How do I install rolled roofing for a deck?

My son built a deck for me joining it to the back of my house and side room. We used tab roofing and now it leaks like a sive. A friend told me I should have used rolled roofing as the roof only has a 6 inch slope. Any suggestions?

Your roof is a little bit to flat for shingle. The roof must have 15 degree- minimum. Bitumen is right.