot earns its place on pass one and pass three, the large flat fields where it covers hundreds of square metres an hour. It is the wrong tool for mortar haze near frames and edge trims, where a person with a pad does a better, more controlled job. A hybrid crew is normal on handover cleans: one operator on the robot for the field, one on the perimeter with hand tools. Trying to do a full construction clean with only a machine leaves the frame lines grey and the client unhappy at handover.
What should the handover clean cost?
Budget 2 to 3 times a routine cycle. A routine clean on a 1,500 m² roof might land near 1.20 to 1.80 USD per m². A first post-construction clean with chemistry, extra passes and perimeter hand work typically runs 3.50 to 5.00 USD per m². On new-build projects that number belongs in the construction contract, not the operating budget, because the dirt was put there by the builder. Get it written in before handover, or the facilities team pays for the contractor’s mess.
Should the builder or the owner pay for the first clean?
The builder. Construction dirt is a defect of the delivery process, and a well-drafted contract puts the handover clean on the contractor. If the contract is silent, the owner inherits a several-thousand-dollar first clean and a permanent stain line if it is done late. Raise it at practical completion, not after you take the keys. If you are the contractor, price the clean into the tender so it is not a surprise later. Either way, somebody has to own it before the glass is handed over, or the argument costs more than the work.
On phased handovers, agree who cleans each zone and when. A roof cleaned once and then re-dusted by the next trade is money thrown away. Sequence the clean after the last dirty trade leaves the roof.
What if the haze is already bonded?
If the residue has been on the glass for months, ordinary chemistry will not touch it. You are into polishing compounds and mechanical correction, which is slow, expensive and carries its own scratch risk on coated glass. On a small area it is worth attempting; on a whole roof, replacement panels are sometimes the cheaper route. This is exactly why the two-week window matters. Every day you wait makes the eventual fix costlier, and there is a point where cleaning stops being cheaper than replacing.
Related reading: CE, IP65 and height access rules, and the cleaning frequency and schedule guide.
Key Takeaways
- Clean new glass within 14 days of dust settling; alkaline cement residue bonds into a permanent haze after that.
- Always lift dry grit first; a wet brush dragging sand scratches glass permanently.
- Use a hybrid crew: robot on the open field, hand tools on frames and mortar haze.
- Post-construction clean costs 2-3x a routine cycle (about 3.50-5.00 USD/m²) and belongs in the build contract.
How do you sequence the clean against other trades?
Do not let the glass clean happen while painters, glaziers or landscapers are still working. Rooftop dust settles for days after the last trade leaves. If the clean must run early for scheduling reasons, do a light final pass after the other trades finish and budget for it. On phased projects, clean each zone only after its last dirty trade is signed off. One well-timed clean beats two rushed ones, and it protects the handover photograph that the client will judge you on.
Also check the water source is not the builders’ temporary supply. Site water often runs cloudy and carries grit that scratches coated glass. Bring a filter or wait for the permanent supply before the handover clean.