Trace the Shade Before You Buy the Beams: Planning a Pergola for a British Patio

Most pergola disappointments are not carpentry failures. The joints hold, the posts stay put, the timber weathers nicely. The problem is that the structure ends up shading the wrong part of the patio at the wrong time of day. A corner that was pleasant at lunchtime becomes a dim tunnel by four o'clock, or the new roof casts a stripe of shade exactly where you wanted to sit with a cup of tea.

The fix is pleasingly low-tech. Before you buy a single length of timber, spend a sunny day watching the shadows your garden already makes. Not the ones you imagine it makes. The real ones, moving across the paving.

Start with the patio, not the catalogue

Pergola planning often begins with a picture of a handsome square frame and a rough idea of size. That is backwards. Begin with the patch of ground you actually use. Where do you put the table? Which corner gets the morning sun? Is there a spot that bakes at midday but turns cold the moment the sun drops behind the house?

Mark the patio's main zones with chalk: seating, cooking, a pot plant or two, the route from the back door. This is your working map. It does not need to be tidy. It needs to be honest about how the space is used, including the bits you avoid because they are always in shadow or always in glare.

A day of canes, chalk and photographs

Pick a clear day and set aside three check-ins: mid-morning, mid-afternoon and early evening. In the UK, 10am, 3pm and the hour before sunset are useful markers, but the exact times matter less than catching the sun at different angles.

Push a cane into the ground at each corner of the proposed pergola footprint. Run string between them if you have it. Then, at each check-in, mark the edge of the shadow cast by the house, fence, shed or existing tree with chalk on the paving. Take a photo from the same spot each time, looking towards the house or the main view. If you have a compass or a phone app, note roughly where south is. If not, note where the sun rises and sets and work from there.

By the end of the day you will have three overlapping shadow maps. They will tell you more about the patio than any glossy brochure.

What the three maps tell you

The mid-morning map shows where the garden is still waking up. The afternoon map is usually the important one for pergolas, because that is when a south-facing patio can become uncomfortably hot or glary. The evening map shows where low sun sneaks under roofs and through gaps, often creating the loveliest light of the day — or the most annoying glare into a kitchen window.

Look for the constant shade. If one corner is dark at all three checks, a pergola roof will not fix it and may make it worse. Look for the moving band of sun. If the patio is bright at 10am but shaded by 3pm, a pergola may only need to take the edge off the middle of the day. Look for the wind funnel, too. A pergola with tight slats can be a lovely sun filter and a terrible windbreak if it is placed across the wrong corner.

Orientation: follow the sun, not the fence

It is tempting to line a pergola up with the house, the boundary or the existing patio slabs. Sometimes that is right. Often it is merely convenient. The sun does not care about your fence line.

Use the shadow map to decide which way the structure should face. A pergola attached to a south-facing house will usually want its open side towards the garden and its shaded side towards the patio doors, so you can step out into cooler air rather than straight into a dark box. A freestanding pergola can be turned slightly to catch the evening sun while blocking the worst of the afternoon glare.

In small gardens, orientation is a compromise. You may not have room to shift the footprint far. That is all the more reason to test it with canes first. Moving a chalk rectangle costs nothing. Moving four concrete footings costs a weekend.

Slats, gaps and the difference between shade and gloom

Slat spacing is where a pergola becomes either a light filter or a lid. Wide gaps give dappled shade and a sense of openness. Tight spacing gives more solid shade, but it can also make a small patio feel closed in, especially under British skies that are not always reliably bright.

As a starting point, slats around 100mm wide with gaps of roughly the same size will cast a soft, striped shadow. Bring the gaps down to 50mm and the shade becomes much stronger. Go tighter still and you are effectively building a roof with cracks in it. Neither is wrong, but they suit different gardens.

Think about the direction of the slats as well as the spacing. Slats running across the path of the midday sun will throw broader bands of shade at lunchtime. Run them the other way and the stripes tend to run lengthwise across the patio, which can look calmer but shade less evenly. The shadow map will show you which part of the day you most want to tame.

Winter sun, low angles and the neighbour's wall

A British garden gets two very different suns. The summer sun is high enough for a pergola roof to do useful work. The winter sun is low, weak and often comes straight in under the beams. That is not a fault. It is a reminder that a pergola is a seasonal shade tool, not a year-round awning.

Even in November, a low sun will draw long shadows across a patio. If winter light matters to you — and in the UK it usually does — do not block the south-facing opening with a solid panel or a dense screen. Keep the low rays coming in. Use the pergola to shade the summer high sun, and use planting, slatted side screens or a removable sail to deal with low glare when it becomes a problem.

Neighbouring walls and fences matter too. A tall wall to the west can steal your evening sun long before sunset. A fence to the east may keep the patio cold until mid-morning. Map those shadows honestly, because no amount of clever slat spacing will bring back sun that a building is already blocking.

Small plots and terraced yards

Terraced-house yards are their own shade case. High walls, narrow footprints and neighbouring buildings mean the sun may only reach the yard for a few hours. In that situation, a dense pergola roof is usually a mistake. Go lighter: fewer slats, wider gaps, maybe a simple frame with a climbing plant rather than a full canopy.

In a small garden, the pergola should also earn its footprint. If it will only be used for three months of the year, make sure it does not turn the rest of the patio into a dark corridor. Leave one side open. Keep the lowest slats high enough not to crowd the space. Choose timber that will silver gracefully rather than demand constant refinishing, especially if access is awkward.

Let the map choose the timber

Once you know where the shade should fall, the buying gets easier. You will know whether you need a compact frame or a long run of slats, whether the posts can sit clear of the main walking route, and whether a side screen is worth adding. You will also have a better idea of how much timber you actually need, which is the most reliable way to avoid a pallet of leftovers.

A pergola built from a shadow map tends to feel settled. It cools the patio without swallowing it. It lets winter light in and takes the sting out of a July afternoon. And it starts not with a catalogue, but with a piece of chalk and a sunny day.