Unit
- Deck: The structural base, typically plywood or OSB, that supports all roof layers. Deck thickness depends on rafter or truss spacing and local codes; common thicknesses range from 1/2 inch to 3/4 inch for residential construction.
- Underlayment: A water-resistant or waterproof layer installed directly on the deck. Felt paper and synthetic underlayment are standard; synthetic offers better tear resistance and UV stability during installation.
- Roof Covering: The outermost layer exposed to weather. Asphalt shingles, metal panels, tiles, and membranes each have distinct lifespans, costs, and aesthetic profiles.
- Flashing: Sheet metal or membrane strips installed at roof transitions (valleys, chimneys, walls) to direct water away from joints. Proper flashing detail is critical; most roof leaks originate at flashing points.
- Drip Edge: A metal strip installed along roof edges to guide water away from the fascia and into gutters. It protects the underlying deck edge from water damage.
- Ridge and Hip: The ridge is the horizontal peak of a sloping roof; hips are inclined intersections where two roof slopes meet. Both require specialized venting and covering details.
- Eave and Rake: The eave is the horizontal lower edge of a roof overhang; the rake is the sloped edge on a gable. Proper overhang dimensions protect walls from water runoff.
- Valley: An internal angle where two roof planes intersect. Valleys collect debris and water; open and closed valley installations each have pros and cons.
- Ice and Water Shield: A self-adhering membrane installed in vulnerable areas such as eaves, valleys, and around penetrations. It prevents water infiltration from ice dams or wind driven rain. Codes often require a minimum width along eaves in cold climates; typical installation extends at least 24 inches beyond the interior wall line.
- Ridge Vent: A vent installed along the roof ridge to allow hot, moist air to escape from the attic. It works with soffit vents to create a continuous airflow path, reducing heat buildup and moisture accumulation. Proper ridge vent installation requires a slot cut in the deck and a baffle to prevent wind driven rain entry.
- Soffit Vent: An intake vent located under the roof eave that draws cool, dry air into the attic. Combined with ridge vents, it enables natural convection. Soffit vents must be unobstructed by insulation; baffles or rafter vents maintain airflow from the soffit to the ridge.
- Gutter and Downspout: Gutters collect rainwater from the roof edge and direct it to downspouts, which carry water away from the foundation. Proper slope of 1/4 inch per 10 feet ensures drainage. Leaf guards and screens reduce clogging; seamless aluminum gutters minimize leak points.
- Skylight Flashing: A specialized flashing system that seals the junction between a skylight and the roof deck. It typically includes a base flange, step flashing, and a curb or integrated flashing. Improper skylight installation is a common leak source; proper integration with the roof covering and underlayment is essential.
Design Principle
Principle: Redundant Water Shedding
Definition: The roof assembly should include multiple barriers that independently resist water penetration, so that if one layer fails, subsequent layers provide backup protection.
Application: In a steep-slope roof, the roof covering is the primary barrier, underlayment provides secondary protection, and the deck serves as a tertiary barrier if both are compromised. Design details like overlaps, sealants, and flashing ensure continuity of the water shedding plane.
Quick Revision
- The roof assembly consists of deck, underlayment, covering, flashing, drip edge, and ventilation components.
- Each layer has a specific function in protecting the structure from water and weather.
- Redundant water shedding ensures that failure of one layer does not lead to leaks.
- Flashings are the most common source of roof leaks and require careful detailing.