A low slope residential roof has a gradual pitch compared to those styles with a steep peak. There should still be sufficient slope for water to drain off the roof following rain or snowmelt, though. If water collects and pools on the roof, you likely have one of the following problems.

1. Cupped Shingles

Cupping is a type of shingle decay. As the material breaks down and moisture soaks into the shingle, the edges begin to curl upward. The result is a cup shape that can hold water. Of course, holding water simply causes the shingles to degrade even more quickly, which can increase the chances of a leak. Cupped shingles must be replaced, as there is no way to repair them. 

2. Rotten Decking

Underneath the shingles are sheets of plywood, which is the decking of the roof. The decking is secured to the roof support trusses, and then shingles are secured to the decking. If water gets into the decking, it can begin to rot and warp, which then leads to low areas where water can collect on the roof. Decking damage means that the damaged board, including the shingles on top of it, must be replaced. 

3. Blocked Gutters

On a steep-pitched roof, water rolls down with enough force that it will simply go over the edge of a clogged gutter. Low-pitched roofs don't provide enough of an angle for forceful water flow, which means a clogged gutter can act like a dam. Instead of running over the gutter, the water backs up onto the roof at the point of the clog. This can cause water to pool on the roof, or it may even backflow under the shingles and cause a leak.

4. Obstructions

Piles of leaves or other debris on the roof, particularly in any roof valleys, also create a dam that can hold pooling water behind it much like a clogged gutter. Low slope roofs are more prone to developing debris piles simply because items don't roll off a low pitch as easily. 

5. Sagging Supports

The supports and trusses that hold up your roof ensure that everything is held level with no low areas or dips where water can collect. If the eaves, gable end trusses, or ridgeline trusses break or develop a sag, then low areas can form where water may collect. This is a major structural and safety concern that needs to be addressed by a roofer right away. 

Contact a roofing contractor if you notice water pooling on your low slope roof.