Laying roof shingles on valleys usually sends the amateur roofer packing, however this is a lot easier to do than it first appears. It is easier to shingle a valley using conventional three tab shingles but the principle is the same no matter what type you use.
You may hear the terms plat or lace the shingles together. You kind of let them overlap in and out on the end tabs. First let’s talk about preparation then come back to this.
All valleys will need roofing felt and possibly flashing. The steeper the roof the less preparation is needed. On a roof that has very little pitch you can use roll roofing to flash the valley and then shingle over it.
Some roofers cut the shingles to the valley and depend entirely on the flashing to protect from leaks. This method, while used in some instances, is not the recommended method for shingling a valley. It does not look as good, and is not as reliable as the conventional method. This is actually the method for hard roofing materials like cedar shakes and tile.
The actual method for placing the shingles is to think of the valley like it is the edge of the roof, but do not cut the amount of the shingle off that is too long. You should have either two or one tabs to extend to the other side of the valley. Never use less than one tab.
You are actually shingling both sides of the valley at the same time alternating from side to side. Place the shingle on one side letting the excess go into the valley then fall in place on the other side. The shingle will turn on a slant and go up and out of line.
When you place the shingle on the other side, they will line up at the lowest point in the valley. Each side is called a course. You can trim off the excess but trim it no lower than the top of the course.
No nails should be placed below the proper line on each side.
You will notice the line on the felt or your chalk line goes into the valley, but use the imaginary line on the shingle, and stay at least one tab above the valley with your nails. If nails are too low in the valley, they can cause leaks.
Now proceed to the other side doing the same thing. As you work your way up the roof, the tabs will appear to interlace or "plat" themselves together. As you are laying the shingles think of the valley being full of water. That is how it will actually be in a driving rain. No nails or gaps should be in the valley.
More experienced roofers can start anywhere and work toward the valley. Less experienced roofers should start at the valley and work out. This way you will be sure to have proper placement in the valley.