The felt is laid out starting at the eaves working to the ridge.  At the eaves the felt lays on top of the edge metal so any water that gets through the shingles will run off into the gutters.

In the valley, you can see that a 8" wide steel reinforcement is installed.  The felt is then laid over the sheet metal which overlaps the shingles at the bottom of the valley.  As the joining of the two pitches on the roof create a "V", the shingles cannot completely seat on the wood at the bottom of the "V".  With out the sheet metal, if someone were to step in the middle of the valley, the material could break creating a potential serious leak.  With the metal in place, the material can transition from  one pitch to the other fully supported in the middle of the valley.  Doubling up the felt works but will not last as the paper dries out and becomes brittle removing the intended support for the area above the "V" in the valley.

Even with the metal we install a second layer of felt in the valley.  This area of the roof accumulates more water when raining and we feel it needs the most protection.

Once the valley is felted in, the material is installed with one side transitioning over to the other side by 2' and the adjoining side cut clean in the middle to create a clean neat appearance, making the roof look nice and smooth.