People try to get more shade from a pergola by cramming in more rafters, then wonder why midday sun still pours through. The amount and timing of shade a pergola casts is governed mostly by the sun’s position in the sky and by the orientation and depth of the rafters, not simply by slat count. Understanding the geometry lets you design shade where and when you actually want it.
Sun angle matters more than slat count
The sun’s position changes throughout the day and across the year in two dimensions: altitude, how high it sits above the horizon, and azimuth, its compass direction. In summer the midday sun is high, so it shines nearly straight down between rafters unless the slats are deep. In winter it stays low, raking in at a shallow angle that deep rafters block easily. You can see the exact numbers for your latitude and date using NOAA’s solar calculator, which gives sun altitude and azimuth hour by hour. Feeding your real sun angles into the design tells you more than any rule of thumb about slat spacing.
Orientation: which way the rafters run
Rafter direction sets when shade falls:
- Rafters running north to south cast shadow bands that sweep east to west across the day, giving shifting shade at midday and less protection when the sun is low in the east or west.
- Rafters running east to west block more of the high midday and afternoon sun, holding shade over the same spot for longer through the hottest part of the day.
If your goal is a shaded lunch spot at noon, orientation and rafter depth do more work than adding slats. Align the run to the hours you use the space.
Slat depth, spacing, and shade percentage
A rafter shades according to its depth relative to the gap between rafters and the angle of the sun. Tall, deep slats set on edge block low-angle sun far better than shallow ones because they present more vertical surface to a raking beam. The ratio of slat depth to spacing, combined with sun altitude, sets the shade percentage at any moment. A practical approach is to pick the hours you most want shade, find the sun altitude for those hours, and size the depth-to-gap ratio so the slats intercept the beam at that angle. This is why two pergolas with the same number of rafters can shade very differently: deeper boards spaced the same distance apart cast far denser shade.
Adding purlins or shade cloth without overloading
When bare rafters are not enough, you can add a second layer of perpendicular purlins to tighten the grid, or stretch retractable shade cloth across the top. Both add shade, and both add load and wind exposure. Shade cloth in particular acts like a sail, transferring significant wind uplift into the frame and footings. Add these only within the structure’s capacity, and account for the extra dead and wind load in the beams and posts rather than assuming the existing frame can carry it.
Span tables set the hard limit
Geometry can suggest longer, deeper rafters than the wood can safely carry. How far a rafter of a given species and size can span before it sags or fails is capped by span tables. The American Wood Council’s span table tutorial explains how allowable span depends on the member size, species, spacing, and the load it carries, and shrinks as any of those grows. Before you widen the spacing to gain a longer clear span or add purlins and cloth, check that the rafter size still falls within its allowable span for the total load. The shade design has to live inside those limits.
