The Pergola
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Pergola Calculator Guide
Rafters, purlins, beams and posts — plus the honest answer about how much shade you'll actually get.
Overhead shade = rafter width ÷ spacing.
2×8 rafters (1.5in wide) at 16in centres block just 9% of the overhead sun.
A pergola is not a roof. It never was.
The Shade Number Is Sobering
With the sun directly overhead, rafters at 16in centres shade about 9% of the area beneath. Even at 8in centres you're at roughly 19%. It's a grid of thin sticks with a lot of sky between them.
What saves it is that the sun is almost never directly overhead. As it drops, you're looking through the rafters at an angle and seeing their depth rather than their width — which is why deep rafters on edge shade far better in the afternoon than the overhead figure suggests. Design for the time of day you'll actually sit there.
Orientation Beats Everything
Run the rafters north–south and they'll block the low morning and evening sun as it swings across them. Run them east–west and you get shade at midday when you probably wanted it least.
This costs nothing to get right at design stage and cannot be fixed afterwards without rebuilding the roof.
Purlins Are the Cheap Upgrade
A second layer running crosswise on top of the rafters roughly doubles the shading and gives the structure that finished, layered look — and it's just a few more sticks.
It's the single best value change you can make to a pergola. It also gives climbing plants somewhere to grab, which is the other honest route to real shade.
Posts Are Structural, and the Base Is the Weak Point
4×4s are usually enough for a small pergola; longer spans and taller posts want 6×6s, which also look right rather than spindly.
Wood sitting directly on concrete wicks water and rots from the bottom. Use standoff post bases that hold the timber clear of the surface. It's a small part that decides whether the structure lasts eight years or thirty.
It Catches Wind
An open pergola seems like it can't catch wind, and mostly that's true — until you add a canopy, a shade sail or a mature vine. Then it's a sail attached to four posts.
If a cover is ever likely, size the footings for it now. Retrofitting a foundation under a finished pergola is not really a thing.
Related: deck cost calculator, lumber calculator, framing calculator.
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