When you are fitting a roof, and have created a valley, you will need to add valley flashing in order to direct water away from the fabric of the roof. Installing valley flashing into an already built roof will allow you to winterize it and ensure that water is kept away from the shingles. However, when you are installing the flashing, you should beware of making silly mistakes which can affect the finished product.
One of the riskiest parts of any valley flashing installation is measuring the roof. Make sure that you have a suitable tape measure, in order to calculate the whole length of your roof valley. Don't under-measure, and don't guess the length, as you need to cover both the beginning and the end.
Not Using Enough Flashing
Another problem is not using enough flashing for your device. Flashing comes in separate sections, and you need to add an overlap in order to ensure that the flashing carries water away. Don't skimp on the extra lengths of flashing; you need at least 5 inches overlap on each piece.
When you are nailing your flashing down, be careful not to pierce the roof below, and don't nail the flashing in such a way that it is not watertight. Use caulking around each nail hole.