When most people think of a pitched shingle roof, they don't think about how poor drainage and water retention can cause damages. Unfortunately, water can collect on even a pitched roof if the conditions are right. Knowing what these conditions are can help you avoid them.
1. Edge Barriers
There are many ways that water can get trapped on a pitched roof. The gutter system is the most common culprit. If your gutters are blocked, too narrow, or misaligned, then water may not flow off the roof and into the gutter trough as it should. Instead it backflows back up the roof plane when it hits the blockage in the gutter, then the moisture can seep beneath the shingles and lead to leaks.
Barriers can occur even without gutters. If there is damage at the roof edge, such as curling shingles, water may not drip off properly. Much like the effects of a clogged gutter, the water will pool behind the damaged roof edge and eventually seep beneath the shingles.
2. Poor Interior Ventilation
Ventilation inside the attic can cause issues that can affect the underside of the roof. Condensation and collected moisture is a major problem in some attics. Warm, moist air from the home rises into the attic, where it can collect and damage the underside of the roof decking (as well as insulation and components in the attic). Attics aren't made for drainage, so condensation has nowhere to go.
Venting is key to preventing this moisture buildup. Vents on the underside of the eaves bring in dry air, which pushes the damp hot air up and through a second vent system in the roof ridgeline. On large or odd-shaped roofs, additional vents may be needed along the main plane of the roof to provide additional ventilation. Some steeply pitched roofs also need a fan to help push through the damp air.
3. Roof Plane Blockages
Blockages on the main plane of the roof can impede drainage in much the same manner as roof edge barriers. Piles of leaves and other debris are the most common type of blockage, so it is important to clean the roof and trim back trees. Moss growth can also cause a blockage, so consider installing zinc strips beneath shingles to kill off potential moss before it takes hold.
Finally, shingle damage can create a blockage. Cupping is when the center of a shingle becomes depressed while the edges curl up, basically creating a shallow bowl that can collect pooling water. When this happens, it is usually time to replace the shingles. Contact a roofing contractor if you suspect drainage issues may be affecting your roof.