A roof slab in full sun can reach 60–70°C surface temperature. Concrete's thermal mass means the heat soaks through for hours, peaking indoors in the evening exactly when bedrooms are occupied. Tin and asbestos sheets are worse — low mass, instant transmission. Rooms below another occupied floor get that floor as free insulation instead.
Our calculator applies a 12% roof factor for top-floor rooms; harsh cases (west-facing + tin roof + hot-dry city) justify rounding up a full half ton. Mitigation helps too: white reflective roof coating can cut slab gain dramatically, and false ceilings with an air gap blunt the evening radiation. Mitigate first, then size — a cool-roof coating is cheaper than half a ton of compressor for the next decade.
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Usually 1.5T with a high ISEER handles it; jump to 2T only with a tin/asbestos roof or strong west sun in a hot-dry city. Oversizing further hurts dehumidification.
Yes — the trapped air gap insulates against slab radiation and reduces the cooled volume, often worth ~10% of the load in top-floor rooms.
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