What is the difference between 'roof' and 'canopy'? And would you describe the sky as the roof of the earth or the canopy of the earth? (Sky meaning everything up to the ozone layer.)
Typically, a canopy is used to provide shade while a roof offers more solid protection; canopies filter and roofs protect. You often hear 'canopy' used to describe the sky-ward protection you experience in a forest or jungle--basically only shade and maybe a little protection from rain. A cave would offer roof-like protection--shade, protection from rain, and protection from big, hard, nasty things that fall from the sky.
These pseudo definitions could be applied to earth's atmosphere in a similar way. Ask, "Does the atmosphere provide a shade-like protection against things from outer space, or does it offer more solid protection?" I think for most of the big, hard, nasty things that fall from outer space that our atmosphere is only a canopy, but if you are specifically talking about types of radiation that our atmosphere completely reflects or absorbs you could claim that our atmosphere (or sky) is a roof.
Though both refer to some sort of surface above something, I think they are distinct in terms of structural support. That is, a roof is generally a rigid object held in place, while a canopy is looser and hangs above something.
For example, big-tops and forests have canopies while houses and caves have roofs.
It is said the Himalayas are the roof of the earth, so perhaps the sky cannot claim that title?
would you describe the sky as the roof of the earth or the canopy of the earth.
Yes, I would describe the sky sometimes as the roof of the earth, sometimes as the canopy of the earth. Of course I would also sometimes describe it as the X of the Y, where X and Y are any other handy words that evoke whatever quality of the sky I am trying to communicate. Playground:Gods. Scarred-battleground:Jetliners. TV-Screen:Cosmos. Note to self: don't give up day job.
So, to answer your question, you first have to know what it is you are trying to communicate. Most people understand the basic (above/below) relationship between sky and earth, so metaphors like roof and canopy might seem a bit, well, banal.
Sky meaining everything up to the ozone layer
Metaphor is poetic, you should avoid seeming like Babbage writing to Tennyson