I am planning on adding about 300 sq feet to my house, a recreation type room. This is be just a shed roof (combined ceiling joists and rafters) extending 12 feet rafter span across about 24 feet wide.
The grade is about 0.5 inch per foot. This will make a ceiling height of about 7 feet six inches at the far end.
People say flat roof will leak, but I really wonder about this wisdom.
I have built 2-part epoxy coated plywood (not OSB though) wood structures (large fish tanks) entirely immersed in water and it has been nearly 20 years. There is still no leak.
If I coat the OSB roof shealthing with a couple of layers of epoxy, and particularly heavily at the seam attached to the rafters, the entire roof should not leak. It should be one whole sheet of waterproof material.
The greatest threat is UV from sunlight that will degrade the epoxy, but the felt and roofing cover will prevent sunlight from reaching the epoxy.
Has this epoxy approach been tried but has failed the test of time?
I will also color the epoxy to reduce sunlight effect during construction.