Soffit and Fascia Systems

Vinyl soffit and fascia systems are designed to be easily installed in remodeling or new construction. Soffit panels are similar to vertical siding. Manufacturers produce panels with both a solid surface and a perforated surface to allow for ventilation in the overhang.

Requirements for Proper Ventilation
Proper attic ventilation is important for any home. The Council of American Building Officials (CABO) one and two family dwelling code (Section R-707 - attic ventilation) covering ventilation of structural spaces furnishes a basic table for determining the 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.

CABO's guidelines require that any attic or space between the top floor, ceiling and roof must be ventilated. It requires that for every 150 square feet of ceiling area, there must be one square foot of ventilation area.

If a vapor retarder of less than one perm has been installed on the warm side of the ceiling, or at least 50% of the required ventilating area has already been provided by gable end vents or ridge vents, you need add only 1/2 of the ventilation area that would be otherwise required. The requirement would then be one square foot of ventilation are for every 300 sq. ft. of ceiling area.

The Free Area Chart
To find the total net free area needed to properly ventilate a home, find the length of the individual ceiling area to be ventilated in a vertical column and width of the ceiling areas in a horizontal column. Where these two columns intersect is the free area of ventilation IN SQUARE INCHES needed for the structure. Adding up all the free areas of all the ceilings will give the total free area required for the home.

Click Here for the Free Area Chart

For more definitive information on ventilation of structural spaces, we refer you to the following sources.

Agricultural Extension Service University of Minnesota St. Paul, MN 55108 Extension Bulletin #399-1976 (612) 625-1915

Small Homes Council University of Illinois 1 East St. Mary's Road Champaign, IL 61820 (217) 333-1801

Calculation of Soffit Required.
A = (B - .5C)/D
Or
A = (Bx2 - .5C)/D

Where:

• A = Lineal feet of perforated soffit needed
• B = Total net free area in sq. inches
• C = Area covered by existing gable or ridge vent in sq. inches (reduced by 50%)
• D = Manufacturer's value for vent area per lineal foot of perforated soffit.

Examples
A house has a ceiling area of 26' x 40'. Using the chart, this equals a total net free area of 499 sq. inches of ventilation needed. The house has a vapor retarder on the warm side of the ceilings and has 2 gable end vents, each 12" x 18", for a total of 432 sq. inches of covered area.

Using the formula, we find 64 lineal feet of perforated soffit is required. (for the sake of illustration, we will use 4.42 sq. in. ft. as a sample manufacturers value for vent area per lineal foot or perforated soffit panel. Consult the individual manufacturer for the vent area angle for your materials.)

Amount of perforated soffit = (499 sq. in. - 216 sq. in.) / 4.42 sq. in./ft. = 64 sq. ft.

Using the same house as above but without a vapor barrier, and gable end vents (432 sq. in.) are less than half of the required area. Using the formula, we find 176 feet of perforated soffit required.

Amount of perforated soffit = ((499 sq. in. X 2) - 216 sq. in.) / 4.42 sq. in./ft. = 176 sq. ft.

The perforated soffit panels should be balanced around the perimeter of the house.

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