metal roof questions

Note: I put this in the rainwater forum because there was no better place for it - it would be great if we had a "retrofit" or "adapting old buildings" section.

We have a century-old "American Foursquare" house - 2 stories high, square, with a pyramidal hipped roof that is very steep (almost 12/12).  There is a two-story porch that also has a hipped roof, a hipped gable in the main roof, and a handful of bay windows etc. with their own small roofs.  The attic is not finished, the attic floor is insulated, and the roof is ventilated.  We do not intend to ever use the attic as living space, because it works well as an unconditioned space, because there is only one window, and because the shape of the attic means that the ceiling is too low to be useful except in a small area in the middle.

The existing asphalt roof needs to be replaced, and we're looking at metal for the following reasons:
- durability
- better cooling
- avoiding chemicals washing off the roof into our rain barrels

The big downside to metal is the upfront cost.

There seem to be two main types of metal roof: panel (standing seam, etc.) or tile.  The latter seems better for our application because we do have a couple of valleys in the roof, and standing-seam type roofs seem to have issues with snow collecting in the valleys.

The materials seem to boil down to steel and aluminum.  Steel sounds like it might be more durable, and since salt air is not a concern we don't need the superior corrosion resistance of aluminum.  Weight isn't a factor, either, since even a steel roof will weigh less than the two layers of asphalt that are on the roof now.  And steel is supposed to be a bit cheaper.

Concealed fasteners are more expensive than through fasteners, but they should last longer, so I think we want concealed fasteners.  Given that our house is 2 stories plus the roof and it's on a slope (making some of it 2 1/2 stories), and that the roof is so steeply pitched, the less we have to get up on the roof to do maintenance the better - and the concealed fasteners should be better in that regard.

So - my take is that we want steel tiles with concealed fasteners.  Does that make sense?  Am I missing any factors?  Do you have any experience with similar roofs?

During my research, the metal supply company said minimum 3/12 pitch recommended for concealed screw. They also did not want it lathed, they want it installed on a solid deck.

Thanks!  We have tons of pitch (10/12 maybe) so that's no issue, and the substrate is solid plywood, almost all of which can be reused according to a couple of roofers we had look at it.

I'd like the roof to be as light-colored as possible to keep it a bit cooler in the summer.  Other than color, do the various paints and coatings really add any functionality to the roof?

I got a 4 Square myself and it too needs a new roof. I was just going to put steel on the roof as it is cheaper and quicker than asphalt roofing. Around here it is $1.99 a linear foot, which is 3 square feet making it pretty cheap. I would not even consider aluminum, I have some on my sawmill and it has punched holes through it for whatever reason...no trees within acre of it. The trick is to buy a few metal cut off wheels for your skillsaw as a hip roof has a lot of angle cuts. A skillsaw makes life a whole lot faster.

I see you live in New England yourself so put the Ice and water shield to the hips, flash the heck out of the chimney if you have one, and just run self-tapping screws in. I live on a hill and my buildings have half the screws they are supposed to and I have yet to have a sheet rip off.

Its pretty forgiving stuff.

Standing seam steel roof has concealed fasteners and an extremely long lifespan. In our area old houses need roofing attention after nearly 80-100 years. I know this because I worked for a local sheet metal company that did roof repairs.