I’ve got a rather battered old tandem garage-rear half is fine as it was added later and is a lot newer, but the front half’s roof (and floor) are buggered.

It looks exactly like this:

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Ridge is all gone, most of the sheets are cracked or missing.
I’ve looked at various replacement options, and the best for condensation/cost appears to be 18mm OSB, membrane, then Coroline. Anyone want to convince me otherwise?
Tin and cement board are expensive and/or drippy, don’t want to do the flat roof thing as the trusses hold the whole thing together!
The garage is sectional, probably 70s, wall construction like this:

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Looked at EDPM but it really won’t suit aesthetically.
My only concerns are the purlin spans (largest is 950mm) and the weight on the walls. But the sheet on there at the moment isn’t exactly light!
Obviously a new garage would be preferable, but it’s not viable right now.
And yes, don’t worry, the asbestos side of things is already covered. 🙂

I did my shed last year with Coroline, so not really on the same scale as your requirements but I found it really easy work work with.

The previous roof was clear corrugated polycarb (previous owner used it as a potting shed) so there were no boards in there – just a cross beam (purlin?) virtually a at the bottom of the roof & right at the ridgeline.
I just added an extra purlin in the mid-point on either side & mounted the sheets directly to that. in your case though, I think you would be better with OSB underneath.
I think that because I didn’t put any board down, I experienced some condensation in the shed. I need to resolve that before the weather cools drastically with some insulation & plywood.

I found a vid on youtube of a guy doing a large shed/garage and he made it look dead easy.
This one:

I see he didn’t use any board underneath, either.

Why boarding under coroline?

I have onduline on my garage it’s been there for 10 years I’ve lived here and shows no sign of letting up.

I am looking to do the same on the new shed.

Won’t be boarding it.

Tbh the main reason for the boarding is to allow for a bit of insulation, and to support the Coroline/onduline. I’m very aware it has a reputation for sagging if not supported well.

Ah mines is on 6inch joists

I’ve also epoxy spray foamed under it….as is the whole concrete garage.

I’ve just built a timber double garage I bought off EBay and have roofed it with Corrapol (marginally cheaper version of Onduline) on top of 25mm celotex which is laid on the original t&g roof panels designed for felt. As said above – easy to work with.

Wiggly tin (or at least pre painted steel) will last longest although it may be pricier. My dad has a similar garage, green painted wiggly tin still like new 10 years on. It also needs less support than some alternatives.