Pergola Shade Calculator
The difference between a comfortable pergola and a sun-blasted one comes down to rafter spacing. Enter your pergola dimensions, rafter size, and spacing to calculate exact shade coverage percentage, number of rafters needed, and lumber requirements. Optionally add shade cloth for enhanced coverage.
Shade Calculator
How Pergola Shade Works
Unlike a solid roof, a pergola creates partial shade through the shadow cast by its rafters. At midday when the sun is directly overhead, shade coverage equals the ratio of rafter width to total span. As the sun moves lower in the sky, rafter shadows elongate and overlap, increasing effective shade in the morning and late afternoon while decreasing it at midday.
This dynamic shading is actually a feature — it provides cooler shade during the hottest hours while allowing warming sunlight through during cooler parts of the day. For consistent, predictable shade, add shade cloth to supplement the rafter shadows.
Shade Coverage Formula
Midday Shade % = (Number of Rafters × Rafter Width) ÷ Total Span × 100
Total Shade % = Rafter Shade + (Open % × Shade Cloth %)
Real-World Example
A 14×10 foot patio pergola with 2×8 rafters at 10" spacing, plus 50% shade cloth:
- Number of rafters: (14 × 12 ÷ 10) + 1 = 18 rafters
- Rafter shade: 18 × 1.5" ÷ 168" = 16.1%
- Open area: 100 - 16.1 = 83.9%
- Cloth shade: 83.9 × 0.50 = 42.0%
- Total shade: 16.1 + 42.0 = 58.1%
- Shaded area: 140 sq ft × 0.58 = 81 sq ft
Rafter Spacing Reference
| Spacing (OC) | 2×6 Shade | 2×8 Shade | Feel |
|---|---|---|---|
| 6" OC | 25% | 25% | Heavy shade |
| 8" OC | 19% | 19% | Heavy shade |
| 12" OC | 13% | 13% | Moderate shade |
| 16" OC | 9% | 9% | Light/dappled |
| 24" OC | 6% | 6% | Mostly open |
Common Pergola Mistakes
- Spacing rafters too far apart. 24" OC looks great in photos but provides almost no shade at midday.
- Forgetting post footings. Pergola posts need concrete footings below the frost line (24-48" deep in most areas) to prevent shifting and leaning.
- Undersizing beams. The beams supporting rafters must handle the combined weight. A 16-foot span needs at least doubled 2×10 or 6×8 beams.
- Ignoring wind loads. Shade cloth acts as a sail. Secure it properly and consider removable options for storm-prone areas.
For related projects, try our deck board calculator, fence stain calculator, or concrete curing calculator for footing work.
Frequently Asked Questions
How much shade does a pergola provide?
A standard pergola with spaced rafters provides approximately 25-50% shade at midday, varying throughout the day as the sun angle changes. The shade percentage depends on rafter width, spacing, orientation, and sun angle. East-west oriented rafters provide more consistent shade. For maximum coverage, space rafters close together or add shade cloth (50-90% shade factor). Our calculator shows exact shade percentages based on your specifications.
What is the ideal rafter spacing for pergola shade?
For moderate shade (40-50%), space rafters at 1.5× the rafter width. For heavy shade (60-80%), space rafters at 0.5-1× the rafter width. For light, dappled shade (20-30%), space at 3-4× the rafter width. The most common spacing is 12-16 inches on center with 2×6 or 2×8 rafters, providing approximately 30-45% shade at midday.
Should a pergola face north-south or east-west?
For maximum shade during the hottest part of the day (midday), orient rafters running east-west. This blocks the high midday sun most effectively. North-south oriented rafters allow more midday sun through but provide shade during morning and late afternoon. East-west is recommended for patios where you want lunch and afternoon shade. North-south works for gardens needing midday sun with morning/evening protection.
How many rafters do I need for a pergola?
Calculate: Number of Rafters = ( Pergola Length ÷ Rafter Spacing) + 1. For a 16-foot pergola with 16-inch spacing: (16×12 ÷ 16) + 1 = 13 rafters. Standard pergolas also need 2 beams (the horizontal members supporting the rafters) and 4-6 posts. Rafters typically extend 12-18 inches beyond the beams on each side for aesthetic overhang.
What size lumber should I use for pergola rafters?
2×6 rafters work for spans up to 8 feet. 2×8 rafters handle up to 12 feet. 2×10 or 2×12 are needed for 14-16 foot spans. The beams carrying the rafters should be one size larger (e.g., 2×8 rafters on 2×10 or doubled 2×8 beams). Posts are typically 6×6 for standard residential pergolas. Always check local building codes for span requirements.
Does adding shade cloth to a pergola help?
Shade cloth dramatically increases coverage. A 50% shade cloth on a pergola with 30% rafter shade creates roughly 65% total shade (the effects don't simply add). Common shade cloth ratings: 30% for light filtering, 50% for comfortable seating areas, 70-80% for intense heat regions, 90% for greenhouse shade. Shade cloth is inexpensive ($0.15-0.50/sq ft) and can be removed seasonally.
How much does it cost to build a pergola?
A DIY pressure-treated pine pergola (10×12) costs $500-1,500 in materials. Cedar costs 2-3× more. Professional installation adds $2,000-5,000+ in labor. Factors: size, wood species (PT pine, cedar, redwood), post footings (concrete piers vs helical piles), and permits. Aluminum pergolas cost $3,000-8,000 installed. Vinyl/fiberglass start around $4,000. Building permits may be required for structures over 120 sq ft in many jurisdictions.