How to Plan Pergola Materials
A pergola is a freestanding outdoor structure with four corner posts, two long beams across the top, and rafters running across the width. Optional top slats provide shade and a finished look. Material counts depend on size, spacing, and overhang preferences.
Rafter Count Formula
Rafters span the width and are spaced along the length. Closer spacing creates more shade; wider spacing creates an airy, modern look:
Rafters=LengthinSpacingin+1
Top Slat Count Formula
Slats=WidthinSlat Width+Gap
Standard Pergola Sizes
Common pergola footprints: 10x10 (small patio), 12x12 (dining area), 12x16 (lounging plus dining), 16x20 (entertaining). Build slightly larger than your furniture footprint to leave 2 to 3 ft of walking space around the perimeter.Post and Footing Sizes
| Pergola Size | Post | Footing Depth |
|---|---|---|
| Up to 8x8 | 4x4 | 24 in |
| 10x10 to 16x16 | 6x6 | 30 to 36 in |
| Over 16x16 | 8x8 | 36 to 48 in |
Below the Frost Line
In cold climates, all post footings must extend below the frost line to prevent heaving. The frost line varies from 0 inches in the South to 60+ inches in northern Minnesota. Check your local code for the required depth in your area.Material Options for Pergolas
| Material | Cost / sq ft | Lifespan | Maintenance |
|---|---|---|---|
| Pressure-treated pine | $15 - $30 | 10 - 20 yrs | Stain every 2-3 yrs |
| Cedar | $30 - $50 | 20 - 40 yrs | Seal every 3-5 yrs |
| Redwood | $40 - $60 | 25 - 40 yrs | Seal every 3-5 yrs |
| Aluminum | $30 - $60 | 50+ yrs | None |
| Vinyl / PVC | $40 - $70 | 30+ yrs | Wash annually |
Rafter Spacing and Shade
The spacing between rafters controls how much sunlight penetrates the pergola. Tighter spacing with wider rafters gives more shade. Common spacings:
- 12 in OC - dense shade, modern look, uses most lumber
- 16 in OC - balanced shade and airflow, standard choice
- 24 in OC - minimal shade, maximum airflow, airy appearance
Add Shade with Slats or Cover
For afternoon shade, add a second layer of slats or a canvas canopy across the rafters. A 2x2 slat at 45 degrees off the rafter angle casts shadows that move with the sun. For guaranteed dry seating, install a retractable canopy or polycarbonate roof panels.