How this calculator works
Fascia is the board along the bottom of the roof edge, and soffit is the board that closes the underside of the overhang. Both run the length of the eaves, so each count is the length with a margin for cuts divided by the length of one board, rounded up. The soffit may need more than one board across its depth, so the calculator multiplies the length by the number of rows (the soffit depth divided by the board width, rounded up).
Worked example: 15 m of eaves with 5 m boards and 5% extra is 4 fascia boards, 4 soffit boards across a 300 mm soffit, and 4 trim lengths, for 4.5 m² of soffit. With 4 m boards the fascia count is 4, and a 600 mm soffit with 300 mm boards needs two rows, 7 boards.
Assumptions and sources
- Assumptions used: 5% waste added; one run of fascia, soffit board across the full depth in rows, and one run of trim along the wall; no corners, joints or vents counted.
- Fascia and soffit boards, trim and corner pieces are sold in different lengths and profiles by different makers, so use the lengths on the pack. Count the ventilated soffit boards or soffit vents separately: a roof space needs ventilation at the eaves, and a vented board supplies it. Replace rotted timber behind the old boards before fitting the new ones. Gutter brackets need solid backing, so allow for the fixing to the rafter ends.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Boards for typical eaves
5 m lengths, 300 mm soffit boards across a 300 mm soffit, 5% extra.
| Eaves length | Fascia boards | Soffit boards | Trim |
|---|---|---|---|
| 8 m | 2 | 2 | 2 |
| 12 m | 3 | 3 | 3 |
| 15 m | 4 | 4 | 4 |
| 20 m | 5 | 5 | 5 |
| 30 m | 7 | 7 | 7 |
Tips before you buy
- Check the rafter ends and the timber behind the old boards before you fit the new ones: a new board over rotten wood will not last.
- Make joints in the fascia and soffit over a rafter or a support, and stagger them so they do not line up.
- Fit a soffit vent or a vented board so the roof space can breathe, and keep the insulation from blocking the airflow.
- Leave a small gap at the ends of boards for the plastic to expand and contract, and cover it with a trim.
- If you are replacing guttering at the same time, do the fascia first and fix the new brackets to the new board.
Tools & materials for this project
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Frequently asked questions
How many fascia boards do I need?
Divide the length of the eaves by the length of a board and add about 5% for cuts. 15 m of eaves takes 4 boards of 5 m.
How many soffit boards do I need?
15 m of eaves with a 300 mm soffit takes 4 boards of 5 m, if each board is 300 mm wide. A deeper soffit needs more than one row.
How long are UPVC fascia and soffit boards?
Most are sold in 5 m lengths, with shorter lengths for small jobs. Read the length on the pack.
Do I need trim as well?
Yes. A J-trim or soffit trim finishes the soffit at the wall, and one length of trim runs the length of the eaves.
Should I replace the soffit and fascia together?
It is cheaper and neater to do both at once, along with the guttering, because the same scaffold is up and the fascia carries the gutter brackets.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.