There are two main categories of roofing. Low slope and steep slope. Low slope is anything that has a pitch of 4/12 or less and Steep slope is everything above that. Slope or pitch is usually noted in a fraction of twelve. Twelve is used as it relates to the old school measurement of 12 inches. Slope is based on rise compared to run. Rise is how vertical the roof climbs and run is the horizontal distance on an equal plane to the flat ground. So a slope of 4/12 means the roof rises 4” on a horizontal distance of 12 inches (or one foot). So if I may jump ahead and offer this example, a pitch of 12/12 would be a 45 degree angle. This is not a pitch someone can walk on without any form of help or fall protection gear.
The general rule of thumb for usage of roof systems is that everything above 4/12 gets shingled with asphalt, cedar, concrete, clay, metal or composite shingles. Anything below 4/12 should get roofed using a membrane system such as SBS torch on, EPDM rubber, TPO, PVC and the quickly disappearing tar and gravel roof systems. Shingles depend on positive watershed and are not entirely self-sealing. They rely on overlap and gravity. A membrane roof is equivalent to enveloping and is a continuous membrane system with no weaknesses. I have waterproofed planters in torch on membrane and they are continuously holding water.
Now shingle manufacturer’s want us to buy more of their product so they say we can put shingles on roofs that have as low a slope as 2 or 2.5/12. They have their low slope requirements and necessitate changes to the roof system other than what would be done on a 4/12 or greater roof. I have seen it done and it has worked. I exclusively use a GAF-Elk shingle because they are so well self-sealing and become almost membrane like once they have had the heat of the sun to activate the patented Dura-grip adhesive on the back of each shingle.
The steeper the slope/pitch of the roof the less water is going to adhere to the surface and erode the waterproofing material. Therefore it goes to say that a flat roof requires a qualitative waterproofing system as it may hold a large quantity of ponding water for extended durations, depending on the effectiveness of the drain placements. Flat roofs commonly settle over time also (or sag in the middle of the roof area where the structural support is at the least) and when the rigid drain, supported by cast iron plumbing pipes, doesn’t move it results in excessive standing water as the drain is now “high”.
The attached photo shows an interlock shingle application on a low slope roof in which the contractor should have chosen a membrane roofing system. Trust Crucial Roof Services to choose and install your low slope roof system. Roof system selection and integrity thereof is of primary regard in all our roofs installed resulting in absolute customer satisfaction and continued referral business.