What a Pergola Actually Does

Most people buy a pergola expecting shade and are mildly disappointed. The structure goes up, it looks good, and at two in the afternoon in July the seating underneath is still in the sun.

That is not a bad build. It is a misunderstanding about what the thing does, and it is almost always the fault of a conversation that never happened.

A Pergola Is Not a Roof

A roof blocks light. It is opaque, so whatever is under it is in shade all day, every day, regardless of where the sun is.

A pergola is a set of spaced slats, and spaced slats do something different. They block light arriving from some directions and let it through from others. What is underneath moves in and out of shade as the sun crosses the sky, and the pattern on the floor shifts through the day.

So the honest answer to “will it be shady under there” is: shady at some times and not others, and which times depends entirely on how it was built.

That is a more interesting answer than it sounds, because it means a pergola can be tuned. You can decide which hours you want covered. Very few buyers are ever offered that choice.

Three Numbers Do All the Work

Forget the overall size for a moment. The shade a pergola casts is set by three things about the slats.

Depth is the one people underestimate. A slat turned on edge and twice as deep shades far more than a wider flat board, for the same spacing and nearly the same material. It is the cheapest way to buy more shade, and it barely changes how the structure looks from underneath.

Spacing is the obvious lever and the one with a cost. Tighter slats give better summer shade and also block the low winter sun that most people actually want on a cold bright day. That trade is worth making deliberately rather than by default.

Direction Decides the Hours

This is the part that should start every pergola conversation and almost never does.

Because the shade moves with the sun, the direction the slats run determines which part of the day sits in shadow. Run them one way and you get your deepest shade around the middle of the day. Run them the other and the structure works harder in the morning and late afternoon, when the sun is low and coming in sideways.

Neither is right. They suit different lives.

Someone who eats outside at six in the evening and is at work at noon wants the opposite of someone whose kids are out there all afternoon in the school holidays. The correct first question from anybody building this is not how big do you want it. It is when do you actually sit out here.

If nobody asks you that, the orientation is being decided by what is convenient to frame, which is a coin toss on whether the structure does the job you bought it for.

The Seasonal Part Is a Feature

The sun sits higher in the sky at midday in summer and lower in winter. A fixed pergola cannot change, so the same structure behaves differently across the year, and that is useful rather than a limitation.

Tuned for high summer sun, it will cut the worst of the July glare and then let a good deal of low winter light straight through underneath. In a climate with gray winters that is exactly the right behavior, and it is why a fully roofed structure is often the wrong answer even though it sounds like the better one.

If you want all day shade in August and sun in February, you want slats. If you want to sit out in the rain, you want a roof. They are different products and the second one needs a different structure underneath it, because a solid roof catches wind and sheds water.

Adjustable Louvers, and When They Are Worth It

Louvered systems let the slats rotate, so you can close them for shade and open them for light. They solve the tuning problem outright.

They also cost considerably more, add moving parts, and need the frame to be built to carry them. Worth it for a space used daily across the whole year. Hard to justify for a corner of the yard used twenty times a summer, where deeper fixed slats at the right spacing get most of the benefit for a fraction of the money.

The useful way to decide is honestly estimating how many days a year you will be under it. People overestimate this by a wide margin.

Posts, Footings and Wind

A pergola looks light and is not structurally trivial. It is a large flat assembly held up on a few posts, and wind pushes on it.

Uplift is the one that catches people out. Wind getting under a slatted top tries to lift the whole structure, which means the connection at the bottom of each post matters as much as the one at the top. A pergola sitting on surface blocks because it seemed like a light structure is a pergola that moves in a storm.

Three things matter here:

• Footings below the frost and movement layer, not pads sitting on the surface
• Post bases that resist lift, not just locate the post
• Enough rigidity across the top so the frame does not rack into a parallelogram

Freestanding structures need all of this more than attached ones, because an attached pergola borrows stiffness from the house. If a structure is going on a deck rather than on the ground, the deck framing underneath has to have been built knowing that, which is the sort of thing that is easy before the boards go down and expensive afterward.

The Material Choice Is Mostly About Maintenance

Metal is the quiet winner on span. Because it is stronger for its size, the slats and beams can be slimmer for the same opening, which is why large modern pergolas tend to be metal. Wood at the same span needs chunkier members, which some people prefer anyway.

Attached or Freestanding Is a Structural Question

Bolting a pergola to the house looks like the simpler option and it changes the engineering in both directions.

An attached structure needs fewer posts and gains stiffness from the wall, so it resists racking better and costs less in framing. It also puts a new load, and a new hole in the weather envelope, onto the side of the building. That connection has to be flashed properly or it becomes the slow leak nobody finds for years, which is the same failure that gives badly attached decks their reputation.

A freestanding structure leaves the house alone entirely. It needs four or more posts rather than two, more bracing across the top, and better footings, because nothing else is holding it square.

The deciding factor is usually not cost. It is where the shade needs to be. A structure pinned to the house can only shade the ground next to the house, and the spot people actually want covered is often a few steps out, where the sun reaches in the late afternoon.

The Mistake That Makes One Useless

Building it where the house already shades the spot.

It sounds too obvious to be common and it is one of the most frequent errors there is. A pergola on the shaded side of a building, or tucked against a wall that already blocks the afternoon, adds a structure and no function.

The fix costs nothing: stand in the space at the hour you actually want to use it, on a sunny day, and look at where the sun is coming from. Ten minutes of that beats any amount of planning on paper. When a pergola is going to form part of an outdoor living area rather than stand on its own, that walk around the site is where the whole layout should be decided.

Questions Worth Asking

1. When do I use this space? If the builder does not ask, say it anyway, because it sets the orientation
2. How deep are the slats and how far apart? These two numbers are the shade
3. What are the footings and the post bases? Uplift, not just weight
4. Fixed or adjustable, and what does the difference actually buy me?
5. Will the structure shade the seating or the paving next to it? They are not the same spot

The Short Version

A pergola is a tunable light filter, not a roof. Depth and spacing set how much shade there is. Direction sets when you get it. The three together are worth more than overall size, and all three get decided in a five minute conversation that mostly does not happen.

Anyone planning one is better served working backward from the hours they want covered. Pergola builders in Kent, WA who open with when do you sit out here are asking the right question, and their Kent base puts them across King County. The ones who open with how big do you want it are going to build you something handsome that you still squint under.