Composition can be used on slopes less than 4/12 down to 2/12, but NOT lower than 2/12. ALL manufacturers require an ice and water shield peel-and-stick membrane on these applications. Otherwise part of their material warranty is void.
The only ice and water product that warrants the ‘Gasket Effect’ is made by Grace. Their product attaches to anything that penetrates it, so leaking and ice damming cannot occur. With this membrane, standard underlayment is not needed. There are cheaper ‘entry level’ membranes, but they can’t touch the performance of Grace Ice & Water Shield.