Hi all

I have an old (1950s?) garage roof that's prone to leaking, so I'd be
glad of any advice on my options.

You can see what it's like from the photos,
inside: http://tinyurl.com/6dkry
on top: http://tinyurl.com/5sdxl

From inside, the roof seems to be made up of concrete beams arranged
like floorboards, with joins in places that make me wonder what
they're attached to.

From on top, it looks like crumbly concrete covered in various layers
of patching and gunge, (some of which I've just peeled off as it was
detaching itself), with several breaks in the level.

From inside there appears to be a consistent slope, but from on top
it's a much messier proposition.

Can anyone tell me a) how the roof is likely to be constructed (and
what's holding those beams up?)
and b) what options I have for making the thing reliably waterproof?

TIA
Ken