Design Spreadsheets

Detail of Roof Truss

Detail of Roof Trust

Roof Design

The roof design and roofing materials have a significant impact on the finished appearance of the residence.

Roof Slope

  • Roof slope is the slant of the roof.
  • Shown on a drawing as a slope ratio diagram or fractional pitch.
  • The slope diagram represents the ratio between the rise and run of the roof.
  • The run is always expressed as 12 units.
  • Fractional pitch is rise/clear span.

Roof Trusses

  • The roof truss is an assembly of members that form a rigid framework.
  • Information needed to purchase a truss includes the span, roof pitch, spacing of trusses, and roof load.
  • Lightweight wood trusses generally can span distances of 20′ to 32′.
  • Residential trusses are generally 2″ x 4″ or 2″ x 6″ lumber.


The cornice is that part of the roof that overhangs the side walls.

Three types of cornices are frequently used in residential construction.

  • Open cornice.
  • Box cornice.
  • Close cornice.

Roof Sheathing and Roofing

  • Roof sheathing supports the roofing material.
  • Sheathing is generally plywood or other approved panel products.
  • Sheathing thickness depends on spacing of the rafters.
  • 1/2″ sheathing-grade plywood is generally used when rafters are 16″OC.

Roofing Materials

  • Asphalt shingles are the most popular type of roofing material for residences.
  • Usual weight is 235 pounds per square.
  • A “square” of shingles covers 100 square feet.
  • A layer of 15-pound saturated-felt building paper is usually placed between the sheathing and shingles.
  • Common Roofing Materials – Associated Roofing Contractors of the Bay Area Counties. Asphalt Shingles: Asphalt shingles are the most commonly used and among the least expensive of materials available for steep slope applications. They are very durable and are available in a wide variety of colors and textures.Roofing material is the outermost layer on the roof of a building, sometimes self-supporting, but generally supported by an underlying structure.

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