Soffit and Fascia Calculator

Last reviewed How we calculate

The soffit is the underside of the roof overhang and the fascia is the board along its front edge, where the gutters hang. Replacing them is usually done by the linear foot, and this turns the length of your eaves into panels, fascia pieces and trim channel.

Soffit panels, fascia and channel

Soffit panels, fascia pieces and J-channel for your eaves and rakes.

How to measure
Section through the eave. The fascia is the board across the ends of the rafters; the soffit closes the underside, and its depth runs from the wall to the back of the fascia. Rafter tail Wall Fascia Soffit Soffit depth

This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.

From the wall to the back of the fascia.

Printed on the product. A 12 in by 12 ft panel covers 12 sq ft.

Around 10% on a simple gable. Allow 15% or more where there are hips, valleys or dormers.

Soffit panels —

Soffit area
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Fascia pieces
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Channel pieces Two runs: one along the wall, one along the fascia.
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Vented soffit panels supply the intake air for most attic ventilation, so use vented panels wherever the attic relies on soffit intake; the attic ventilation calculator gives the area needed. Rotten fascia and rafter tails often only show once the old covering is off.

How the calculation works

The soffit area is the length of the eaves and rakes times the depth of the overhang. Divide that by what one panel covers, after waste, for the panel count. Fascia is counted by length. Soffit panels are held by a channel at each edge, one along the wall and one along the fascia, so the channel run is twice the eave length.

A worked example

150 ft of eaves and rakes with a 16 in soffit is 200 sq ft of soffit. With 10 per cent waste and panels covering 12 sq ft each, that is 19 panels. The same length takes 14 pieces of 12 ft fascia and 28 pieces of 12 ft channel.

Soffits and ventilation

In most houses the soffit is where the attic takes in air. Vented soffit panels let that air in; solid panels shut it off, and blocking the intake is one of the fastest ways to shorten the life of a roof. Use vented panels wherever the attic relies on them, and check that insulation in the attic is not pushed against the vents from above. The attic ventilation calculator gives the intake area needed.

Frequently asked questions

Should I use vented or solid soffit panels?

Vented, wherever the attic takes its intake air from the soffits, which most do. Solid soffit panels cut off that airflow and can shorten the life of the roof.

Can the fascia be replaced without taking the gutters down?

Usually not. Gutters hang from the fascia, so they normally have to come down while the fascia is replaced and go back up afterwards.

What is J-channel used for?

J-channel is the trim that holds the cut edges of soffit panels, along the wall and along the fascia. F-channel does the same job at the wall on some systems.

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