☀️ Skylight Sizing Calculator
Calculate optimal skylight size for natural light and code compliance
📐 Room Details
💡 Expert Installation Tips
🎯 The 5-15% Rule: Not Just a Number
Everyone quotes "skylight should be 5-15% of floor area" like it's gospel. Reality? It depends on EXISTING light. I calculate actual daylight factor: (window area + skylight area) ÷ floor area. Target: 15-20% total for bright room. 150 sq ft room with 15 sq ft of windows already has 10% coverage. Add 15 sq ft skylight (another 10%) = 20% total – bright but not a greenhouse. North-facing kitchen with tiny window (3%)? Go 15% skylight. South-facing room with patio doors (25% windows)? 5% skylight is plenty. One client ignored windows, added 15% skylight to room that already had 20% glazing – summer heat gain cost $480/year extra in AC until we added solar shades.
📏 Framing Reality: 14.5", 22.5", 30.5" Rough Openings
Standard rafters are 16" or 24" on-center. A 2x4 skylight (actual skylight, not rough opening) needs 30.5" × 51.5" rough opening – that's TWO rafter bays at 16" OC. You're cutting TWO rafters and installing a header. For one-bay installation (no header), skylight width must fit WITHIN one bay: max 14.5" actual width for 16" OC framing (skylight between rafters). Popular mistake: buying 22.5"×46.5" skylight thinking "it'll fit" – nope, you're cutting a rafter. Curb-mount units add 2-4 inches to rough opening. I always ask rafter spacing BEFORE ordering. $850 Velux skylight had to be returned because framing was 24" OC (22.5" actually fit!), but header would've cost $600. Went with two 14.5" units instead.
🔨 Low-Pitch Roofs: The 4-Inch Curb Rule Nobody Follows
IRC R308.6.7: Roofs under 3:12 pitch MUST have 4-inch minimum curb or skylight will leak. Period. I've torn out 20+ failed installations where contractor skipped curbs on 2:12 roofs "because it's faster." Water pools, finds a crack, bam – $4,200 water damage. Even manufacturer warranty voids without proper curb on low slopes. Here's the trick: most curb-mount skylights come with 4" curbs INCLUDED. But some deck-mount units (designed for steep roofs) don't – you have to build custom curb on-site. Check specs. On any roof under 4:12, I add curb regardless of code (4:12-ish is borderline). Extra $120 for pre-fab curb vs $3,800 leak repair? Easy math.
⚡ Egress Skylights: The 5.7 Sq Ft Trap
IRC R310 says egress skylights need 5.7 sq ft NET CLEAR OPENING – not gross skylight size. A 24"×48" skylight (8 sq ft) has maybe 5.0-5.5 sq ft net opening once you subtract frame. FAILS code. You need minimum 30"×46" operable unit (9.6 sq ft gross) to guarantee 5.7 sq ft net. Also: max 44" sill height from floor, must open from inside without tools. I've seen inspectors red-tag bedroom egress skylights because homeowner didn't read fine print. Had to upgrade $780 skylight to $1,450 egress-rated model. Pro tip: If room has compliant window egress, skylight doesn't need to be egress-rated – saves $400-700 per unit. But if it's sole bedroom egress? Pay for proper unit or fail inspection.
💸 Energy Costs: The Dark Side of Oversizing
Client wanted "maximum natural light" in 200 sq ft home office. Installed TWO 4×4 skylights (32 sq ft, 16% of floor). Summer: room hit 88°F by 10 AM despite AC at 72°F. Added $67/month cooling (vs $18 before skylights). Radiant heat gain through glass is BRUTAL – even low-E glass transmits 55% solar heat. We added motorized solar shades ($340 each), dropped temp to 76°F, bills to $32/month. Net effect: skylights cost $50/year MORE in energy vs savings from reduced daytime lighting ($30/yr). Should've done 10% glazing (one 2×4 skylight), saved $220/year in energy, still plenty of light. Rule: South/west-facing rooms stay under 10% skylight. North/east can go 12-15% safely.
❌ Common Mistakes to Avoid
🚫 Ignoring Rafter Spacing Before Ordering
The Error: Measuring rough opening ceiling dimensions, ordering skylight to fit, not checking rafter bay width.
Real Impact: Customer measured 30" between drywall seams, ordered 30"×30" curb-mount skylight ($890). Opened ceiling – rafters were 24" OC. Actual space BETWEEN rafters: 22.5". Skylight wouldn't fit without cutting rafter and adding header ($680 framing). Returned skylight (20% restocking fee: $178), ordered 22"×27" instead ($720). Total waste: $178 + 3 weeks delay. Proper measurement from attic BEFORE ordering would've saved everything.
Fix: Before ordering, go in attic with tape measure. Measure rafter-to-rafter spacing (typically 16" or 24" OC). Skylight width must fit BETWEEN rafters (14.5" max for 16" OC) OR span 2+ bays and require header. For header-width skylights, budget extra $400-800 for structural work. Check manufacturer rough opening specs, not just "skylight size."
🚫 Skipping Curb on Low-Pitch Roof (Under 3:12)
The Error: Installing deck-mount skylight directly on 2:12 pitch roof to save $120 curb cost.
Real Impact: Beautiful $1,200 skylight install. 4 months later, brown water stain appeared on ceiling. By month 8, drywall sagging from water weight. Leak source: water pooled against skylight bottom lip (nearly flat roof), seeped under flashing. Damage: $4,200 (new drywall, repainting, mold treatment), plus $850 to remove skylight, build proper curb, re-install with new flashing kit. All to save original $120 curb cost. Warranty voided – manual stated "curb required for pitches under 3:12."
Fix: On roofs 3:12 pitch or less, use curb-mount skylights with 4" minimum curb per IRC R308.6.7. Pre-fab curbs cost $90-150. Custom-built curbs: $180-250 (2×6 lumber, waterproof membrane). For 3:12 to 4:12 ("borderline"), I still recommend curb – cheap insurance vs $4K leak damage. On 5:12+, deck-mount is fine per code.
🚫 Using Standard Skylight for Bedroom Egress
The Error: Installing non-egress 24"×46" fixed skylight in bedroom with no other egress window, thinking "it's big enough."
Real Impact: Final inspection FAIL. IRC R310.1 requires EVERY bedroom to have egress opening. Inspector measured skylight – fixed unit, can't open, doesn't count. Existing window: too small (4.2 sq ft vs required 5.7). Had 3 options: (1) Replace skylight with egress-rated operable unit ($1,450), (2) Enlarge window opening ($2,800), (3) De-classify room as "bonus room" and lose bedroom count (hurt resale value). Chose option 1. Total waste: $780 (original skylight unusable) + $1,450 new + $400 second install labor = $2,630 overage.
Fix: Check egress requirements BEFORE skylight spec. Bedrooms need min 5.7 sq ft NET CLEAR opening (R310.2), sill height max 44" from floor. Egress-rated skylights cost $1,100-2,000 vs $600-1,200 standard. BUT if room already has compliant window egress, skylight doesn't need to meet R310 – use cheaper fixed unit. Get permit/inspection plan approved before ordering.
🚫 Oversizing for "Maximum Light" Without Energy Analysis
The Error: Installing 15% skylight glazing in south-facing room based solely on "natural light" goal, ignoring heat gain.
Real Impact: 180 sq ft primary bedroom, south-facing. Installed THREE 2×2 skylights (27 sq ft total, 15% glazing) for "bright wake-up light." Summer reality: room hit 82-86°F by 7 AM despite central AC. Portable AC added ($340) to maintain 72°F, ran 6 hours daily (May-Sep: $94/month electric vs $28 before). Annual extra cost: $396 cooling, minus $40 lighting savings = $356 net LOSS. Should've done 8% glazing (TWO 2×2 units), would've stayed comfortable with $180/year cooling, $15 lighting savings = $165 net, still very bright.
Fix: Calculate solar heat gain BEFORE installing. South/west rooms: max 8-10% skylight. North/east: up to 12-15% safe. Use low-E glass (reduces heat gain 30-40%). For high-sun-exposure rooms, add manual or motorized shades ($150-450). Rule: Annual cooling cost increase should NOT exceed 3x lighting savings or you're net-negative. Ask for energy modeling ($200) if unsure.
🚫 Centering Skylight in Room Instead of Over Work Area
The Error: Placing skylight dead center of ceiling for symmetry, not considering furniture layout or task areas.
Real Impact: Kitchen remodel: installed 4×4 skylight centered in 12×14 room. Looked perfect architecturally. Reality: light beam hit dining table (great), but NONE hit primary work zone (sink/stove area – north wall in shadow). Still needed under-cabinet lights on 8 hours/day. Later added second 2×4 skylight above sink ($1,850 install). Should've offset FIRST skylight 3 feet toward work zone initially – one skylight would've done both. Wasted $1,850 + original positioning was suboptimal.
Fix: Plan skylight location around TASK AREAS first, aesthetics second. Kitchens: over sink/island. Bathrooms: over shower/vanity. Home office: above desk (but avoid glare on monitor – 60° from screen angle). Bedrooms: offset toward reading chair, NOT over bed center (wakes you at dawn). Use cardboard template on ceiling, live with it 2-3 days, test with sun angles. I use "sun path" apps to simulate light at different times. Small move (2-3 feet) can double useful illumination.
❓ Frequently Asked Questions
What size skylight do I need for my room?
Use the 5-15% rule: skylight area should be 5-15% of room floor area, adjusted for existing windows and orientation. Formula: Target total daylight = 15-20% floor area (windows + skylight). Example: 120 sq ft room with 12 sq ft window (10%) needs 6-12 sq ft skylight (5-10%) for 15-20% total. Adjustments: North-facing or few windows → use 12-15% skylight. South-facing with windows → use 5-8% skylight. Standard sizes: 2×2 ft (4 sq ft), 2×4 ft (8 sq ft), 4×4 ft (16 sq ft). For 100-150 sq ft room, one 2×4 skylight (8 sq ft, 6-8%) is typical. Check manufacturer rough opening requirements vs rafter spacing before ordering.
Do I need a curb for my skylight installation?
YES if roof pitch is 3:12 or less per IRC R308.6.7 – minimum 4-inch curb required. On steeper roofs (4:12+), curb not mandatory but still recommended for better waterproofing. Curb types: (1) Pre-fab metal/wood curbs ($90-180, easiest install), (2) Site-built curbs ($40 materials + labor, custom fit). Why curbing matters: Curb raises skylight 4-6 inches above roof plane, allowing water to sheet past without pooling. On low-pitch roofs, lack of curb causes 90% of skylight leaks. Deck-mount (no curb) skylights are designed for steep roofs ONLY (6:12+). Don't skip curb to save $100 – leak repairs cost $2,000-5,000.
What are IRC egress requirements for skylights in bedrooms?
If skylight serves as PRIMARY bedroom egress per IRC R310, it must meet: (1) Minimum 5.7 sq ft NET CLEAR opening (not gross size – measure operable opening, subtract frame). (2) Minimum 24" high × 20" wide clear opening. (3) Maximum 44" sill height from finished floor to bottom of opening. (4) Operable from inside without keys/tools (manual crank, electric motor with backup). (5) Opens to exterior grade or roof (for rescue access). Reality: A 24"×48" skylight (8 sq ft gross) might only provide 5.0 sq ft net opening → FAILS. Need 30"×46" or larger. Exception: If bedroom has compliant window egress, skylight doesn't need to meet R310 – can use cheaper fixed unit. Egress skylights cost $1,100-2,200 vs $600-1,200 standard.
Will a skylight increase my energy bills significantly?
Depends on SIZE, orientation, and glazing. Heat gain: Skylights transmit 2-3x more solar heat than walls due to overhead angle. South-facing skylight: +800-1,200 BTU/hour heat gain per sq ft in summer (vs 300 BTU for south wall). Cost impact: Typical 2×4 skylight (8 sq ft) in moderate climate: +$8-15/month cooling cost (May-Sep), -$3-5/month lighting savings = net $3-12 INCREASE. Oversized (15% floor area) south-facing skylight: +$40-80/month cooling common. Mitigation: (1) Use low-E coated glass (reduces heat gain 30-40%), (2) Add solar/blackout shades ($120-400), (3) Limit south/west skylights to 8% floor area, (4) North/east skylights safer – less heat gain. Best ROI: North-facing 2×4 skylight with low-E glass: +$5-8/month cooling, -$5/month lighting ≈ cost-neutral with comfort gain.
Can I install a skylight myself or do I need a professional?
DIY-able for skilled homeowners IF: (1) Comfort cutting rafters/installing headers, (2) Roof access safe (not too steep), (3) Standard installation (no complex flashing). Steps: Cut ceiling opening, remove roofing shingles, cut rafter(s), install header (if multi-bay), build/install curb (low pitch), mount skylight, install flashing kit (critical!), seal/waterproof, patch interior. Time: 8-16 hours for first install. Cost DIY: $600-1,500 skylight + $150 tools/materials vs Professional: $1,800-4,500 installed. Risks: Improper flashing = leaks ($2,000-5,000 damage), structural errors = sagging roof, code violations = failed inspection. Recommendation: DIY if experienced with roofing + have 2+ helpers (heavy lifting). Hire pro for: low-pitch roofs (tricky flashing), egress units (code compliance), cathedral ceilings (complex framing). Permits/inspections required regardless – budget $150-300.
