OK the manufacturer's don't recommend placing metal roofing on a slope less than 3/12. Just like everyone else " I just need a little more". Is it the water back up that they are concerned with or the weight load or combination of snow/water. Maybe Bob will pick up an enlighten me or anyone else who had installed one on a lesser pitch.
Wierd-- I built about 300 steel buildings with either a 1/12 or 2/12 pitch depending on the brand. I'd say they are telling you that to limit their liability.
Get strip caulking for all the seams -- flash and caulk properly and I don't see how it could leak except in the case of poor craftsmanship.
I don't see weight of water, etc being a problem. The metal probably weighs less than roll asphalt roofing that's recommended for low pitches. I know it weighs less than asphalt shingles.
Glenn may be onto the real reason.
I think you are getting the standard low pitch rule. Do as Glenn suggests. The seals will most important if you have some heavy wind driven rain and/or ice dams in the winter/spring. The lower the pitch the less gravity a leak has to overcome.
Use ice and water seal under it , also called peel and stick. The whole roof that is . The tricky parts are at wall step ups , if you have any, and wind driven rain , IF , it can stack up / be blown up and under any counter flashings.
Extend your lower eave line / facia over hang at least 2 " so water can't capalary / wick back up and get to the facia and or roof sheathing. In other words a nice long drip path right into the center of the gutter.
And Glenn and John ideas as well.
3/12's as low as we've done.
Any vents or skylights ? Those could be trickie. On that low a pitch MTL not recommended .
If you have to have any of those you might need to rethink or relocate them to another section of roof , IF, possible that is.
Peg I am going to use perlins instead of sheeting on the porch roof so I guess storm/ice guard is out. The lesser porch roof will be placed under the overhang of the main cabin roof(10/12). There will be a 10-12" drop from the roof edge of the 10/12 to the porch roof. Maybe I can get a picture posted that will illustrate what I am doing. Well I am up to the mountains to work on the soffit. This has to be done before the roof is addressed anyway.
If there is snow it may try to dam and water could back up and flow over the high end so I would get external rubber seals and preferably a longer leg flashing from the wall out. The rubber seals go over the sheeting and under the flashing. I would also caulk them top and bottom. If no snow that probably would not be as much of a problem. The buildings I did were in a no snow area. The steel roofing company should have seals available as an accessory.
Most 26 ga. or 29 ga. high tensile will span 4 feet at 20 lbs psf. Purlins at 2 feet would probably take that up to 40 psf.
I'd say sheet it and use the Ice and water shield.
If your in a snow area you will have snow slides off the main roof and the metal alone will not take that without support , it's cheaper to do it once right than it is to do it twice. d*
Peg This is the area that I was speaking of. The 3/12 or lesser will be put up under the overhang of the main roof. There will be a 10-12" drop from the main roof to the porch roof. The porch roof will start approximately 12" back from the edge of the main roof. If I would use a pitch diverter or flashing then my pitch would have to be less than 3/12 which will not give enough headroom on the rafter tail end. By using a 2/12 and a rim joist for the rafters to sit on I can probably live with the distance.
Another alternative would be to use a pitch diverter with a narrow top(2 or 3" elevation) and a seal on the diverter half the distance of the overhang attached to a 2X2 or 2X4 filler between the porch roof and the soffit of the overhang. Make sense?
I wanted to stay away from sheeting and use perlins 16"OC. I could however use sheeting for the upper 4' to reduce the damage by the snow/ice slides.
Hope these photos help
John
(https://i220.photobucket.com/albums/dd161/redoverfarm/100_1550-1.jpg)
(https://i220.photobucket.com/albums/dd161/redoverfarm/100_1549-1.jpg)