All steep-slope roof systems (i.e., roofs with slopes of 25 percent or more) have the following basic components:

1. Roof structure: rafters and trusses constructed to support the sheathing.

2. Sheathing: boards or sheet material that are fastened to roof rafters to cover a house or building.

3. Roof underlayment: typically asphalt felt applied in rolls directly on top of the roof sheathing. The underlayment is a second line of weather resistance and helps shed water. 

4. Flashing:  a water impervious component used to 1) diverts water away from openings in- or vulnerable areas of- the enclosure, and/or 2) directs water from behind cladding in an assembly to the exterior.  In a roof assembly, flashing is often sheet metal or rubberized membrane material. Flashing occurs in a roof system at valleys, in locations that the roof abuts a wall (step flashing), and at chimneys.

5. Roof covering: asphalt shingles, tile, slate or metal that protect the underlayment and sheathing from weather.

6. Drainage: a roof system's design features, such as shape, slope and layout that affect its ability to shed water.

7. Insulation: Insulation in attics to meet the following minimum R-values, and to be installed evenly while not blocking soffit vents:

   • R-30 in climate zones 1, 2, and 3

   • R-38 in climate zones 4 and 5

   • R-49 in climate zones 6, 7, and 8

8. Ventilation:  ridge vents, eave vents, and gable-end vents move air through an attic, reducing the amount of heat and moisture that contributes to mold issues. Baffles should to be used along eaves to promote air movement. Roofs for a vented attic or vented roof configuration include provisions for ventilation.  A roof that meets the requirements for an unvented roof does not need ventilation.

9. Airsealing: Ideally all attic penetrations (light fixtures, plumbing vents, etc) are sealed to isolate attic from space below.