can roll roofing be put over shingles on a nearly flay roof... it does something like slope a quarter inch per foot.
A Happy New Year to you and let's see if we can provided some insight on the situation.
Asphalt and other roof covering like them are water shedding materials and not a waterproof barrier as they do not seal together to form such a barrier. These product simply overlap and allow rain water to be shed form the surface as it runs across the surface. Minimum slope for shingles and roof coverings that overlap such as this is 2 inches of slope per foot of rise and with that usually require a double underlayment beneath the roof covering. Roof slopes 2/12 and steeper are known as "steep-slope" roofs. Those less than 2/12 are known as "low-slope" roofs.
Low-slope roofs require a membrane type roof which is adhered to the surface and the laps to form a waterproof barrier. Rool roofing is a common generic term used by people which often are referencing a wide range of products. Roll Roofing that you see in the typical home center is a fiberglass cap sheet is often mis-sold by home center employees that are unaware of what is required by Building Code for roofing and for proper performance. A patio that is open to weather and not enclosed is not limited by Code in most areas as to the type of "roofing" that can be used. If it is enclosed or in any way has any occupied area such as a sun room or other partially enclosed structure must comply with Building Code and have a fire rated roof on it. This is going to require 2 or more membranes depending on deck type and membrane type. Simply adhering a cap sheet over shingles to an occupied space would not be compliant with Building Code.
If you have a patio (non-occupied space) that has shingles and you want to cover over it with a roll roofing from a home center, and fully adhering to the shingles can improve the situation, however, covering over a shingle roof is unlikely to last for more 3 to 5 years if it does not have foot traffic. The roll roofing will bridge over the irregular surface of the shingles and be vulnerable to damage if walked on.
The best do-it-yourself application would be to remove the shingles to the wood deck and clean free of debris, then secure any loose wood deck. Apply a #30 felt with larger head nails, attaching to the wood deck after allowing it the warm prior to nailing. Do not allow to get any moisture to it as it will wrinkle as it is an organic felt product. Fully adhere a second #30 felt using an appropriate adhesive (also available at the home center) to the nailed felt and allow to set overnight. Fully adhere the roll roofing in the same adhesive as the finished surface. Adhesive should be used at a rate of 1.5 to 2 gallons per 100 square feet (10ft x 10ft area). This will provide a reaosnable roof system which could perform for a longer time period.
Consider the above and if you have additional questions or other information to provide more information please feel free to provide.