A pergola has no roof. It has rafters with sky between them. So the shade under it is a pattern of stripes, and the stripes get wider and narrower as the sun moves.
How much shade is that? It can be worked out with one simple rule. This guide shows the rule, a table for common rafter sizes, and what to add when the answer is not enough for your patio.
The plain answer
An open pergola gives partial shade that moves with the sun. It does not give full shade, and no rafter size changes that.
In the Carolinas the summer sun at noon is very high. Light falls almost straight down between the boards, so open rafters give the least shade exactly then, at the hour when many people want it most. Morning and late afternoon are when a pergola does its best work. In spring and fall the midday sun is lower, and the same rafters shade more.
If you need shade for lunch in July, plan for more than rafters. It is better to know that before the build than after it.
How a rafter makes shade
Swipe the drawing sideways to see all of it.
Each rafter throws one strip of shade. The strip is as wide as the board is thick, plus a part that comes from the depth of the board. That second part is the depth divided by the tangent of the sun's height.
In plain words: when the sun is low, the tall side of the board blocks a lot of light and the strip is wide. When the sun is high, light slips down between the boards and the strip is hardly wider than the board.
Take a typical 2x6 rafter. Its real size is 1.5 by 5.5 inches. With the sun at 45 degrees it throws a strip about 7 inches wide. Put a rafter every 16 inches, and 7 of every 16 inches are in shade. That is 44 percent. When the sun climbs to 75 degrees, the strip shrinks to about 3 inches and the shade drops to 19 percent.
Three things follow from the rule. Deeper boards, closer spacing and a lower sun all mean more shade.
Shade by rafter size and spacing
The table shows the share of the ground in shade for typical wood rafters. Read across a row to see the same pergola as the sun climbs.
| Rafters | 30°low sun | 45°halfway up | 60°high sun | 75°summer midday |
|---|---|---|---|---|
| 2x6every 24 in. | 46% | 29% | 19% | 12% |
| 2x6every 16 in. | 69% | 44% | 29% | 19% |
| 2x6every 12 in. | 92% | 58% | 39% | 25% |
| 2x8every 24 in. | 59% | 36% | 24% | 14% |
| 2x8every 16 in. | 88% | 55% | 36% | 22% |
| 2x8every 12 in. | 100% | 73% | 47% | 29% |
The top row is the height of the sun in degrees above the horizon. The left column is a low sun, in the morning or the late afternoon. The right column is a high sun around the middle of a summer day. The numbers are for a clear day, for rafters alone, with the sun crossing them. Trees, the house and the season change the result in a real yard, so use the table to compare, not as a promise.
- Half the spacing, twice the shade. Going from 24 inches to 12 inches doubles the number, until the ground is fully shaded.
- A deeper board helps most when the sun is low. At 30 degrees, a 2x8 every 16 inches shades 88 percent against 69 percent for a 2x6. At 75 degrees it is 22 percent against 19 percent.
- The right column stays low. Under a high summer sun, the best row in this table gives 29 percent.
Swipe the drawing sideways to see all of it.
These sizes are typical, not a fixed list. The written quote gives the real rafter size and spacing for your pergola.
Which way the rafters run
The table assumes that the sun crosses the rafters. It does not always. When the sun shines along the rafters, the depth of the board does nothing, and each board shades only its own thickness. For 2x6 rafters every 16 inches that is about 9 percent.
- Rafters that run north to south shade best in the morning and the late afternoon. The sun is then in the east or the west, low, and it crosses the boards.
- Rafters that run east to west do a little better around midday, when the sun is in the south and crosses them. The midday sun is high, so "a little better" is the right wording.
On many patios the shape of the space decides the direction. Then the other tools do the work. Top slats run across the rafters, so they catch the sun from the side the rafters miss.
Six ways to get more shade
In order of cost, from the least to the most:
- Closer rafters. The same boards, more of them. With the sun at 45 degrees, 2x6 rafters go from 29 percent at 24 inches to 58 percent at 12 inches.
- Top slats across the rafters. Typical slats are 2x2 boards. They add a second layer that works when the sun runs along the rafters.
- A fabric shade canopy or shade cloth. It covers the open gaps.
- Climbing plants. They take seasons to grow, so they are a plan for later years, not for the first summer.
- A clear or tinted polycarbonate cover. It also keeps the rain off.
- A pavilion roof. Full shade and rain cover. It is a different structure: heavier, on footings, and it usually needs a building permit. See pavilions.
Swipe the drawing sideways to see all of it.
The first two are choices you make when a wood pergola is drawn, because the wood is cut on site. An aluminum pergola uses the sizes and spacing of its system. Not sure if you want a pergola or a roof? Read pergola, pavilion or gazebo.
Plan the shade before the build
Tell us when you use the patio most: breakfast, lunch or after work. Tell us which way it faces, or send a photo and the time of day you took it. Then the rafter size, the spacing and any extras can be planned for that hour.
Send photos and rough sizes on the photo quote page. We send a free written quote. Every pergola, pavilion, privacy wall and repair is quoted after we see photos.