People have commented to me that when they have been painting their galvanised roof they were surprised at how much cooler the iron was after painting, or even during painting when they have come back from lunch and put one hand on the unpainted iron and the other hand on the newly painted iron right next to it.

I suppose how long an unpainted galv roof would last depends to an extent on the local climate. For example if you lived in a dry climate then it would last longer, if you live in a humid warm climate it would corrode more quickly and need replacing earlier, and it is well known that if you live near the sea it will corrode faster due to the salt deposits from the sea air. Since emissions are generated when the roof is manufactured, the lifetime of the roof becomes a factor when calculating the annual emissions cost over the lifetime of the roof. I don’t have the numbers on this or for emissions from paint manufacture, but it is on my “to do” list when I have time.

By comparison, a number of scientific studies have found that a flat white roof reflects enough sunlight back into space to have a cooling effect that cancels the global warming of roughly 10 tonne of carbon dioxide emissions. And if you have air conditioning then you may also have energy savings from reduced air conditioning use which in turn reduces CO2 emissions. A 2010 study into four types of commercial buildings across the USA, looked at overall energy and emissions savings from replacing a dark roof with a white roof (summertime cooling savings minus winter heating penalty equals overall savings) and found that savings were greatest in the warmer zones in the south of the country and that even up in the cooler zones near the Canadian border there were savings in energy and emissions. This study did not also take into account the added benefit of cooling the planet by reflecting sunlight back into space, as that has been covered in some depth in other papers.

Ian

There is probably a lot to be said for the energy and other environmental savings in never painting a roof, however. This kind of environmental accounting gets complicated.

I added two rooms to my house well over thirty years ago, and left the roof unpainted. 34 years have passed and there’s no rust to be seen! Not even around the lead-head steel nails holding the roof down (the more modern screw-bolt system involves galvanised screws which might be even better). I do recommend no painting of roofs, and certainly no ‘etching’ which removes the galvanised protection, not even where the iron sheets lap each other. And, since that leaves a shiny steel surface the reflective quality would surely equal a white-painted surface.