Roofing
Slope drives everything
Determine slope as rise over run and use it to judge the covering. Asphalt shingles are the norm at 4:12 and steeper; between 2:12 and 4:12 they need special low-slope underlayment; below 2:12 expect a membrane or roll roofing. Shingles laid over a low-slope porch roof are a classic defect photograph - water backs up under the laps.
Flashings and drainage
Flashing is where roofs leak. Know step flashing at sidewalls, counterflashing, headwall or apron flashing, valley treatment, drip edge, and kick-out flashing where a roof meets a wall above a gutter. A chimney 30 inches wide or more needs a cricket behind it to divert water. Report missing drip edge, exposed fasteners, unsealed penetrations, and any roof-to-wall joint sealed with caulk instead of flashing.
Condition versus cause
Asphalt defects include curling, clawing, blistering, granule loss, fishmouthing, and shingles that are overdriven or have exposed fasteners. Blistering often points to poor attic ventilation rather than age. Ice dams form when warm attic air melts snow that refreezes at the cold eaves; the real fix is air sealing and insulation, not more shingles.
Worked example
A roof has a ridge vent, no soffit vents, and staining on the underside of the sheathing near the rake. The likely cause is inadequate intake ventilation at the eaves; report the visible staining, the missing intake vent, and recommend correction. Describe how you observed the roof - walked, from a ladder, or with binoculars.