Soffit & Fascia Calculator

Q: How much soffit and fascia board do I need?

Measure the total length of all eaves runs on your building, including garage, bay windows, and any lean-to or extension roofs. Add 10% for joins, mitres at corners, and trimming waste. Fascia boards run along the length of the eaves at the foot of the roof and are typically 150-200 mm wide. Soffits fill the horizontal gap between the fascia and the wall beneath and are typically 200-400 mm wide depending on the eaves overhang. Both are available in 2.5 m and 5 m lengths.

Q: What is the difference between soffit, fascia, and barge board?

The fascia board is the vertical board fixed to the ends of the roof rafters along the eaves, to which the guttering is fixed. The soffit is the horizontal board that closes off the underside of the eaves between the fascia and the wall. The barge board (also called verge board) is the board fixed along the sloping edge of a gable end roof, parallel to the roof slope. All three are usually installed as a matched set for a consistent appearance. They may be timber (painted or stained) or uPVC (low maintenance).

Q: Should I choose timber or uPVC soffit and fascia?

uPVC soffit and fascia is the most popular choice for replacements as it requires virtually no maintenance, does not rot, and is available in a wide range of colours and wood-effect finishes. It typically lasts 25-40 years. Timber soffit and fascia has a more traditional appearance and is preferred on period properties and conservation areas where uPVC may not be appropriate. Painted timber requires repainting every 5-8 years and must be inspected for rot. If the existing fascia boards are rotten, they should be replaced entirely rather than overboarded.

Q: How do I ventilate the roof space through the soffits?

UK Building Regulations Approved Document F (Ventilation) requires roofspaces to be ventilated to prevent condensation and moisture damage. For a cold pitched roof, continuous ventilation at eaves level equivalent to a 25 mm gap across the full eaves length is required. For deeper spans (over 10 m) or attic conversions, additional counter-ventilation at ridge level is needed. Ventilated soffits (either with pre-drilled holes or a continuous mesh soffit strip) provide this ventilation. Insulation must not block the eaves air path - use eaves baffles or insulation stops to maintain the air gap.

Q: Can I fit new fascia and soffit over the existing boards?

Overboarding (fitting new uPVC boards over existing timber) is a common and faster installation method, provided the existing timber boards are structurally sound - free from rot, delamination, and insect damage. However, if the existing boards are rotten, overboarding conceals the problem and can trap moisture, accelerating further decay. Always probe the existing fascia boards with a bradawl to check for rot before deciding to overboard. Replacement (stripping out the old boards entirely) is more work but allows inspection and repair of the rafter ends and roof structure.

Q: What waste allowance should I use for soffit and fascia?

Allow 10% waste on the total linear metre quantity for joins, mitres, and end trimming. Corner joints and mitres in uPVC are typically made with purpose-made external and internal corner trim pieces rather than mitre cuts, which simplifies installation and reduces waste. On complex rooflines with hips, valleys, or multiple eaves levels, increase waste to 15%. Always use lengths that minimise the number of joins on a single run - 5 m lengths are better than 2.5 m where the run is longer than 2.5 m.

Fascia, soffit, barge board, and accessories - by elevation with joint cover and corner counts

Material

Low maintenance, available in white and woodgrain finishes. Standard board length 5 m. No painting required.

Fascia Board Width (mm)

Soffit Board Width (mm) Eaves depth - wall face to back of fascia. Measure on site.

Board / Plank Length (m)

Include in calculation

Fascia boards Soffit boards Barge boards (gable ends) Ventilation strip

Waste / Cutting Allowance (%)

10%

10% standard; 15% for complex rooflines with many hips or valleys

Elevations

Accessories Enter total counts for the whole job. Corner pieces are sold separately from board lengths.

External Corners Outside corners of the building

Internal Corners Inside corners (e.g. bay windows, re-entrant angles)

Stop Ends Caps at open ends of fascia / soffit runs

Job Costing (optional)

Fascia board per metre

Soffit board per metre

Barge board per metre

Joint cover (each)

Corner piece (each)

Labour cost

Per linear metre of eaves run

Delivery (flat fee)

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Results

Linear Metres & Board Counts

Total Eaves Run
-
linear metres

Fascia Boards

lengths

Soffit Boards

lengths

Barge Boards

lengths

Accessories

Joint Covers

fascia + soffit

External Corners

pieces

Internal Corners

pieces

Stop Ends

pieces

Elevation Breakdown

Elevation	Eaves Run	Fascia Lengths	Soffit Lengths	Barge LM	Joint Covers

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Frequently Asked Questions

How much soffit and fascia board do I need?

Measure all eaves runs (including garage, bays, and extensions) and add 10% for joins and mitres. Fascia boards: typically 150-200 mm wide. Soffits: 200-400 mm wide depending on overhang. Both available in 2.5 m and 5 m lengths.

What is the difference between soffit, fascia, and barge board?

Fascia: vertical board at the rafter ends along the eaves - guttering fixes to this. Soffit: horizontal board closing off the eaves underside between fascia and wall. Barge board (verge board): board along the sloping gable end edge. All three are usually installed as a matched set in timber or uPVC.

Should I choose timber or uPVC soffit and fascia?

uPVC: most popular for replacements - virtually no maintenance, no rot, wide colour range, lasts 25-40 years. Timber: traditional appearance preferred on period properties and Conservation Areas, but needs repainting every 5-8 years and rot inspection. If existing boards are rotten, replace entirely - do not overboard.

How do I ventilate the roof space through the soffits?

Approved Document F requires eaves ventilation equivalent to a 25 mm continuous gap across the full eaves length for a cold pitched roof. Use ventilated soffits (pre-drilled or continuous mesh strip). Use eaves baffles to prevent insulation blocking the air path. Deeper spans (over 10 m) also need counter-ventilation at ridge level.

Can I fit new fascia and soffit over the existing boards?

Overboarding is possible if existing timber boards are structurally sound - probe with a bradawl first to check for rot. If boards are rotten, overboarding conceals the problem and traps moisture, accelerating decay. Full replacement is more work but allows inspection of rafter ends and roof structure - recommended when any rot is found.

What waste allowance should I use for soffit and fascia?

Allow 10% waste for joins, mitres, and end trimming. uPVC corners use purpose-made corner trim pieces rather than mitre cuts, reducing waste. Complex rooflines with hips or multiple levels: 15%. Use 5 m lengths where runs exceed 2.5 m to minimise joins.

UK Building Regulations Part C • BS 5534 • BS 8000-0

Fascia, Soffit & Barge

Fascia board - The vertical board fixed to the rafter feet along the eaves. The guttering brackets fix to this board. Width typically 100-225 mm depending on rafter depth.
Soffit board - The horizontal (or near-horizontal) board that encloses the underside of the eaves between the fascia and the wall. Can be vented or solid.
Barge board - The diagonal board at gable ends of a roof, running along the verge. Not present on hipped roofs. Measured along the slope.
Joint cover - A clip-on strip that covers the join between two board lengths. Required wherever two boards meet end-to-end.

Material Guide

uPVC - By far the most common choice for residential. Zero maintenance, no painting, rot-proof. Standard 5 m lengths. Available in white, cream, and woodgrain foils. Typically 18-25 mm thick.
Painted timber - Traditional appearance. Requires painting every 5-7 years. Typically 25 mm ex softwood. Better thermal performance than uPVC.
Aluminium - Powder coated in any RAL colour. More rigid than uPVC, longer lengths available. Popular on contemporary and commercial projects.
Composite - Cellulose and PVC composite. More rigid and stable than uPVC, often used to replicate timber appearance. Lengths typically 3.6-5 m.

Measuring Tips

Measure each elevation separately - front, rear, left side, right side
For gable ends, measure the sloping length of the barge board, not the wall height
Soffit width: measure from the outside face of the wall to the back of the fascia position
If the soffit is wider than one board, the calculator accounts for this automatically using the board width you enter
Add an external corner piece wherever two fascia runs meet at an outside corner of the building
Allow one joint cover wherever two board lengths butt together - the calculator estimates these from board count

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Soffit & Fascia Calculator

How much soffit and fascia board do I need?