Corrugated metal does sound like something that will provide long lasting support. Off the cuff you’re looking at about 5 kg (10 lbs) per 1 m² (square yard) for 0.5 mm which seems to be a common thickness. The more zinc you have per area the longer it’ll last, a good paint cover will help as well - especially against condensation that you’ll probably get a lot when building in Nepal. You could probably go with less thickness and reduce weight (and lifetime), but not make a big dent in cost.
Here are a number of Chinese and Indian suppliers of corrugated sheet metal. I ticked off the trade assurance box in the hope to get reliable suppliers with short lead times. You can check those in the supplier infos (but take em with a big chunk of Himalayan salt). I think in this case the “last mile” also merits particular attention - I’ll get to that later. The response rate will probably indicate how quick they are in reacting at all. You may need a bit of back forth to avoid ordering a bunch of containers of the wrong product. I believe what you are looking for is something like 35/207 trapezoidal sheet metal or at least something that plays well with it. I assume your best bet is to make a long list and get a lot of Mandarin speakers on telephones if you want to avoid misunderstanding and make rapid progress. In Europe the stuff sells for around 7$ per square yard (10 sqft) in small batches. I’d assume you’d get it significantly cheaper in somewhat lower quality. Let’s see of the cuff there are 1E5 (new?) homeless people who need 15$ worth of roof each. That is $1.5 Million + transport. The former should not be a problem, the latter will be.
Another product that may help is GFRP roofing sheets. Here you’d want glass fiber with some transparent plastic (not PVC). Alternatively there is cellular pc - but that stuff is a little sensitive mechanically (and expensive). It is extremely light though and would probably be the first choice for air-lifting/dropping. But even GFRP will probably weigh significantly less (perhaps air-lifting would be realistic) and both will allow some light through the roofs. They won’t last forever (Nepali UV load will be high, but my gut says you can expect 5 years minimum - perhaps 10) but it may be possible to re-purpose them into some sort of cold frame later.
Regarding the last mile here is a thought: The stuff will probably arrive through the main roads on truck/20ft containers. It’ll go the last mile either by donkey or human in many cases. So what I would do is bundle them with other supplies that are typically needed in the following way. You need some sturdy long poles (approx 60-70 mm (2"-3") in diameter), perhaps a few shorter cross-beams, and some cordage and then turn the sheets into improvised stretchers with other supplies secured on top of them. This could be done in the population centers, or perhaps better in temporary distribution centers, where the supplies could just be dropped off.
Until this gets rolling I’d like to throw in this idea for consideration again. It would be much cheaper than tents (and ultra-light) - and would probably be relatively easy to integrate with existing improvised shelters. And of course this could be mixed with hexayurts as well, which would make the roofing a breeze.
I’m not sure if someone already contacted Ralf, but if there is someone on the ground who has a little background or interest in this - now is a good opportunity to get the ball rolling. It is not rocket science and it works very nicely. I helped build something like it myself.
Oh yeah almost forgot - If you need money for steel, talk to Lakshmi. But honestly, I think he is very much on this already.