1) Opening roof stuff
2) Water pressure stuff
These two things bring on the challenges. First the second. I had a dreamy vision of 4 feet of head working with a small on-demand propane system, thereby keeping the whole plumbing gravity fed. Basically I'm bummed to learn that to meet the 15 psi requirement for the on-demand propane water heater I was given I would need 35 feet of head. Not gonna happen. At least not in my next location. This makes me consider a small water pump. Or some other kind of small on-demand propane water heater. If I want to use this water heater, I need to learn about pressurized water systems. Hmm... hot water... that luxury... how / when / why / hmm...
OK, I NEED TO THINK MORE IF I CARE. "Tankless heaters are limited to a choice between expensive and CO2 problematic energy sources: gas and electricity. This makes it impossible to include other heat sources, including renewable energy. Tank-type systems have a much wider choice of heat sources available, such as district heating, central heating, solar heating, geothermal heating, micro CHP and ground-coupled heat exchangers." (source) I need to have some conversations with some local visionaries.
Second the first. Which directions to put the hinges? Use a 2" x 2" angle iron as a frame to fend off warping problems in the future? Some kind of pinion gear (used to convert rotation into linear motion)? How to fully waterproof? As my dad said, you want strength, precision, and waterproofness. Thus the challenge.