Pergola Shade & Slat Calculator
Calculate Slat spacing, Slat Counts & Solar Shade Percentages
Building a pergola to create a shaded outdoor living space? Pergolas are highly popular because they filter hot sun rays without fully blocking cool summer breezes. Slat/rafter height, thickness, and spacing directly dictate how much shade is cast at solar noon. Use this calculator to estimate slat counts and shade density.
How Pergola Slats Cast Shade and Why It Matters
Unlike gazebos or pavilions that have solid shingled roofs, a pergola features open rafters (slats or purlins) running across the roof beam structure. The amount of shade cast depends on the **width, height, and spacing** of these slats. When the sun is directly overhead (90° angle at solar noon), only the width of the slats blocks sunlight.
However, as the sun moves lower in the sky (e.g. 45° or 60° angle), the height of the rafters blocks significant wind-blown light, throwing much longer, wider shadow lines. According to geometric formulas: $$\text{Effective Shadow Width} = \text{Slat Width} + \text{Slat Height} \times \cot(\theta)$$ Where **$\theta$** is the sun's angle relative to the horizon. Shorter spacing and taller slats produce maximum shade density.
The standard spacing for pergola slats (usually 2x2 or 2x4 lumber) is **3 to 6 inches**. Spacing closer than 3 inches creates heavy shade but requires significantly more lumber, while spacing over 6 inches offers very light shade.
Pergola Slat Spacing vs. Solar Shade Density
| Slat Spacing | Shade at Noon (90°) | Shade at Afternoon (45°) | Lumber Volume Needed |
|---|---|---|---|
| 2 inches apart | 43% shade | 100% full shade | Very High (tight look) |
| 4 inches apart | 27% shade | 91% shade | Standard / Moderate |
| 6 inches apart | 20% shade | 67% shade | Low / Lightweight |
| 8 inches apart | 15% shade | 53% shade | Minimal (mostly decorative) |
How to Use This Pergola Shade Calculator
Measure the length of the pergola framework parallel to the rafter spacing. Input the thickness/width and height/depth of the wood slats (e.g., a standard 2x4 board measures 1.5 inches wide by 3.5 inches deep). Specify your preferred slat spacing gap and the sun angle. Click Calculate.
Frequently Asked Questions
Should pergola slats run North-South or East-West?
For optimal shade control, run your pergola roof slats in a **North-South direction**. This allows the slats to cross the path of the sun (which travels East to West), maximizing shadow coverage during the hot midday and afternoon hours.
What is the difference between rafters and slats?
Rafters are the main structural boards (usually 2x6 or 2x8) that support the roof span. Slats (or purlins, usually 2x2 or 2x4) are smaller boards laid perpendicularly on top of the rafters to create the shading grid.
Can I add fabric shades to my pergola?
Yes. Many homeowners attach sliding canvas shade sails or retractable canopy covers underneath the pergola rafters, allowing them to open the roof for full sky views or close it for weather protection.
- American Wood Council (AWC) - Structural timber span limits and outdoor pergola framing specifications.
- North American Deck and Railing Association (NADRA) - Safety guidelines for outdoor timber frames and structures.