I built a collector for my
hot water system and used .018" aluminum I
think. I don't know the grade of metal. It was Amerimax(?) flashing, painted. I probably overdid the silicone, but the 36 sf collector easily produces 21,000+ BTU on a 70F day in 4-5 hours of sun. Copper tube spacing was about 5.75" if I remember. I did not do the "coil in a tub" like many/most builtitsolar projects. Used a plate exchanger. Waiting to see how it holds up while exposed to
city chlorine. Seems ok after 9 months.