Building a Pergola: Rafter and Purlin Layout That Divides Evenly

What Carries What, Counting Down From the Slats

Five things stack up between the footing and the sky, and each is bought and set out differently. Footings take the load into the ground. Posts take it down. A pair of beams — one either side of each post line, or a single beam notched over — spans between the posts. Rafters cross the beams. Purlins cross the rafters, and where the client wants denser shade a lath layer crosses the purlins in turn. Each layer runs at ninety degrees to the one below it, which is what makes the thing rigid in both directions and also what makes every layer visible from the ground.

The order of decisions runs the other way from the order of assembly. Post positions come first, because they are the only thing that has to agree with the paving, the drain, the neighbouring wall and whatever is buried in the lawn. Beam span follows from post spacing, beam depth follows from beam span, and only then does the rafter grid have a fixed number to divide.

Post spacing itself is not a taste decision. It is the clear span of the beam sitting on top, and that span comes from a published table for the species, grade and section you are buying — the AWC Span Tables for Joists and Rafters in North America, the span tables in AS 1684 Residential timber-framed construction in Australia and in NZS 3604 Timber-framed buildings in New Zealand, or a supplier's tables derived from the strength classes in BS EN 338. A pergola beam is usually loaded far more lightly than a roof beam, which is exactly why it is tempting to guess, and exactly why the guess is unverifiable.

The connection at the top of the post deserves the same attention as the span. A beam bolted to the side faces of a post transfers its load in shear through the bolts; a beam notched into the post head and bearing on the shoulder transfers it in bearing, and the bolts only stop it moving. The second is stronger and much less fussy about bolt sizing, and it is the reason so many pergolas are built with paired beams straddling the post. Whichever you use, the notch has a limit: cutting a bearing shoulder more than a modest fraction into the post or the beam takes real section away, and the notch provisions in the AWC National Design Specification for Wood Construction and in BS EN 1995-1-1 (Eurocode 5) are the places that limit is written down.

Square the Post Grid, Then Never Measure From a Post Again

A pergola set out from four corner pegs that were eyeballed square will look wrong in a way nobody can name. The rafters are parallel to each other because you marked them off one beam, and the purlins are parallel to each other because you marked them off one rafter, but the two grids are not at right angles and the shadow pattern on the paving shows it as a slow drift. Pull both diagonals before the holes are dug and again after the posts are concreted, because a post takes a nudge from a barrow far more easily than anyone expects.

The tape reading you are checking against is not a judgement call. For a rectangle it is the square root of length squared plus width squared, and a 4.8 by 3.6 layout has a diagonal of exactly 6.0 m, which is a pleasant coincidence and not a general rule. What matters more is knowing how far apart the two readings may be before the frame is visibly out: a 20 mm difference between diagonals on a structure this size is a couple of millimetres of skew per bay, invisible; 80 mm is not. Set out to the post centres, not the post faces, because a 100 mm post rotated a quarter turn moves its faces and does not move its centre.

1. Establish one control line first — usually the face of the house, the edge of the paving or a boundary you have to stay parallel with — and set the two posts on it.
2. Square the other pair off that line with a scaled 3-4-5 struck as large as the site allows, not at arm's length; a triangle set out over three metres carries a fraction of the angular error of one set out over one.
3. Pull both diagonals and adjust the free pair until the two readings agree, then re-read the sides, because moving a corner to fix a diagonal has just changed a side.
4. Mark post centres with pegs offset clear of the holes, so the setting-out survives the auger.
5. Re-read the diagonals with the posts plumbed and braced, before the concrete goes off, while it is still a five-minute correction.

The Rafter Count Is Arithmetic; the Remainder Is a Decision

With the beams up and measured — measured, not taken off the drawing, because the built length is the one the rafters have to fit — the count comes out of one division. Take the beam length, divide by the largest spacing your span table allows for the rafter section you are buying, round up to get the number of bays, and add one for the rafter that closes the far end. A 4.8 m beam at a 450 maximum gives 10.67, which rounds to 11 bays and 12 rafters.

Now look at what the rounding did. Eleven bays across 4.8 m is 436 mm on centre, not 450. Rounding up never widens the spacing past the limit — that is the point of it, and it is why the count is a ceiling rather than a nearest — but it also means the number you typed is a maximum you will not actually build at. Everyone who sets out a fixed 450 and lets the last bay take the remainder has misunderstood which of the two numbers is the constraint. The spacing is the constraint. The layout is free.

So spread the remainder across every bay instead of dumping it in one. Divide the measured beam length by the bay count, then mark every rafter position from one tape pulled from a single end before any timber is offered up. Measuring each bay off the previous mark adds a saw kerf and a pencil width every time and delivers the whole accumulation to the far end. Mark the side of the line the timber sits on with a cross, too: a 45 mm rafter set on the wrong side of its mark has moved 45 mm, and on a bay of 436 that is a visible tenth.

Where the beam does not divide kindly and the client has fixed the spacing for a reason — matching an existing structure, or lining rafters up with fence panels or paving joints — the remainder has to sit somewhere, and it goes into the two end bays equally. Set out from the centre line of the pergola outward in both directions and let the two ends absorb half each. A symmetrical pair of slightly wide end bays reads as a deliberate margin. One narrow bay at the right-hand end reads as running out of beam.

The other thing worth deciding before cutting is whether the end rafters sit flush with the beam ends or inboard of them. Flush is the usual choice and makes the beam length the same number as the rafter spread, which keeps the arithmetic honest. Inboard by a hundred millimetres or so gives the beam a visible return past the last rafter, looks more finished on a heavy-sectioned frame, and means the length being divided is the beam minus 200 — which is a different division with a different answer, so make that call before you run the numbers, not after.

Shade Is Made of Gaps, and the Gap Is Not the Spacing

Centres are how carpenters talk and gaps are what people see. A 45 mm planed rafter at 436 centres leaves 391 mm of sky between neighbours; the same centres with a 75 mm rough-sawn section leave 361. That is an eight per cent change in the amount of light coming through, achieved without touching the layout, and it is the reason a pergola drawn at one section and built at another never looks like the visual. Decide the clear gap you want first if shade is the point of the structure, then add the actual timber width back on to get the centres, rather than the other way round.

Actual width is not nominal width, and the difference is standardised rather than approximate. Under PS 20, the American Softwood Lumber Standard, a nominal 2x6 is dressed to 1.5 by 5.5 in — 38 by 140 mm — and a 4x4 post finishes at 3.5 in square. Metric markets are messier in a different way: rough sawn 50 by 150 is 50 by 150 until it is regularised to 47, planed all round it lands nearer 45 by 145, and the three of those in one order will not line up on a beam. Ask which one is being delivered and set out to that figure, because a 5 mm error in assumed width across twelve rafters is 60 mm that has to come out of the end bays.

Purlins Cross Back, and They Are a Different Division

Purlins are the second grid — the run of smaller sections laid over the rafters at ninety degrees, tying the tops together, stiffening the whole plane against racking and doing most of the shading work. They are counted along the rafter, not across the beam, which means the number being divided is the pergola's depth plus any overhang at each end, not the beam length that governed the rafters. On the 3.6 m frame in this article, with 300 mm of rafter tail projecting each side, the run to divide is 4.2 m, and forgetting the tails is how a purlin order comes up two rows short.

The count works the same way as the rafters — bays rounded up, plus one to close the far end — and it carries the same remainder decision, but it is worth resisting the instinct to make the two grids match. A pergola with rafters and purlins at identical centres reads as a coarse square net; the same frame with purlins tighter than the rafters reads as a woven ceiling, and it costs nothing extra structurally because purlins carry very little. The reason to tighten them is shade, and the reason not to is that every additional row is another line of fixings driven overhead.

One caveat on the input, because the calculator's field asks for a run measured along the slope. A pergola with a nominal fall for drainage — a rafter dropped 60 mm over 3.6 m to stop water sitting on the top faces — has a sloped length within a millimetre or two of its plan length, and you can ignore the distinction. Pitch the frame properly, at 15 degrees for a slatted awning look, and the sloped run is 3.5 per cent longer than the plan dimension: on a 4.2 m run that is 148 mm, enough to change the row count at some spacings. Measure the timber, not the footprint.

The maximum spacing belongs to whatever the purlins are supporting, not to the purlins themselves. Bare, they carry nothing but their own weight and can be spaced to taste. Under a polycarbonate or metal panel they have to sit inside the panel manufacturer's published span for the load case, which is a figure a supplier will confirm in writing if asked. Under a retractable shade fabric they are taking a live tension load that pulls sideways as well as down — a different structure, and a conversation to have before the timber is ordered.