The thing nobody tells you before you build: a pergola's shade depends as much on the time of day and which way the slats run as on how far apart they are. The same structure that throws deep, comfortable shade at noon can let the late sun pour straight through underneath at 5 p.m. Shade isn't one number — it's geometry between your slats and the sun's angle, and once you see how the two interact, you can design for the hours you actually use the patio instead of hoping it works out.
I've watched people build a gorgeous pergola with widely spaced rafters, then sit under it in July wondering why they're still squinting. The slats were too far apart, too thin, and running the wrong direction for their afternoon-use patio. Every one of those is a design decision you make before the first post goes in — and every one comes down to math you can do at the kitchen table.
How pergola shade actually works
A pergola shades by interception: the slats (and the rafters under them) block part of the sky, and the fraction of direct sun they block is your shade percentage. But the sun moves. In the morning and late afternoon it sits low, raking in at a shallow angle; at midday in summer it's nearly overhead. The shadow each slat casts shifts and stretches across the day, so the ground coverage you get at noon is not what you get at 6 p.m.
Two geometric facts drive everything. First, when the sun is directly overhead, shade percentage is simply the fraction of the opening your slats cover from above — slat width divided by the spacing between slat centers. Second, as the sun drops toward the horizon, the depth (height) of each slat starts to matter: a taller slat casts a wider shadow at low sun angles, so deep slats on edge shade far better in morning and evening than thin flat boards. That's the whole secret to good pergola shade — play the slat depth and spacing against the sun angles during your use hours.
The slat-spacing and shade-percentage relationship
Start with the overhead case, because it's the simplest and the worst-case for coverage (least shade). If your slats are 2 inches wide on top and spaced 4 inches apart center-to-center, then at high noon they cover half the opening: 2 ÷ 4 = 50% shade. Tighten the spacing to 3 inches and you're at 67%; open it to 6 inches and you drop to 33%. The relationship for the overhead sun:
Midday shade % = slat top width ÷ slat spacing (center to center)
But that's only at noon. The real lever for the rest of the day is the gap-to-depth ratio — the open gap between slats divided by how deep (tall) each slat is. When the sun's altitude angle is low enough that its shadow throw exceeds the gap, the slats fully shade the ground. Specifically, a gap is fully closed by shadow once the sun's altitude drops to where the slat depth times the tangent relationship covers the gap — in plain terms, deeper slats and smaller gaps close off the low-angle sun. A 2-inch gap between slats that are 6 inches deep blocks sun whenever it's lower than about 70 degrees, which is most of the day outside high summer. The Pergola Shade Calculator runs these angles for your latitude and slat dimensions, but the principle is: closer slats plus taller slats equals more shade, and depth is what buys you the morning and evening.
Orientation: which way should the slats run?
This is the decision people get wrong most often, and it's free to get right. The direction your rafters and slats run determines when you get shade, because slats block the sun best when it's crossing them broadside rather than sighting straight down the gap.
Slats running north-south (so the gaps run N-S) give the best midday shade. As the sun crosses from east to west through the middle of the day, it moves broadside across the slats, and each one keeps casting a shadow. This is the right choice for a lunch patio or anywhere you want relief from the high noon sun.
Slats running east-west give better morning and late-afternoon shade. When the low sun is in the east or west, it crosses these slats broadside and they block it well; but at midday the high sun sights down the gaps and slips through. Choose this if your patio gets its heavy use at breakfast or at the end of the day, when the low, hot sun is the real problem.
For most people who use the patio for dinner in summer, the low western afternoon sun is the enemy, and no slat orientation fully fixes a sun that's nearly horizontal — that's where a side screen, a vertical louver, or a retractable canopy on the west side earns its keep. The NREL sun-position data and the solar geometry tools from the U.S. Department of Energy let you look up the sun's altitude and azimuth for your latitude and the exact hours you care about — genuinely worth ten minutes before you commit to a slat direction.
Slat spacing vs shade percentage
Here's the table to design from. It assumes nominal 2-inch-wide slats (a 2x board on edge, ~1.5 inch actual) and shows midday overhead shade; deeper slats raise the all-day average well above these midday figures.
| Spacing (center to center) | Open gap | Midday shade | Feel |
|---|---|---|---|
| 2 in | 0.5 in | ~75% | Deep shade; nearly a roof. |
| 3 in | 1.5 in | ~50% | Strong dappled shade. |
| 4 in | 2.5 in | ~38% | Light, airy shade. |
| 6 in | 4.5 in | ~25% | Mostly sun, gentle pattern. |
| 8 in | 6.5 in | ~19% | Decorative only; minimal shade. |
A traditional open pergola with rafters every 16 to 24 inches and no top slats gives almost no usable shade — it's an architectural frame, not a sunshade. To actually feel cooler underneath, you need top slats at 4-inch spacing or tighter, or you need to add a cover. People are routinely surprised by this; a bare rafter pergola looks substantial but blocks maybe 15% of the midday sun.
The geometry: gap-to-depth ratio and sun altitude
To shade more than the midday minimum, lean on slat depth. Picture a slat standing on edge: the taller it is, the longer the shadow it throws sideways for a given sun angle. The gap between slats is fully shaded when that shadow reaches across the gap, which happens when:
Gap fully shaded when: slat depth × tan(sun altitude from horizontal-to-slat) ≥ gap
The practical takeaway without the trig: a 2x6 laid on edge (5.5 inches deep) at 3-inch spacing throws deep shade across most of the day, because even when the sun's fairly high the deep slat covers the narrow gap. The same board laid flat (1.5 inches deep) only shades well when the sun is nearly overhead. So for real shade, run your top slats on edge, not flat — same lumber, same spacing, dramatically more shade, and as a bonus the on-edge boards are stiffer over a span.
Louvered and adjustable vs fixed
Fixed slats are a permanent compromise — you pick one spacing and orientation and live with it year-round. Louvered (adjustable) pergolas let you rotate the slats: open flat for full sun on a cool morning, tilt to throw shade as the day heats up, and close fully to shed a rain shower. The aluminum motorized versions are the priciest option but the most flexible, and they solve the afternoon-sun problem a fixed roof can't — you just angle the louvers against the low western sun.
| Option | Shade control | Typical cost | Notes |
|---|---|---|---|
| Open-rafter (no slats) | ~15% fixed | $3,000–$6,000 | Architectural; little real shade. |
| Fixed wood slats | 25–75% by spacing | $4,000–$9,000 | Set it once; on-edge slats shade best. |
| Fixed + fabric canopy | up to ~90% | $4,500–$10,000 | Retractable cloth between rafters. |
| Louvered aluminum (manual) | 0–100% adjustable | $8,000–$15,000 | Tilt to track the sun; sheds rain. |
| Louvered (motorized) | 0–100% adjustable | $12,000–$25,000+ | Remote/auto; rain sensors available. |
If a louvered system is out of budget but afternoon sun is your problem, the cheapest fix is a retractable fabric canopy or shade sail mounted under fixed rafters — it blocks up to 90% on demand and rolls back when you want sun. For the full structure cost and material breakdown, the Deck Cost Calculator helps if your pergola sits over a new deck, since the two projects usually get priced together.
Pergola shade quick facts to remember
- Midday shade % ≈ slat width ÷ spacing — tighter spacing, more shade.
- Run slats on edge, not flat — depth buys morning and evening shade.
- Slats N-S = best midday shade; E-W = best morning/afternoon shade.
- An open-rafter pergola blocks only ~15% of midday sun.
- Low western afternoon sun needs a side screen or canopy, not just roof slats.
A worked example: targeting ~50% midday shade
You've got a 12-by-14-foot patio you use mostly for lunch and early afternoon, and you want comfortable dappled shade — roughly half the sun blocked at midday — using 2x4 cedar slats run on edge.
- Slat actual width on edge: 1.5 in
- Target midday shade: 50% → spacing = 1.5 ÷ 0.50 = 3 in center to center
- Open gap: 3 − 1.5 = 1.5 in
- Run slats north-south for best midday coverage
- Slats over 14 ft span: 14 ft ÷ 3 in spacing ≈ 56 slats
Those 3-inch-spaced 2x4s on edge (3.5 inches deep) hold around 50% at noon and climb well past that morning and evening as the deeper dimension blocks the low sun. If midsummer noon still feels too bright, you can add a retractable canopy later without rebuilding. Measure the patio first with the Square Footage Calculator so you size the frame and count the slats correctly, and price the posts-and-beams structure with the Pergola Calculator.
Material and board sizing
Span drives your lumber. Top slats on edge can run further between rafters than flat boards before they sag — a 2x2 flat slat looks delicate but flexes over anything past a few feet, while a 2x4 or 2x6 on edge stays straight over 8-to-12-foot rafter bays. Rafters themselves typically span 8 to 12 feet between beams depending on size; the span tables from the American Wood Council give the allowable runs for each lumber size and species so your roof doesn't sag in year three. Cedar and redwood resist rot and stay light; pressure-treated pine is cheaper but heavier and wants sealing. Whatever you pick, run the load-bearing rafters in the larger dimension and reserve thinner stock for the decorative top slats.
Common questions about pergola shade
How much shade does a pergola actually give?
It depends entirely on slat spacing, depth, and the time of day. An open-rafter pergola with no top slats blocks only about 15% of the midday sun — not much. Add top slats at 4-inch spacing and you're around 38%; tighten to 3 inches and run them on edge and you'll hit 50% or more at noon and considerably more in the morning and evening. For deep, near-roof shade you need 2-inch spacing or a fabric cover.
Which direction should pergola slats run for the most shade?
Run slats north-south for the best midday shade, since the sun crosses them broadside through the middle of the day. Run them east-west if your patio gets heaviest use in the morning or late afternoon, when the low sun crosses east-west slats broadside instead. No fixed orientation fully blocks the low western afternoon sun — for that you need a vertical side screen, louvers, or a canopy.
What slat spacing gives 50% shade?
For nominal 2-inch slats (about 1.5 inches actual), 3-inch center-to-center spacing gives roughly 50% shade at midday, leaving a 1.5-inch gap. Running those slats on edge rather than flat pushes the all-day average well above 50%, because the deeper dimension blocks the lower morning and evening sun. Tighter spacing or deeper slats raise the percentage from there.
Are louvered pergolas worth the extra cost?
If you want shade you can change through the day or year, yes — adjustable louvers solve the afternoon-sun problem that no fixed roof can, and they shed rain when closed. The tradeoff is cost: manual louvered aluminum runs $8,000 to $15,000 installed and motorized systems well beyond that. If budget is tight, fixed on-edge slats plus a retractable canopy gets you most of the flexibility for far less.
The bottom line
Pergola shade is geometry, not guesswork. Midday shade is roughly slat width divided by spacing, but the real performance comes from running slats on edge so their depth blocks the low morning and evening sun, and from orienting them north-south for noon or east-west for the ends of the day. An open-rafter pergola barely shades at all; for comfort you want 3-to-4-inch spacing or a canopy. Set your target, check the sun angles for your latitude, and run the numbers through the pergola shade calculator so you build it right the first time.