With the bare steel roofing exposed inside, you will get rain in there, as some steam hits the colder steel and condenses. You have at least 2 options, maybe more. Add a layer of plywood or OSB or similar on the rafters before the steel goes on or a second option, make a hood with steam stack and run it either into the cupola or out thru the roof. I had 3 different evaporator set ups with home made hoods before my current one with a factory hood. If you make a hood, design it with a gutter system and drain, because lots of steam will condense and run back into the pan to be boiled again and again without it. Also, under the steam stack make a shallow funnel to catch the stack condensate and send it to the gutter you make. My gutters were made of 1x1x1 aluminum channel, mitered at the corners and "welded" using aluminum rod and a map gas torch. The drain was added the same way, another 5-6" long aluminum channel "welded" to a good place to run the drain and catch some hot water. One hint on the drain, slope it slightly away from the hood so the drips fall off rather than follow the channel back on the underside and then run down the pan and wet the gasket under the pan. A 2-3 degree slope prevents that.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.