Roof Drainage, Insulation, and Ventilation
Drainage
Low-slope roofs need at least 1/4 in. per foot of slope to drains, scuppers, or gutters, with saddles between drains and crickets behind large penetrations so water cannot pond. Size the drainage components: a useful rule of thumb is about 1 sq in. of leader or downspout area per 100 sq ft of roof area, adjusted for rainfall intensity. Gutters run about 1/16 in. per foot toward the outlet. Where parapets exist, a secondary or overflow drain set slightly above the primary drain protects the structure if the primary clogs. Keep strainers clean - maintenance, not design, causes most ponding complaints.
Insulation
Insulation is placed above the deck (rigid board or tapered), between framing, or below, depending on climate and venting strategy. Tapered insulation can create slope but must not be crushed at crickets or drains. In cold climates the primary control for ice dams is air sealing plus insulation plus ventilation, not more underlayment.
Ventilation
Attic ventilation balances intake (soffit) and exhaust (ridge or static vents), roughly 50/50. The common rule is 1 sq ft of net free vent area per 150 sq ft of attic floor, relaxed to 1/300 when a vapor retarder is present and intake and exhaust are balanced. Provide baffles so insulation does not block soffit vents.
Traps
Adding ridge vent without soffit intake, installing a vapor retarder on the wrong side, and confusing condensation with a roof leak.
Worked example
A 1,500 sq ft attic at 1/150 needs 10 sq ft of net free area: about 5 sq ft of soffit vent plus 5 sq ft of ridge vent.