BuildingPergolaAdvanced

Bolted Pergola

A 3,600 x 2,400 mm cedar pergola whose beams are notched over the posts and bolted through, with rafters bolted to the beams and every fastener in A4 stainless because it lives outdoors.

Designthe dimensions everything else is derived from

Footprint

3600 x 2400 mm

Height

2400 mm to the underside of the beams

Posts

4 off 100 x 100 mm

Beams

2 off 3600 x 100 x 200 mm, notched over the posts

Rafters

12 off 2400 x 45 x 120 mm at 300 mm centres

Exposure

fully external, so every fastener is A4 stainless

Cut list sizes are finished sizes. Add 5 mm to the width and 25 mm to the length of every solid-wood part for saw kerf and planing, and buy boards oversize by one thickness so a bowed board can be flattened rather than relegated.

Materialswhat each part is made from, and why

All materials

Cedar (Western Red) · 4 posts, 2 beams, 12 rafters

One of the few timbers that resists rot without a treatment. Treated softwood will also last, but it has to be re-cut and re-treated at every notch, and a pergola is nothing but notches.

Stainless Steel A4 (316) · every bolt, washer and nut

A4 (316) rather than A2. Plain steel or zinc plating will rust within a season outdoors, and a rusting bolt stains the cedar around it as well as losing section.

Cut listfinished sizes, worked back from the design

Part	Qty	Finished size	Stock
Post	4	100 x 100 x 2400 mm	Cedar (Western Red)
Beam	2	3600 x 100 x 200 mm	Cedar (Western Red)
Rafter	12	2400 x 45 x 120 mm	Cedar (Western Red)

Fastenersquantities and lengths that follow from the joints

All standards

Item	Qty	Grip	Standard
M12 x 220 hex bolt, A4 stainless	8	200 mm	ISO 4014
M10 x 160 hex bolt, A4 stainless	24	145 mm	ISO 4014
M12 plain washer	16	—	DIN 125 A
M10 plain washer	48	—	DIN 125 A
M12 and M10 nylon-insert nut	32	—	DIN 985

Grip is the sum of the clamped thicknesses. A bolt has to clear the nut by two full threads, which is what sets the length that appears in each row — not a preference.

M12 x 220 hex bolt, A4 stainless · grip 200 mm

Two at each of the four beam-to-post crossings. Grip is the beam (100) plus the post (100). Partial thread rather than full thread, so the plain shank bears in the holes and the joint resists shear instead of the threads cutting into the timber. ISO 4014 →

M10 x 160 hex bolt, A4 stainless · grip 145 mm

One at each rafter-to-beam crossing, twelve rafters over two beams. Grip is the beam (100) plus the rafter (45). ISO 4014 →

M12 plain washer

One under each head and each nut. Cedar is soft, and an unwashered nut pulls straight into it under the bolt load. DIN 125 A →

M10 plain washer

One under each head and each nut across the twenty-four rafter bolts. DIN 125 A →

M12 and M10 nylon-insert nut

Eight M12 and twenty-four M10. A structure that dries and shrinks will loosen a plain nut on its own; the insert keeps the joint in tension. DIN 985 →

Toolsand what each one is for on this job

All tools

Circular Saw

Crosscutting the 100 x 200 beams, which are too deep for a single pass from one side.

Drill

Boring the 13 mm and 11 mm clearance holes.

Twist Drill Bits

Clearance one size over the bolt: 13 mm for M12, 11 mm for M10. An oversized hole is a joint that moves.

Socket Set

Thirty-two nuts in an awkward overhead position.

Combination Wrench

Holding the head while the nut is run up, which is the only workable way on a through-bolt.

Tape Measure

Setting the 300 mm rafter centres from one end rather than cumulative from the last.

Torque Wrench

Bringing every structural bolt to the same tension instead of whatever felt tight.

Order of workthe sequence, and where it is not free to change

Set the four posts, 3600 mm apart one way and 2400 mm the other, and brace them plumb in both directions before anything goes on top.
Cut a 100 x 100 notch in each beam where it crosses a post. A beam that sits beside the post instead of over it hangs the whole roof on the bolts.
Drop the beams into their notches and clamp.
Bore each crossing through and bolt up, snug only, until the frame is square.
Check the diagonals, then bring the eight bolts to torque.
Lay the twelve rafters on at 300 mm centres, bore and bolt each one.
Only now remove the temporary bracing.

Where it goes wrongthe three failures worth designing out

Zinc-plated fasteners. They look identical in the packet and will be rusting by the second winter, with a rust stain running down the post.
Bolting a beam to the side of a post rather than notching it over the top. The bolts then carry the entire roof load in shear and withdrawal, and a bolt in withdrawal is the one thing a bolt is poor at.
An impact driver on the bolts. It gets the joint to a torque nothing specified, crushes the cedar, and strips the thread from the hole it is in.

Frequently asked questions

Why A4 stainless rather than A2?

Because pergolas live in the places A2 struggles with. A2 (304) is fine inland; near the coast, or anywhere salt spray or chlorides reach, only A4 (316) reliably resists pitting. The price difference is small next to replacing rusted bolts inside a finished joint.

Should the posts go into concrete?

The posts should stop above the concrete, on a base that keeps the end grain clear of standing water. A post cast into concrete has a permanently wet end grain, and end grain is where rot starts in every timber.

Do the rafters need two bolts each?

No, and this project uses one because they sit on the beams rather than being clamped between them; their weight holds them and the bolt stops them sliding. The beams, which carry the whole roof, get two per crossing for that reason.