It is harder for water to shed off of shingles the more flat they lay. The bigger problem is when it creeps back underneath the shingle and finds its way into your home through a nail hole. With flat roofing, there are no nails, there are no screws, there are zero penetrations and avenues created for water infiltration. When it comes to low pitched roofs, this is a must. Otherwise you will be paying for roof and interior repairs for years to come.