It’s not all black and white: why roof colour matters

Dark roofs in a Melbourne suburb

Many houses in the Australian suburbs have dark roofs. But in a land as hot as ours, why is this so? Lance Turner tries to answer this question.

It seems that almost every new home built in Australia has a dark coloured roof, with the majority ranging from grey to black. While they may blend into the landscape better than a light coloured roof, they are certainly not as good at keeping temperatures in the home livable.

The reason goes back to basic school physics of course—black (or dark coloured) surfaces absorb a great deal more heat energy than do white (or light coloured). Indeed, it’s not unrealistic for a black roof to reach 80°C on a hot day.

The real problem though comes from the fact that the under surface of the roof re-radiates this heat into the roof cavity of the home. Unless the home has very high levels of insulation, this heat rapidly migrates into the living spaces through the ceiling. This can be felt on hot days by touching the ceiling. It’s not unusual for the ceiling of a poorly insulated home to reach 50°C on a very hot day.

Now, all this is pretty obvious, but the reason so many homes in Australia suffer from this problem is less so. After doing some research, it became clear that there are at least three main factors in the black roof issue.

The first is that many people prefer the look of a dark roof because, as mentioned earlier, it blends into the surrounds better, although this depends on the surrounds of course. However, as can be seen in the photo above, in areas with lots of greenery, darker roofs do indeed stand out less.

The second factor seems to be the building industry. Unfortunately, the industry does tend to set trends based on what they perceive the customer wants (which is not always what they actually want or need). A great example of this is halogen downlights. Builders install these environmental disasters because they are cheap to buy and because they believe customers want a flush-fitting trendy light. The fact that they increase the running cost of the average house by hundreds of dollars a year doesn’t enter the equation.

Dark roofs come about from the same beliefs. Builders install them because they think customers want them, and customers want them because ‘the building industry always uses dark roofs, so they must be the best option’. Unfortunately, not many people put much thought into such decision processes.

The third factor is local government. Many councils have restrictions on how light a colour you can use on a roof. The main reason for this seems to be that the majority of the housing stock already has a dark coloured roof, so new buildings have to blend in to some degree.

The lack of foresight is plain to see—if no-one changes the equation, we will be stuck with dark-roofed, thermally inefficient houses for evermore.

This article first appeared in ReNew 110. Click here to read the full article including DIY options for a cool roof.

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