🏠 Soffit and Fascia Calculator
Calculate materials needed for your roof eave trim installation
📏 Eave Measurements
💡 Expert Installation Tips
🎯 Measure Twice, Order Once
I always tell homeowners: walk the entire perimeter with a tape measure. Don't estimate sections – I've seen people run short 20 feet into a job because they eyeballed one gable end. Measure each wall separately, then add them up. For overhang width, check EVERY corner. Old houses settle – I've found 2-inch variations on the same roofline. Use the largest measurement so you're not stuck with gaps.
🔧 The Trim Channel Dance
Here's what 18 years taught me about trim installation: J-channel goes up FIRST along the wall, perfectly level with the bottom of your fascia. Use a chalk line – eyeballing will haunt you at the corners. F-channel (or undersill trim) secures the outer edge. Leave 1/4" gap at all channel ends for expansion – vinyl moves 3/8" per 10 feet in summer. I watched a crew nail everything tight; by July, the whole soffit was buckled waves.
🌬️ Ventilation Math Matters
Code requires 1 sq ft of net free area per 150 sq ft of attic (IRC R806.2). "Net free area" isn't the panel size – vented soffit typically has 8-10 holes per square foot. A 12"x12' panel might only give you 0.6 sq ft NFA. For a 1,500 sq ft attic, you need 10 sq ft NFA total. If using continuous vented soffit with 0.05 NFA per linear foot, that's 200 LF of vent. Don't mix it with solid panels randomly – do full runs for consistent airflow.
📦 Material Type Reality Check
Vinyl: Cheapest upfront ($2-3/LF installed). Works great in moderate climates but
gets brittle below 10°F – I've replaced cracked panels after polar vortexes. Dark colors fade fast
in Arizona sun.
Aluminum: My go-to ($4-6/LF). Won't rot, dent-resistant (but not dent-proof – hail
will ding it). Factory finish lasts 20+ years. Expands MORE than vinyl though – use proper
fastening.
Wood: Beautiful for historic homes ($6-10/LF), but you're signing up for
maintenance. Repaint every 5 years, inspect for rot annually. I've pulled down 8" cedar fascia
boards eaten hollow by carpenter ants.
⚠️ The "Waste Factor" Truth
10% isn't some random number – it's survival insurance. Every inside corner needs a double-channel piece. Every window or door means custom cuts. That 12-foot panel? You're cutting it to fit an 11'3" run, and the 9" scrap is useless for anything. I budget 15% on homes with lots of angles or valleys. One customer argued "why waste money?" – ended up reordering and paying $200 delivery for three panels.
❌ Common Mistakes to Avoid
🚫 Forgetting the Frieze Board Height
The Error: Measuring from the wall top to fascia edge without accounting for the frieze board (decorative trim between wall and soffit).
Real Impact: Ordered 16" wide soffit panels for an 18" overhang measured from roof sheathing, not from where J-channel actually sits. Had to piece in 2" filler strips that looked terrible. Wasted $340 on wrong panels.
Fix: Measure from the BOTTOM of fascia to where J-channel will mount (usually 1-2 inches below wall top). Subtract 1/2" for fit tolerance. If there's a frieze board, start from its bottom edge.
🚫 Using Non-Vented Soffit Everywhere
The Error: Installing solid soffit to "save money" ($0.50/panel cheaper) without calculating attic ventilation needs.
Real Impact: Customer called 6 months later with ice dams in winter, 140°F attic in summer. Insurance denied mold claim because "inadequate ventilation." $4,800 to add ridge vents + tear out and replace soffit with vented. The "savings" was $87.
Fix: Use the 1:150 rule. For a 1,200 sq ft attic, you need 8 sq ft NFA minimum (preferably split 50/50 between soffit intake and ridge exhaust). Mark vented panel runs on your sketch BEFORE ordering.
🚫 Over-Nailing or Tight-Nailing
The Error: Driving nails/screws flush against panels or spacing them every 8 inches "for extra security."
Real Impact: Inspected a 3-year-old job that looked like a wave pool. Vinyl had expanded 1/2" per 12-foot run in summer heat but couldn't move because it was pinned tight. Panels cracked at every fastener. $1,950 to replace 180 LF of soffit.
Fix: Leave 1/32" gap between nail head and panel (thickness of a dime). Fasten in the CENTER of slots, spacing 12-16" apart. Never nail through flat field – only at edges where trim captures it. Test one panel in summer heat; it should slide 1/4" side-to-side.
🚫 Ignoring Fascia Board Condition
The Error: Wrapping rotten wood fascia with new aluminum/vinyl without replacing the substrate.
Real Impact: Wrapped 86 LF of 2x8 fascia that "looked okay from the ground." During a windstorm 8 months later, entire sections ripped off – the wood underneath was punky. Gutter brackets had zero holding power. Fell and damaged client's new deck ($2,100 deck repair + $980 re-fascia with proper lumber).
Fix: Before measuring for materials, poke every foot of fascia with a screwdriver. If it sinks more than 1/4", that section needs replacement. Budget for 1x8 or 2x8 pressure-treated or Azek (PVC) boards as backer. Wrapping is cosmetic only – the structure must be solid first.
🚫 Not Planning for Corners and Transitions
The Error: Calculating only linear footage without noting inside/outside corners, step-downs, or pitch transitions.
Real Impact: Needed 12 outside corner pieces and 4 inside corners (double-channels) but only ordered material for straight runs. Each corner piece is $8-12. Made two trips to supplier 40 miles away. Lost a full day, burned $60 in gas, customer annoyed by delay.
Fix: Sketch your roofline. Mark every corner with "IC" (inside) or "OC" (outside). Count gable ends separately – they need starter strips. Add 1 extra corner piece per 10 actual corners (because you WILL mess one up cutting the angle). Order these specialty pieces with your main material – they're the holdup items.
❓ Frequently Asked Questions
How do I measure soffit if my overhang width varies around the house?
Measure the overhang at multiple points along each wall (every 10-15 feet). Old houses settle unevenly, so you might find 16" on one side and 19" on another. Always use the LARGEST measurement when ordering soffit panels – you can trim panels narrower, but you can't stretch them wider. For example, if you get 14", 16.5", 18", and 17", order 18"-wide soffit for the entire run. The slight overhang in narrower sections won't be noticeable once installed. Don't try to order different widths for different walls; you'll create visible seams and waste time custom-fitting.
What's the difference between vented and non-vented soffit, and how much of each do I need?
Vented soffit has perforations (tiny holes) allowing air intake into the attic; non-vented is solid. Building codes (IRC R806.2) require 1 square foot of net free area (NFA) per 150 sq ft of attic space, ideally split 50/50 between intake (soffit) and exhaust (ridge/gable vents). "Net free area" accounts for screen blockage – vented soffit typically delivers 8-10 sq inches NFA per linear foot. For a 1,800 sq ft attic, you need 12 sq ft NFA total = 6 sq ft from soffit intake = roughly 100 linear feet of vented soffit (assuming 0.06 NFA per LF). You CAN mix vented and solid panels, but don't scatter them randomly; run full vented sections where intake is needed.
How much waste factor should I add for a complex roofline with multiple gables and corners?
For a simple rectangular home with 4 corners, 10% waste is adequate. But for homes with multiple gables, dormers, hips, or valleys, bump it to 15-20%. Here's why: every inside corner requires a double-channel piece and custom-cut soffit at an angle – those cut-offs are rarely reusable. A panel that's 12 feet long might yield only 8'4" of usable material after corner cuts, leaving you with awkward scraps. I've worked on Victorian homes where waste hit 22% because of turrets and angled bays. The financial hit of under-ordering (delivery fees, project delays, mismatched dye lots) far exceeds the cost of one extra box of panels sitting in your garage.
Can I install new soffit and fascia over my existing wood trim, or must I remove it first?
You CAN install vinyl/aluminum over existing wood IF the wood is structurally sound (no rot, minimal warping). This is called "capping" or "wrapping." First, test the wood: poke it with a screwdriver every 2 feet. If the driver sinks more than 1/4 inch, that section is too deteriorated and must be replaced – wrapping rotten wood just hides a problem that will worsen. Assuming it's solid, you'd install J-channel and F-channel directly over the wood, then slide in new soffit panels. The advantage: saves labor tearing off old material. The risk: you're adding weight to potentially aging fasteners, and you can't inspect what you've covered. Personally, I remove old wood on any home over 30 years old – I've found hidden wasp nests, squirrel damage, and water stains that would've caused issues later.
What's the actual coverage area of one soffit panel, and how do I convert that to the number of panels I need?
Soffit panels are typically sold in 12-foot lengths, with widths like 12 inches (single), 16 inches, or 24 inches (double). A 12" x 12' panel covers 12 square feet. HOWEVER, you lose about 1/2 inch on each side for channel insertion, so effective coverage is closer to 11.5 sq ft per panel. To calculate: (Total soffit area ÷ Panel coverage) x (1 + Waste%). Example: 180 sq ft area ÷ 11.5 sq ft/panel = 15.65 panels, round up to 16, then add 10% waste = 18 panels. Always round UP on final counts. Also check if panels are sold individually or in bundles (often 12 panels/bundle) – you may need to buy 2 bundles even if you only need 18 panels.
