A common location for a roof leak is in the valleys, or low areas, of your roof. A shingle roof is designed so that water flowing into the valleys is quickly shed, but there are three issues that can compromise this design.
1. Debris Blockages
The valleys of your roof tend to collect all sorts of debris. The more valleys in the design, the more likely it is that there will be a blockage. Dead leaves and fallen branches tend to wash off the pitched peaks of the roof and collect in the valleys, where they act as a dam. When it rains, moisture flows down the valley, hits the blockage, and then flows backward and underneath the shingles. Moss growing on the roof can cause a similar problem, or it may even get underneath a shingle and lift it up so water flows under more easily. A roofer can put an end to the leak by cleaning the roof and replacing any shingles that have suffered water damage.
2. Poor Seals
Valleys have extra seals in place since they have a lot more water running down them compared to the other surfaces of your roof. Generally, there is a bitumen-impregnated roof seal that is installed underneath the shingles, and then special overlapping valley shingles are installed over the top of the seal. If the seal fails, your roofer will need to remove the shingles so that they can install a new seal. In some cases, particularly on a roof that keeps developing valley leaks, they may also put in aluminum flashing over the top of the valley to further minimize the chances of a leak.
3. Gutter Backups
Not all roof leaks are a result of issues with the roofing. Sometimes, clogged or malfunctioning gutters can cause water to backflow underneath the shingles, particularly beneath a valley. Gutters usually have a corner piece installed at a valley, which is a shape that is more prone to blockages. Cleaning the gutters may solve the leak, but a roofer will need to assess the valley to make sure the shingles and seal are undamaged. Installing leaf guards over the top of the gutters may help prevent future backups and leaks. Gutters that are sized too small for your the water runoff may also cause backflow issues and valley leaks, but upgrading to larger gutters fixes the issue.
Contact a roof repair service in your area if you have a leak.