Roof Deck, Underlayment, Flashing, and Waterproofing
The water-shedding envelope
The deck is the foundation. Know minimum thickness and span ratings for wood sheathing (e.g., 15/32 in. or 7/16 in. OSB, depending on rafter spacing), the required gap between panels for expansion, and fastener schedule (typically 6 in. on edges, 12 in. in the field for sheathing). Know when a drip edge is required at eaves and rakes, and how it interacts with the underlayment (underlayment laps over the drip edge at eaves, but the drip edge laps over underlayment at rakes in many details - verify with local code).
Underlayment: asphalt-saturated felt (#15 or #30) vs. synthetic (lighter, stronger, more slip-resistant). Know the lap requirements (typically 2 in. side laps, 4 in. end laps) and the slope-dependent rules - some underlayments are doubled or replaced with a peel-and-stick membrane below certain slopes.
Flashing: step flashing at sidewalls (each course interlocks with the shingle course), apron/headwall flashing at the top, valley flashing, pipe boots, counterflashing at masonry, and kick-out flashing at the bottom of a sidewall to divert water away from the wall. Kick-out flashing is a favorite exam trap.
Worked example: a sidewall intersecting a roof needs step flashing at every shingle course, a kick-out at the bottom, and counterflashing where the wall is masonry. Missing the kick-out causes water to run behind the siding - a common failure.
Rule of thumb: water always follows gravity and capillary action - detail every penetration and transition.