Explainer: cool roofs for dummies

Poppy Johnston

Tower House by Austin Maynard Architects. Image: Peter Bennetts, Tess Kelly

As the days get warmer, we tend to forgo our cosy dark layers for airy pastels and whites. That’s because most of us know that lighter colours absorb less heat from the fierce summer sun.

The same rule applies to buildings: the lighter the roof, the cooler the building, and vice versa. This is hardly news to human-kind – the Greeks have been painting their buildings white for centuries – but in other warm climates, Australia’s included, dark roofs continue to reign supreme.

This may be on the cusp of change. In a city fringe suburb in Sydney’s south west, which already suffers through unbearably hot 50 degrees Celsius days in summer, dark roofs have been given the flick in favour of lighter alternatives. Note that these design guidelines don’t constitute a ban, however, but fans of slate grey roofing will have to ask for an exemption.

The arguments against lighter roofs tend to boil down to glare, which can be uncomfortable for an overlooking neighbour, and aesthetics. The winds of fashion have been blowing in dark roofs direction, with one provider of roofing tiles reporting that around 70 per cent of tile sales were black or grey back in 2014. In some more established suburbs, white roofs have been considered at odds with the overall character and heritage of the area.

For homes that do opt for pale roofing, the benefits can be huge. There are many factors that influence exactly how much cooler a light-coloured roof will be compared to their darker counterparts, but most researchers agree the differences are stark. The Fifth Estate has gone into more detail here but some light-coloured roofs have been found to reflect up to 70 per cent of summer heat gain, which is around 50 per cent more than a dark roof.

That can be imagined as the cool roof being comfortable enough to walk on and the dark version hot enough to fry an egg on, as one expert has explained.

Why this happens comes down to some pretty basic science. When a surface appears a colour when it is illuminated with white light, like sunlight, it means it is reflecting light of that colour and absorbing all other colours. Light is energy, so the more light it absorbs, the warmer it becomes. A black object absorbs no wavelengths of light so will be warmer than any other hue.

It’s also worth noting that the inverse is true. In colder climates, heating-centric climates like Tasmania, heat absorbing dark roofs may be preferable. Check out the Australian government’s YourHome guide to get a better idea of what roof is best for your climate, but don’t forget that climate change will, ahem, change climates.

Save money and emissions with a cool roof

But why all the fuss about a hot tin roof? Well, that heat radiates down into your home and will demand a lot more work from your air conditioner to stay comfortable on a hot day. That means higher energy bills, and unless you’re relying solely on renewable energy, higher emissions.

Dark roofs also increase the surrounding air temperatures, which can worsen what’s known as the urban heat island effect. This is the cumulative effect of warm surfaces such as concrete and building materials that can send city temperatures soaring on a hot day.

For those who truly can’t stand the sight of a light roof, fortunately, it’s not our only weapon in the bid for cooler roofs and subsequently cooler suburbs. Insulation helps slow the heat transfer process, and green roofs covered in soil and plants have also been shown to act as an insulating layer. You can also put those sun rays to work rather than reflecting them back into space by capturing them with a rooftop solar system.

There are also specific heat reflecting products on the market, some that even come in darker colours that still claim to reflect up to 50 per cent of sunlight.

Couldn’t care less about the colour of your roof? Perhaps one day you can coat your roof in a new paint dubbed “the whitest paint in the world” by its creators, which contains barium sulfate (used in photo paper and cosmetics) and could do more to cool your home than your airconditioner.

How cool.