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Why even slat spacing changes a pergola
Uneven pergola slats catch the eye at once. One gap wide, the next narrow, and the whole roof looks homemade in the wrong way. Even spacing does more than look good: it spreads shade evenly, it makes the structure read as deliberate, and it stops arguments about where each slat goes. The maths is not hard, but doing it by hand for seventeen slats invites a small error that shows up at the far end. This calculator does it exactly: it takes your span, your slat width and your target gap, then returns the slat count and the even gap that fits, including equal gaps at both ends.
The method matters. You cannot simply divide the span by slat plus gap, because the two end gaps count too. A run has one more gap than it has slats: a gap before the first slat, a gap between each pair, and a gap after the last. The calculator first finds how many slats fit near your target gap, then re-divides the leftover space into equal gaps. For a 3,000 mm span with 90 mm slats and a 90 mm target gap, it fits 17 slats, and the even gap becomes about 81.7 mm. Seventeen slats cover 1,530 mm of the span, which is about 51 percent shade. That shade figure is a useful bonus: it tells you how much sun the roof will block at midday.
How to use the calculator
- Measure the span the slats must cover, in millimetres, from inside face to inside face.
- Enter your slat width, the actual width of the boards you bought, not the nominal size.
- Enter the gap you would like, as a starting target rather than a fixed demand.
- Read the slat count and the even gap, then mark from one end using the even gap throughout.
A worked example
Take a span of 2,000 mm, slats 100 mm wide, and a target gap of 50 mm. The calculator fits 13 slats: thirteen times 150 is 1,950, which fits inside 2,000 plus one target gap. The even gap is then 2,000 minus 1,300, which is 700 mm of total gap space, divided by 14 gaps, giving exactly 50 mm. In this case the target and the even gap agree, which happens when the numbers divide neatly. In the 3,000 mm example they did not: the target was 90 mm but the even gap came out at 81.7 mm. That is normal, and it is the right answer. Forcing 90 mm gaps would leave a stray 8 mm somewhere, and stray millimetres are exactly what the eye notices.
Marking out is where care pays. Measure the first gap from the inside face of the beam, mark the slat, measure the next gap, and continue. Check the total at the far end before fixing anything: the last gap should match the first. If it does not, one mark drifted, and it is far easier to re-mark now than to move fixed slats later.
Tips for a clean pergola roof
- Measure real board widths. A board sold as 100 mm is often 90 or 95 mm after planing.
- Keep the end gaps equal to the middle gaps. A half gap at one end looks like a mistake.
- Pre-drill near board ends to stop splitting, especially in hardwood.
- Fix slats with the best face up, and keep any bow facing the same way.
- Think about sun angle: a roof that shades at noon may admit low evening sun, which is often welcome.
Common mistakes
The most common mistake is spacing from the centre of one slat to the centre of the next without accounting for the ends, which leaves one end gap wrong. The second mistake is using nominal board sizes instead of measured ones, which shifts every gap by a few millimetres. The third mistake is forgetting that slats shrink and swell with weather: leave the calculated gaps as they are, because wood movement is small compared with a well-chosen gap, and tight slats will bind and cup. Remember the scope of this tool too. It is an educational estimator for spacing and shade on an ordinary garden pergola. It does not size beams, posts, footings or fixings, and it does not judge wind loads or snow loads. Structural questions belong to a qualified professional and to local building rules.
Who spaces pergola slats
Garden builders use this maths on every pergola, deck builders use it for shade roofs over seating areas, and homeowners use it when replacing rotten slats on an existing frame. The drawing the tool produces shows the layout at a glance, so you can check it before cutting. Measure the span, enter the real board width, and let the even gap do the work.
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