Thermal shock cracks glass when a cold liquid hits a pane that is over roughly 60-70 °C. On a summer afternoon, dark-tinted skylights in Dubai or southern Spain can reach 65-80 °C surface temperature. Cleaning then means the robot must use warm water, low flow, or wait for early morning.
How hot does skylight glass really get?
Measured surface temperature depends on tint, coating and airflow. Clear low-E glass in a temperate city peaks near 45-50 °C. Tinted or heat-absorbing glass over a sealed atrium can top 75 °C. Spots matter more than averages: a shaded edge of the same pane may be 20 °C cooler, and the crack starts where hot glass meets cold water first.
Why does it crack?
Glass is brittle and a poor heat conductor. The wetted surface contracts while the bulk stays expanded, building tensile stress at the edge or around any pre-existing chip. Tempered glass handles a larger gradient than annealed; laminated is in between. A 25-30 °C difference between water and glass is generally tolerated, 50 °C is gambling.
- Water at 20 °C on glass at 70 °C: 50 °C delta, high risk on annealed panes.
- Water at 40 °C on glass at 70 °C: 30 °C delta, usually safe on tempered.
- Pre-dampening a small test pane first lowers the spike dramatically.
What should the cleaning schedule look like?
Clean before 09:00. In hot climates that is the whole answer. Where night cleaning is impossible, split the job: west-facing glass in the morning, east-facing after 16:00, never the sun-facing pane at 14:00 in August. Lingyun Y3 with a 20 L tank lets you run a controlled pre-wet pass, pause two minutes, then scrub – a slower method that avoids the thermal spike.
Which buildings should stop robot cleaning in peak heat?
Old annealed or single-glazed skylights, dark-tinted roof glass, and any pane with visible edge chips. For those, either clean at dawn or contract rope access crews who can inspect and stop. A chip you can see is a crack waiting for a temperature swing.
There is a cost angle too. Cracks mean glass replacement, sometimes a delivery lead time of six to twelve weeks for coated insulated units. One cracked atrium panel can cost more than a year of cleaning.
Does water quality change the thermal risk?
It does. Hard water leaves minerals that flash-dry into spots as the surface cools, and cold hard water sprayed on a hot pane can also trigger localised stress at the impact point. Soft or deionised water at a moderate temperature is the safer combination. Keep the tank out of direct sun too, so the water does not bake between runs.
How do you measure surface temperature quickly?
An infrared thermometer is enough. Take three readings per zone – centre, edge and near any mullion – and record the highest. If any reading is above 55 & deg;C, hold off or pre-wet. Repeating the check across the season builds a simple heat profile for the roof and turns scheduling into a decision instead of a guess.
Should hot-climate sites clean at all in summer?
Yes, but earlier and with warm water. Skipping summer means three months of dust baked on by heat, which is harder to remove later and looks worse. The practical answer is a dawn shift two or three times through the hot months, plus a light pre-wet. Some sites accept a mid-summer pause and a deeper clean once temperatures fall, which also works if the dust load is low.
How do you brief operators on the risk?
Give them one rule and a thermometer: if the pane reads above 55 & deg;C, stop and either pre-wet or reschedule. Explain what a stress crack looks like – a sudden sharp line from an edge or a chip – and what to do: stop, photograph, report. Operators who understand why the rule exists follow it far more reliably than those given a number alone.
How do you handle glass with existing damage?
Photograph and document it before cleaning, whether or not you think it is related. A chip or edge scratch is a stress riser, and a thermal cycle can turn it into a crack. If a pane already shows damage, either skip it in peak heat or clean it first thing in the morning when the temperature gap is smallest. Never be the last person to touch a damaged pane without a record.
Does the machine’s water temperature matter?
It does, within limits. Warm water at 35-45 °C closes the temperature gap you are trying to avoid, but heating water on every job costs energy and time. Most sites settle for ambient water used early in the day rather than carting heaters onto a roof. Where a site genuinely needs large volumes, warm water is worth the hassle only if dawn cleaning is impossible.
If in doubt, delay. A one-day delay costs nothing. A crack in an atrium pane costs a replacement, weeks of lead time and a conversation about whether your cleaning process caused it. The conservative choice is always available and almost always cheaper than the alternative, so take it early rather than learning the lesson on the roof.
There is also a habit worth building: keep a small thermal log for the roof. Two or three readings a week through the first summer, recorded with the cleaning dates, tells you exactly when the risk window opens and closes at your site. That beats any generic rule of thumb, because two buildings a street apart can differ by ten degrees depending on tint, airflow and shadow.
Key Takeaways
- Keep the water-to-glass temperature gap under about 30 °C.
- Tinted atrium glass can hit 75 °C in summer; clear glass rarely passes 50 °C.
- Clean before 09:00 or split east/west faces around the solar peak.
- Pre-wet then scrub when you must work in heat, never a cold single pass.
- Visible edge chips plus peak heat equals stop-and-inspect, not clean.
For weather-matched schedules and machine specs, use the contact page. More operational notes are in the news archive.