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Category Roofing

Dimensions & Specs

Total Roof Eave Perimeter

lin. ft

Overhang Depth

inches

Attic Floor Area (for Vent Check)

sq. ft

Soffit Panel Width

Panel NFA Rating

sq.in / sq.ft

Check manufacturer spec (usually 4-12)

Material List

Soffit Panels (12ft length)

Includes 5% waste factor

J-Channel / F-Channel

Total linear feet required

Ventilation Compliance (1/300)

Required Intake 0 in²

Provided Intake 0 in²

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About

Proper roof overhang finishing serves two distinct purposes: aesthetics and attic ventilation. This calculator addresses the complexities of estimating material needs for eaves while ensuring compliance with the 1/300 ventilation code. Insufficient intake ventilation can lead to ice dams in winter and superheated attics in summer, significantly shortening shingle lifespan.

This tool integrates a "Smart Waste Factor" algorithm. Unlike simple area dividers, it accounts for the geometric reality that soffit panels (typically 12ft long) must be cut to specific overhang depths, often leaving unusable off-cuts. It simultaneously calculates the Net Free Area (NFA) to verify if the selected perforated panels provide adequate airflow relative to the attic's square footage.

Formulas

Material Estimation:
The number of panels is derived from the total linear footage of the overhang divided by the effective coverage width of a panel, adjusted for cutting waste.

Panels=PerimeterPanelWidth×(1+Waste%)

Ventilation Code (1/300 Rule):
The total required Net Free Area is the attic floor area divided by 300. Ideally, 50% of this is intake (soffit).

ReqIntake=AtticArea600

Reference Data

Material Type	Typical Size	NFA per sq.ft	Usage
Solid Vinyl	12in × 12ft	0 in²	Gable ends, non-vented areas
Fully Vented Vinyl	12in × 12ft	6-14 in²	Intake areas (eaves)
Center Vented	12in × 12ft	2-6 in²	Partial airflow
Aluminum Vented	16in × 12ft	8-16 in²	High durability, high flow

Frequently Asked Questions

The 1/300 rule applies if you have a balanced system (50% intake at soffits, 50% exhaust at ridge) and a vapor barrier. If you do not have balanced ventilation, building codes often default to the stricter 1/150 rule, requiring double the ventilation area.

J-Channel is a trim piece used to secure the soffit panel against the house wall. F-Channel is essentially a J-Channel with a nailing flange, often used on the fascia side (outer edge) to support the soffit panel, though sometimes J-Channel is used there as well.

Measure from the exterior wall of the house to the inside of the fascia board. This is the "clear span" the soffit panel must cover. Do not include the thickness of the fascia itself.