D.M. Procida wrote:
This may turn out to be a stupid question, but I have nothing to
prove...

I don't understand why it isn't possible to make roofs cheaply, using
board materials instead of tiles or glass fibre. It's possible to make
boats waterproof, with appropriate sealing materials and pain, so why
not roofs, which have to deal with a lot less water?

They do, but they have a comparatively short life compared to the
'traditional' coverings - and glass fibre coverings on a close boarded roof
are generally more expensive and time consuming to lay, particularly on a
'pitched' roof (there may be differing opinions on this BTW).

It would be convenient and cheap to put large plywood boards on a
roof, make sure that there's some kind of overlap or flexible seal
where their edges meet, and finish them with thoroughly waterproof
paint.

Again they do - but with the same results as above. As a matter of
interest, some roofs are 'timber sheeted' with traditional coverings such as
slate and tile and are called Close Boarded Roofs - and are very expensive.

I'm assuming that I'm not a genius who has thought of something that
has never occurred to anyone else, and there is a reason why this
isn't done - what is it?

A great deal of exposure to the sunlight and the UV rays tend to destroy the
'painted' coverings over a relatively short period of time (around 10 years
or so).

When you consider that Welsh slate, Belgium slate and stone tiled roofs can
have a life in excess of 100 years (cement slates are less than this) and a
concrete or clay tiled one of around 30 - 100 years, all with a good, long
lasting aesthetic appearance - then a 'painted' or felted roof really is a
no-brainer both cost wise and aesthetically. Again, opinions will differ,
but the test of time has proved which are best to use.

Cash