The roof pitch is only 1:12 That won't work for shingles. I work with a company that makes thousands of tons of APP (atactic polypropylene) and SBS (styrene-butadiene-styrene) modified bitumen roofing membranes. The membrane is reasonably priced and is most often used in commercial applications. The membrane sheets are usually joined by melting the overlapped seams forming a continuous barrier. So It may not be a DIY roof. Your options are limited due to roof slope, but the roof is cheaper than adding trusses and a steeper pitch.