Gable Roof Area & Rafter Calculator
Calculate framing specs for simple gable roofs. Includes rafter cut list, ventilation requirements, and total shingle squares.
Total Roof Area
0 sq ftMaterials Estimate
0 Squares
0 Bundles (w/ 10% waste)
Rafter Specs
Run: 0 ft
Common Rafter Length: 0 ft
Rise: 0 ft
Ventilation Required
Total NFA: 0 sq ft
(Split 50/50 Soffit/Ridge)About
The gable roof is the most common residential roof style, characterized by two sloping sides that meet at a central ridge. While seemingly simple, accurate framing requires precise calculation of the "Common Rafter" - the diagonal member connecting the ridge board to the wall plate. Builders must also account for the "Overhang" (eaves), which not only protects the siding from rain but significantly increases the material surface area.
This tool computes the theoretical rafter length (line length) and the total surface area for sheathing and shingles. Crucially, it estimates the Attic Ventilation requirement based on the "1/150 Rule" (1 sq ft of net free vent area for every 150 sq ft of attic floor), ensuring the roof system breathes correctly to prevent moisture buildup and ice dams.
Formulas
The Rafter Length Lraft is the hypotenuse of the triangle formed by the Run (half the span) and the Rise. Note: Real rafters need adjustment for the ridge board thickness.
Total Roof Area accounts for the rake overhangs (ends of the house) and eave overhangs (bottom of the slope). Lhouse is the building length.
Ventilation Vreq (sq ft) calculation using 1/150 rule:
Reference Data
| Roof Pitch | Angle | Run Multiplier | Valley Multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 1.003 | 1.416 |
| 2/12 | 9.46° | 1.014 | 1.424 |
| 4/12 | 18.43° | 1.054 | 1.453 |
| 6/12 | 26.57° | 1.118 | 1.500 |
| 9/12 | 36.87° | 1.250 | 1.601 |
| 12/12 | 45.00° | 1.414 | 1.732 |
| 18/12 | 56.31° | 1.803 | 2.062 |