The best way to insure no leaks is to apply a roof covering barrier that will not allow water that has accumulated on top to seep inside. Modified Bitumen is a great roof covering for low pitched roofs. Bitumen is modified by adding one or more polymers, and the sheets are typically reinforced with various types of mats or films. The sheets can be either smooth or have mineral granules on the surface, similar to the normal composition shingles you see. Modified Bitumen looks similar to 90lb roll roofing but it is very different. It is thicker and much, much stronger.
They will start with the bare roof sheathing (plywood decking) and first apply a layer of asphalt saturated felt usually 15lb, and then mop over this with hot tar. Then they will add a second layer of felt 30lb and mop again. Then they will apply the modified on top.
Modified comes in a 39” wide role and it will be overlapped 4” and sealed with a tar adhesive. At the junction point of the flat roof and the pitched roof it is attaching to, the roofers should remove the lower 3 rows of comp shingles and run the modified bitumen up to the lower edge of the fourth row. Then the 3 rows should be placed back into position over the bitumen.
It is never a good idea to have regular roofing shingles such as 3 tab or architectural shingles applied on a flat or low pitch less than 4/12. This is because water cannot run off as readily and the roof will stay wet longer or water will pool. This will allow water to seep under the shingle and begin to destroy the underlayment (felt ), and then the roof sheathing itself. Also high winds can cause the shingle tabs to blow up allowing water under the shingle with the same results of damage.
The proper roof covering will protect your investment and your wonderful new addition.