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Soffit and Fascia Calculator — Boards, Trims & Lengths
Calculate soffit boards, fascia boards and joining trims for your eaves. Covers uPVC and timber with UK standard sizes.
Total perimeter of eaves (all sides)
Typical UK range: 200-300mm
Adds 10% extra for cuts and joints
10% is standard for soffit and fascia work
Enter your supplier price for a cost estimate
Enter your supplier price for a cost estimate
Default: 300mm standard
Default: 4 corners
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How We Calculate This
Our soffit and fascia calculator works out how many boards and trims you need based on your total eaves length, soffit depth and chosen board size. It covers both uPVC and timber options.
The formula
Fascia boards = Total eaves length ÷ Board length
Soffit boards = (Total eaves length ÷ Board length) × Rows needed
Soffit rows depend on your soffit depth vs. the standard 300mm board width. If your soffit is 250mm deep, one row of boards suffices. If it is 500mm, you will need two rows.
Standard UK sizes
- Fascia boards: 150-225mm wide, 18mm thick, lengths 2.5m/3m/5m
- Soffit boards: 300mm wide, 9mm thick, lengths 2.5m/3m/5m
- Joining trims: H-section connectors at every board joint
- Corner trims: Internal and external corner pieces
The joining-trims figure is an estimate: it assumes each fascia line is one continuous run and counts an H-section joint at every in-line board join. Where the eaves is broken at corners, some of those joins fall on the corner trims instead, so reduce the H-trim count accordingly and order corner pieces separately.
The calculator assumes 4 corner trims as standard. Adjust based on your roof shape — a semi-detached house may need fewer, while an L-shaped roof needs more. Always add 10% wastage for cuts and fitting adjustments.
Frequently Asked Questions
The most common UK fascia board sizes are 150mm (6"), 175mm (7"), 200mm (8") and 225mm (9") wide. The correct width depends on the depth of your rafter feet at the eaves. For most modern UK houses, 175-200mm is standard. uPVC fascia boards are typically 18mm thick and come in 2.5m, 3m or 5m lengths.
Standard soffit depth (the horizontal projection from the wall to the fascia) is typically 200-300mm on most UK houses. Bungalows and older properties may have deeper soffits of 300-400mm. uPVC soffit boards are usually 300mm wide (to cover the most common sizes in one piece) and 9mm thick.
uPVC is by far the most popular choice for UK homes. It requires no painting, does not rot, and lasts 20+ years with minimal maintenance. Timber fascia and soffits need regular painting (every 3-5 years) and are prone to rot, but some conservation areas or listed buildings may require timber. uPVC is typically more cost-effective over its lifetime.
On a cold pitched roof without a vapour-permeable (breathable) underlay, you need ventilated soffit boards to move air through the roof void and prevent condensation. The figures come from BS 5250:2021 (referenced by Approved Document C as the means of compliance for roof-void condensation; Approved Document F gives only minimal roof-void coverage): for a roof pitch of 15° or more, provide ventilation equivalent to a continuous 10mm gap along each eaves; for a shallower pitch of 10–15°, this rises to a 25mm equivalent gap; and where the pitch exceeds 35°, or on a mono-pitch/lean-to roof spanning more than 10m, add a continuous 5mm high-level (ridge) vent as well. Vented soffit boards have perforations along their length to deliver this airflow. Use our Felt and Batten Calculator to check if breathable membrane is included in your roofing spec. For gutter sizing alongside your soffit work, see our Gutter Sizing Calculator.
You need one H-section joining trim at every joint between boards (where two board ends meet). You also need external and internal corner trims at each corner of the house. For a standard detached house, this is typically 4 corners. Do not forget J-trim or F-trim where soffit meets the wall.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.