There is one skill you will never hear talked about that is vital to successfully completing virtually any roofing project. It is not mentioned because it is not something anyone ever thinks about. It is a skill that can be learned by most, but some seem to come by it naturally. It is the ability to think like water.
It is certainly important to read the application instructions that will come with any roofing product and follow them closely. But application instructions will rarely be able to cover every situation that arises. Nor can instructions be in the field to explain all the variations of a given application difficulty, or to look at the finished product to verify that it is correct.
It seems obvious to us that we will understand how water flows, we see it every day. We watch how overflow runs down a street, how a river runs, how drops fall. It is not hard to predict how water will act in those situations because they are familiar to us. However, when dealing with a new situation like how water will flow down a valley or around a chimney, the truth might not seem so obvious.
It may be that the details of how to deal with a particular situation get lost in all the other details of roofing. How to apply the product, how to fasten it, or which goes under and which goes over.
The importance of understanding water flow is the same whenever roofing is applied, but here are some examples specific to the application of asphalt shingles.
There is an obvious concept that gets missed surprisingly often. Vertical splices between the shingles must never line up. When they do, water running off the surface of the top shingle will flow right through the splice in the shingles below it and to the substrate. This includes being aware of the splice in the starter course. That splice is especially easy to forget because it is completely covered by the first course of shingles.
When the rows of shingles meet a vertical wall, metal shingles need to be applied. These are light weight squares of metal that are bent to go up the sidewall and on to the roof. Each metal shingle needs to overlap the metal shingle below it. In addition, the water that flows onto each metal shingle from the asphalt shingle above needs to flow out onto the asphalt shingle below it. If this simple rule is not followed (and it's surprisingly tricky), the water will again flow under the shingles an onto the roof deck.
When dealing holes in the roof, such as those covered by roof vents and plumbing stacks, take extra care to pay attention to water flow. Be sure that the water will flow off the shingles above onto the cap and then flow off the cap onto the shingles below. Keeping this simple sequence in mind will result in a leak proof installation. It might seem odd, but sometimes inexperienced roofers will fasten the cover to the roof and then shingle right over the flanges all around. A moment's thought will show why this is guaranteed to send a flood of water under the shingles rather than onto them. This error can cause a leak large enough to destroy a ceiling in no time.
One other example. If installing an exposed metal valley, be aware that the water will flow down one plane of the roof and hit the valley at speed. It will actually wash up onto the other plane under the shingles for a short distance. When nailing the metal valley in place, and when nailing the shingles that lap the valley, keep the nails as far from the center of the valley as possible.
As you can imagine, specific examples are never-ending, but the principle remains the same. Keep in mind how the water will flow from shingle to shingle, how it will work its way around obstacles in its path. Note that the farther up the roof you go, the less water there will be. And the wider the obstruction, the more water that will have to be dealt with. Pay special attention to situations where water flows around corners - water can work its way into small spaces when flashing does not cover corners effectively.
So, to recap. Whenever meeting a situation that isn't covered by instructions or training, think like water, and the solution will come.
Once you master this, you can think about water flowing up the roof.