The pergola is a feature of the outdoor environment. That being said, it is always exposed to the elements. And the very structure and size of the pergola does not always make it practical for the hard-pressed homeowner to cover it with an awning. Indeed, the way pergolas are designed and built and installed these days does make provision for the weather. And it need not be inclement to cause gradual damage to the pergola’s materials.
Metal-proofing is still possible but it is hard labor through and through. Unless you are prepared to coat the pergola’s metal roof with a form of rust guard that might just work every other month. You save yourself time and hassle by just sticking up aluminum pergolas for once and for all. And by the way, it is a lot cheaper and lighter too. And that also makes it less cumbersome to clean and maintain. You did not think you were going to get away with that now did you?
Of course you must still clean and maintain your pergola, just like everything else. It does not matter whether its roof is constructed from aluminum or not. Nevertheless, with or without coating, aluminum does have the ability to withstand the vagaries of the weather, it can be mild, it can be inclement, a lot better than other materials, certainly a whole lot better than wood or tarpaulin anyhow.
And why would anyone wish to put a wooden roof over their pergola deck? That people do is a matter of choice and is probably more of a luxury than anything else. And you can just about forget about the tarpaulin. Because by the time the next great big storm comes, it will be ripped to shreds.