If you have to shingle around a dormer, how are you going to make sure that the tabs on each side of the dormer keep their pattern? A slight variation in each course will show up when you meet the top. It’s called “tying in”, and here’s how to do it. Shingle along the bottom of the roof as you would normally. When you reach the dormer, continue shingling up to the ridge on one side of the dormer. Meanwhile, snap horizontal chalklines on the other side of the dormer, measuring up from the eave. Carry the course in line with the top of the dormer all the way across the roof, nailing near the top of the shingle, so you can slide a course underneath it. Shingle from this course up to the ridge. Now, stretch a chalkline vertically from the top to the bottom along the gaps between shingles. Do this every 3 feet. Using your vertical and horizontal chalklines, you can maintain your pattern perfectly.