Installation Instructions
Allura Fiber Cement Siding Installaon Manual
67
To t a l At t i c A r e a C a l c u l a t i o n
Determining Soffit Ventilation (Example)
1. Determine the local code requirement for total attic ventilation:
1:300 requires 1 sq. ft. of ventilation for every 300 sq. ft. of attic space.
2. Determine the total area of the attic (sq. ft.) to be ventilated (see illustration):
40 ft. x 30 ft. = 1,200 sq. ft.
3. To t a l f re e a r e a o f ve n t i l a t i o n r e q ui r e d fo r t h e a t t i c:
1,200 sq. ft. / 300 = 4.0 sq. ft.
4. Convert square feet into square inches (sq. in.):
4.0 sq. ft. x 144 = 576 sq. in.
5. Factor in the location of vents:
50% at ridge, roof vent or gable vent=
576 x.50 = 288 sq. in.
50% at soffit/eave = 576 x .50 = 288 sq. in.
6. Determine the total soffit ventilation area required:
Area of the soffit available for ventilation= 80 lineal feet.
Ventilation area required per lineal foot of soffit= 288 sq. in. / 80 lin. ft. =
3.6 sq. in / lin. ft.
7. Determine the amount of vented soffit required. The NFA information for Allura
products is available at AlluraUSA.com/resources/soffitspecsheet.pdf
8. Divide the required net free determined for the eave locations by the net free area
of the soffit product.
9. Install the required amount of vented soffit accordingly.