Drawn in plan, a pergola reads as a shopping list. Parallel lines, gaps between them, a beam across each end. The lines look like the part that stops sun and the gaps look like the part that lets it through, and neither of those is quite true. What you get on the patio is the depth of those rafters measured against the width of the gaps beside them, read at whatever angle the sun happens to be crossing them at the hour you actually sit out there.
Turn the same top the other way on the same posts and you have a different structure. Same lumber, same footprint, different hours.
How a rafter makes shade
A rafter shades by presenting its depth. The shadow it drops lands offset sideways from the rafter itself, and how far over it lands depends on how low the sun is sitting across the run. Low sun, long offset. High sun, short offset — the shadow falls nearly straight down, and the rafter covers little more than its own width.
Continuous shade happens at the moment each rafter's shadow reaches the next rafter. That is a ratio: depth against the clear gap beside it. Deepen the rafter or narrow the gap and you have moved the same ratio, which is why "what spacing should the rafters be" has no answer standing on its own. Spacing means nothing without depth. Neither means anything until you say which hours you want covered.
There is a second piece of the geometry, and it catches people. When the sun lines up with the run of the rafters — shining down their length rather than across them — each rafter casts its shadow onto itself and the gaps are wide open. Depth stops counting altogether. All the shade left in the structure comes from the beams, the posts and whatever is growing on it.
Which way the top runs
Rafters running north and south present their depth to the sun's east-to-west travel. Early and late, when the sun is low and off to one side, the shadows lap over each other and the shade goes solid across the whole floor. Around the middle of the day the sun swings south, runs down the length of the rafters, and the top stops working. Coffee in the morning and dinner in the evening: good. Lunch: bands of sun crossing the table.
Rafters running east and west do the reverse, with a seasonal twist. Midday, the sun crosses them and their depth counts — but in high summer the noon sun sits close enough to overhead that the offset is short, so you still get stripes unless the gaps are tight. In the cold months, when the noon sun is low, the same top goes almost solid. Shade in November, stripes in July, which is backwards from what most patios are for. And early and late, the sun runs down their length and the top does nothing at all.
Neither orientation touches a sun that is low and lined up with the run at the same time. That light arrives flat, from off the corner, under the beam, at eye level. Nothing overhead intercepts it.
Start from the hours, not the patio
The useful question is not which way the patio faces. It is which hours you expect to be under the thing. A breakfast patio and a dinner patio want opposite tops on the same house.
Late afternoon on a west-facing patio in summer is the case that sends most people looking for a pergola, and it is the case an open top handles worst. Around midsummer the sun sets well north of due west, so the last hours come in almost horizontally, off the corner of the house, at an angle no arrangement of rafters can catch. Adding rafters does not fix it. The light is not coming from above.
Worth saying plainly before anyone spends money: if a builder tells you a standard open top will make a west-facing patio comfortable at dinner time in July, they are answering a question you did not ask.
The low sun is a side problem
Everything that actually solves late-day sun is vertical, and all of it changes the structure.
- A slatted or louvred panel on the west side. Fixed slats set at an angle cut the sun while keeping some of the view through. It is also sail area — wind load that arrives at the posts and travels down into the ground.
- A retractable screen or a canopy on a track. Dropped for the hours that need it, rolled away the rest of the time. Moving parts outdoors, which means maintenance.
- Rafters extended past the beam. A cantilever on the sunset side gives low light more to cross before it reaches the seats. It costs length, not complexity.
- Planting. A tree solves it permanently and takes seasons to do it. A vine over the top solves it partly, adds weight, holds moisture against the wood underneath, and drops leaves into every gap you were careful about.
Deeper rafters, tighter gaps, or a layer laid across
Once you know the hours, there are a few ways to move the ratio, and they cost different things.
Deeper rafters buy cut-off with fewer pieces, and depth is also what lets a rafter span between beams. The trade is visual. Deep members read heavy from underneath and lower the apparent ceiling, and a pergola that feels like a lid is a common regret.
Tighter gaps get you there with the members you already like the look of. That means more pieces, more dead weight onto the beams and down through the posts, more places for leaves to lodge, and a top that behaves like a roof in a gust well before it behaves like one for shade.
A layer laid across — slats or purlins running perpendicular over the rafters, usually shallower and closer together — gives you cut-off in the direction the rafters cannot cover. It is the standard cure for the midday stripe, and it is where most of a pergola's character comes from anyway. It is also the layer people add later, so it is worth asking whether the beams and posts were sized for it at the start.
Every one of those adds load: dead weight all the time, wind load in a storm, and — once a top gets dense enough to hold snow — a load nobody sizes for on an open frame. It all arrives at the posts and then at the holes under them, which is a separate conversation with its own rules, covered in footings, frost depth and things that move in spring.
When the honest answer is a roof
If you want dependable shade at a fixed hour, an open frame may simply be the wrong instrument. Adjustable aluminum louvre roofs rotate closed and shed water. Solid or translucent panels do it permanently. Both are legitimate, and both stop being a pergola in the ways that matter to the rest of the job.
- Water. A closed top collects it, which means gutters, a downspout and a discharge point. Ending that line next to a footing or against the foundation reverses the point of the exercise — where the roof water goes applies to a small roof as much as a large one.
- Attachment. Against the house, a solid roof becomes a ledger and a flashing detail, and that connection is where these structures usually fail. The reasoning is the same as on a deck: the ledger board is where a deck usually fails.
- Uplift. A roof over an open frame is a wing. Post bases and beam connections have to be rated to hold down as well as up.
- Review. A solid roof is more likely to need a permit, and much more likely to interest an association. Both are cheaper to find out before the design than after — check the HOA before you draw anything.
- Heat. A closed top over a paved patio holds warm air under it unless the sides are genuinely open. Shade and airflow are not the same purchase.
What is under it, and what is behind it
A light-colored concrete patio bounces glare up into your eyes from outside the footprint of the structure, so a pergola over pale hardscape can measure as shaded and still feel bright. Dark material overhead reads denser than pale material at the same spacing, because almost none of the light that hits it comes back down.
There is a room on the other side of the wall, too. A pergola over west-facing glass shades the glass, and that is doing as much for the room behind it as for the seats outside. Worth knowing if the kitchen behind that glass is getting redone in the same year.
Wood moves, and a tight top shows it
A pergola is a lot of long, exposed, unrestrained pieces, and they behave the way all wood here behaves — taking on moisture through a humid summer and giving it back through a dry heating season. Indoors that shows up as seasonal movement in floors and cabinet doors. Outdoors it is bigger, because nothing is moderating it.
What that means for spacing is simple. The tighter the gaps, the more obvious a rafter that cups or twists becomes, because the eye reads it against its neighbor. It is also why the top faces of everything deserve attention before assembly — those surfaces take the sun and the standing water, and nobody is reaching them again once the top slats are on. Species and treatment change the rate, not the mechanism — the same argument as deck materials against Indiana weather. And treated lumber has to give up its mill moisture before it takes a finish at all; staining a new deck covers how to tell.
Test it before anyone cuts lumber
The whole thing can be checked in an afternoon, and almost nobody does it.
Set stakes where the posts would go and run string between them at beam height. Lay a board flat across the string, oriented the way the rafters would run, and hold another one beside it at the gap you are considering. Then look at the ground. What you see under those two boards is what the whole top will do, scaled up, at that hour of that day.
Photograph it at the hours you already know you use the yard, not a convenient one. And do it in the season you are building for — a test in April flatters every design on the table. The shadow the house or the fence throws at your target hour tells you the same thing for free.
What to remember
Shade is depth over gap, read against the way the sun crosses the run. Pick the direction the top runs from the hours you want covered, not from the shape of the patio. Handle the low sun at the sides, because it is never coming from above. And every step toward denser shade is weight and wind load handed down to the posts and the ground beneath them.
Photographs are not much help here — a pergola shot at the right moment looks like whatever the photographer wanted — but you can see how the tops are layered on the ones in our gallery. If you want somebody to stand in the yard with you at the hour that actually matters and work backward from there, ask for an estimate and we will walk it.
