Worth considering is that the roof is the part of the building most on view even if your railway is ground level and its worth it to get it looking as good as you can.
Brandbright is back in business though their 'Materials' section is still not stocked. I've used their brilliant roofing sheets before and can recommend them especially their corrugated iron. I'm assuming they will be producing these sheets again.
Failing that the National Show is in early April and there will be plenty of options available there. If you can't wait then I'm sure you will get some advice on building some of your own.
I would suggest you don't attempt to use the jigstones roofing sheets as they are very small and you would need to assemble the units and camouflage the joins.
Also, as you add to the building keep checking the weight of the whole thing. I have a terrace of 2 storey miner's cottages and I need my wife's help to get them onto the workbench.
Here is an old photo of a building I made for a friend. The roof is cut from a sheet of fibreglass moulded roofing slates and is robust enough not to require any supports or sub layer. This makes the job easier, if you can get hold of the stuff.
Good luck