We're building a roundwood tractor barn for a small tractor. The ground measurements are 6m by 2.50m (about 18x7.5 ft). It's around 5m (15ft) high to the ridge, with a steep pitch on the roof to fit in with the surrounding buildings. The wood has been fairly expensive to construct the frame but still reasonable, and locally sourced. The problem comes from the roof, which is about 36 square metres (108 sq ft). Due to the steep pitch it's probably impractical to attempt something like a mud/adobe finish, or a living roof. Tiling it, even with reclaimed tiles, is incredibly expensive. Tin/metal sheeting looks ugly, plastic doubly so.
We've been pondering an alternative covering, and my husband had been thinking about fabric. Then the thought came to him that world war 1 and 2 aeroplanes often had a wooden frame covered in doped fabric, so could a similar principle be applied to a roof? Some basic research shows that dedicated aeroplane skin is fairly reasonably priced, and some types (like Ceconite) appear to have a decent lifespan when exposed to the elements. Older dopes tended to be nasty chemicals and highly flammable, but there might be a more modern, environmentally friendly alternative ??. He saw banana oil mentioned, though where you'd get industrial amounts of banana oil from is another thing! Ignoring dedicated aeroplane skins, we were wondering if it might be possible to source a large sheet of fabric fairly cheaply, perhaps an old boat sail, or used marquee.
In terms of attaching it, it woud be very easy to put slatting across the existing roof chevrons and adhere the fabric to that. The dope finish would then aid the waterproofing and tighten the roof hard against the structure. I guess the downsides would be the possible fragility of the roof and the fact that a tear would be difficult to repair. It also might look quite odd stretched hard against the underlying wood - a bit like skin over a bony hand.
I had some friends who moved in the late fall and needed to put a roof on a shed in a hurry. They used what they had: a pile of threadbare sheets and mixed colors of latex paint. They hoped it would last the winter. As I recall, they had three layers of sheets. After the first, they sheets were pressed onto wet paint. At some point they repainted the roof and added another layer or two of sheets. The roof was about 10 years old when I saw it, and was still holding up and waterproof.
Have you considered using canvas painter's drop cloths? They are fairly inexpensive. The doping used on aeroplanes was made to be very light weight and slick, for a shed roof you really don't have that worry so I would recommend using an elastomeric latex coating which would be a more natural less toxic product. The coatings are used for sealing flat roofs, or old metal roofs on trailer homes etc... They can be tinted different colors, and life expectancy would be more than twenty years. You could even start with a layer of burlap, (burlap has large mesh openings which would allow the latex to soak through and adhere to the roof sheathing) roll the elastomeric coating onto it then put the canvas on while the latex is still wet, and another layer of the latex over the canvas. The canvas layered with latex has often been used on wooden boat roofs, so it must work pretty well. One other consideration is that using this method you could do some interesting shapes, you could even give the roof a thatched look.
Recycled billboard vinyl; 18 mils thick , water resistant/proof, lasts 5 years untreated in UV exposure, painted probably much longer. 14 x 48 piece is under $50, you can get various sizes. Yes, this is recycled, so one side is printed advertising. I cover my patio with it, shade in summer, water proof in winter.
The roof of our house is ferro-cement. Concrete. It's a barrel vault. Works great. We are building a USDA meat processing facility for our farm and using the same techniques to build concrete ceilings and roofs. Very solid and long lasting. Minimal to no maintenance.