Craftsman CTS205 manual Porch Ceilings, Soffit, Fascia, Requirements for Proper Soffit Ventilation

CertainTeed Vinyl Siding Installation Guide

Section 9 — Porch Ceilings, Soffit, Fascia

Requirements for Proper Soffit Ventilation

Proper attic ventilation is important for any home. The 2009 International Building Code (IBC) Section 1203 Ventilation furnishes a basic guide for determining proper ventilation for any home. The information provided here may under certain circumstances not result in enough ventilation. Therefore, the calculation provided should be used as a guide only.

The IBC guideline requires that any attic or space between the top floor, ceiling and roof must be ventilated. It requires one square foot of ventilation area for every 150 square feet of attic space.

If a vapor retarder of less than one perm has been installed on the warm side of the ceiling or if at least 50% of the required ventilating area has already been provided by gable and vents or ridge vents, you need add only one-half of the ventilation area that would otherwise be required. The requirement would then be one square foot of ventilation area for every 300 sq. ft. of attic space.

How to determine soffit ventilation

1.Determine the local code requirement for total attic ventilation. 1:150 requires 1 sq. ft. of ventilation for every

		150 sq. ft. of attic space.
2'-0"	40'-0"
		1:300 requires 1 sq. ft. of ventilation for every 300 sq. ft. of
		attic space, if a vapor barrier having a transmission rate not
		exceeding 1 perm is installed on the warm side of the ceiling.

2.Determine the total area of the attic (sq. ft.) to be ventilated.

30'-0"40 ft. x 30 ft. = 1,200 sq. ft.

3.Total free area of ventilation required for the attic. 1,200 sq. ft. / 150 = 8.0 sq. ft.

			4. Convert square feet into square inches (sq. in.).
2'-0"
			8.0 sq. ft. x 144 = 1,152 sq. in.
	ridge	eave

5.Location of vents.

50% at ridge, roof vent or gable vent =

1,152 x .50 = 576 sq. in.

50% at soffit/eave = 1,152 x .50 = 576 sq. in.

6.Total soffit ventilation area required.

Area of the soffit available for ventilation:

80 lineal ft. x 2 ft. soffit depth = 160 sq. ft. Ventilation area required per sq. ft. of soffit = 576 sq. in. /160 sq. ft. = 3.6 sq. in. /sq. ft.

7.Soffit product selection.

Compare the soffit ventilation area required to the manufacturer’s soffit product literature.

8.If the roof pitch is 9/12 or higher, add 30% more ventilation to accommodate the higher volume of air moving through the air.

85