How the soffit and fascia calculator works
The soffit closes the underside of the roof overhang; the fascia is the upright board at the end of the rafters that carries the gutter. They are measured differently. Fascia is a simple length: every eave plus every gable rake. Soffit depends on how the material is installed, so the calculator offers two methods.
Cut across: panels = eave ÷ panel width ÷ ⌊panel length ÷ overhang⌋ × 1.10 · Along the eave: boards = eave × rows ÷ board length × 1.10Vinyl and aluminium panels in North America run from the wall to the fascia, so each long panel is cut into short pieces the depth of the overhang, and any offcut shorter than the overhang is lost. In the UK and much of Europe, uPVC or timber soffit boards run parallel to the wall in 5 m lengths, one board wide if the board is wider than the overhang. Both counts add 10% for mitres at corners and damaged pieces.
Worked example
A gable house 40 × 26 ft has 80 ft of eaves with an 18 in overhang and four rakes of 15 ft. A 12 ft vinyl panel, 12 in wide, cuts into 144 ÷ 18 = 8 pieces, and each piece covers one foot of eave. Eighty feet therefore needs 10 panels, 11 with the allowance, for 120 ft² of soffit. Fascia runs 80 + 60 = 140 ft, so ten 16 ft boards with waste. The attic floor of 1,040 ft² needs 1,040 ÷ 300 = 3.47 ft² of net free vent area, half of it at the eaves: 249.6 in², or 3.1 in² for every foot of soffit. In metric, 24 m of eaves with a 45 cm overhang covered by 50 cm uPVC boards in 5 m lengths takes 24 ÷ 5 × 1.1 = 5.3, so six boards.
Standard soffit and fascia sizes
| Material | Common size | Laid |
|---|---|---|
| Vinyl soffit (US, Canada) | 12 in or 16 in wide × 12 ft | Across the overhang |
| Aluminium soffit | 12 in or 16 in wide × 12 ft | Across the overhang |
| uPVC soffit board (UK) | 100–600 mm wide × 5 m | Along the eave |
| Timber or plywood | 2.44 × 1.22 m (8 × 4 ft) sheets ripped to width | Along the eave |
| uPVC fascia | 150–400 mm high × 5 m | Along every edge |
| Wood fascia | 1×6 or 1×8 in, 12–16 ft lengths | Along every edge |
With 12 ft cut-across panels, the overhang decides the waste: 16 in gives 9 pieces, 18 in gives 8, 20 in gives only 7 with a 4 in offcut each time, and 24 in gives 6.
Ventilation through the soffit
Most cold, ventilated roofs pull outside air in at the eaves and let it out near the ridge. US practice uses 1 ft² of net free area per 300 ft² of attic floor when vents are split between high and low, with about half at the soffit. UK guidance for pitched roofs asks for eave openings equal to a continuous 10 mm gap, which is 100 cm² per metre of eave, rising to 25 mm on low pitches. Use vented panels or vent strips to reach the figure, and fit rafter trays so insulation does not block the airflow.
Mistakes to avoid
- Covering rotten rafter ends. New fascia over soft timber fails within a few years; probe and repair first.
- Forgetting J-channel and F-trim. Cut-across panels sit in trim at the wall and fascia; order it by length, roughly twice the eave length.
- Ignoring expansion. Vinyl moves with temperature; leave the gaps the maker specifies and do not nail tight.
- Working off a ladder at height. Eave work above one storey is safer from a scaffold tower.
Frequently asked questions
How many soffit panels do I need?
Divide the eave length by the panel width, then by the pieces each panel gives. With 12 × 12 ft panels and an 18 in overhang, each panel covers 8 ft of eave, so 100 ft of eaves takes 13 panels, 14 with waste.
How much soffit ventilation do I need?
Under the 1/300 rule, a 1,200 ft² attic needs 4 ft² of net free area, with about 2 ft² (288 in²) at the soffits. UK guidance asks for 100 cm² per metre of eave on most pitched roofs.
What size is fascia board?
Wood fascia is usually 1×6 or 1×8 in, sold in 12–16 ft lengths; uPVC fascia comes in 5 m lengths from about 150 to 400 mm high.
Do rakes need soffit as well as fascia?
Only on boxed gables with an overhang. Many gables just have a fascia or barge board, so the calculator counts rakes for fascia only.
How do I measure a soffit overhang?
Measure horizontally from the face of the wall to the back of the fascia board, not along the sloping rafter.
Last reviewed: 2026-09-27 · How we calculate