1. Using Aluzinc (aluminum-zinc alloy-coated steel) roofing because it is rustproof, which can make the roof long lasting for decades (30 years warranty as the company’s term)
2. Using the proper roof profile and fixing system, nowadays, it is popular for roofing as Lean-to-Roof type which the slope angle is lower than 5 degrees, 1 degree also has seen. If the building located in the heavy rain area, it has to consider about the roof profile and boltless or seam system which will be better for anti-leakage. You should discuss with the engineer for suggestions and technique information.
3. Using the anti-leakage roofing system, which there are the anti-leakage roofing accessories and strengthen equipment to fix the roof profile, anti-leakage for the entire roofing area and seal the joints, which can make the building look nice (there are flashing, end closer, clip lock, and special flashing for the opening gap on the roof such as solar cells, lightning rod, smokestack etc.)
4. Using the experienced and skilled roof installer with installation warranty, including has the special techniques professional teamwork to make the house look nice, strong, and durable.

To fix flashing on the roof by using C-clip support, so screw doesn’t not pass through the roof sheet for anti-leakage, is truly boltless system.

Profile selection is using the proper Aluzinc coated steel roof sheet, roof profile, fixing system, anti-leakage roofing system as suggested above, including using the experienced and skilled roof installer. Those will eliminate the problem in water leakage, humid, or rust. So, definitely no worries for the (steel) roofing in rainy season.