What Size Post for a Pergola? 6x6 Unless It Is Small

4x4 works for a small low pergola in a sheltered spot. Everything else wants 6x6 — posts fail by leverage at the base, not by crushing.

By Zakaria October 8, 2026 3 min read Save to Pinterest

Looking up at a white painted pergola post where the cross beams are notched and lapped over it — Photo: josstyk, Pixabay Content License, via Pixabay

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6x6 unless the pergola is small, low and sheltered. The reason is not weight — a 4x4 carries the load easily. Pergola posts fail by bending and racking under wind leverage at the base, and that is a stiffness problem, not a strength one.

The short answer

Use 6x6 posts for most pergolas and 4x4 only for a small, low structure in a sheltered position. Pergola posts rarely fail by crushing — they fail by bending and racking under wind leverage, and a 6x6 is far stiffer against that than a 4x4. Height is the factor that matters most, because a taller post applies the same wind force with more leverage at the base. Space posts 8 to 10 feet apart, set footings below the frost line, and prefer galvanised standoff brackets over burying posts directly in concrete.

6x6 for most builds, 4x4 only when small, low and sheltered
Failure is bending, not crushing — stiffness is what you are buying
Height drives it: 4x4 fine at 8 ft, marginal at 10
Post spacing 8–10 ft, 12 ft about the limit
Footings below the frost line; width resists overturning
Standoff brackets beat burying posts in concrete

Sizing the post

This table answers the question on its own

Situation	Post	Why
Under 8 ft, spans under 8 ft, sheltered	4x4	Adequate, and cheaper
8–10 ft, normal garden	6x6	Stiffness against racking
Over 10 ft, or exposed site	6x6 + bracing	Leverage grows fast with height
Heavy climbers planned	6x6	Mature wisteria is a serious load
Attached on one side	6x6 free side	House carries the other

Notching eats a 4x4

Cut the beam notches into a 4x4 and there is very little section left at precisely the most stressed point. A 6x6 takes the same joint and still has material around it. This is where a lot of undersized posts actually fail.

Why leverage, not weight

A pergola roof is open. It carries almost no vertical load — no snow lying on it, no tiles, no ceiling.

What it does catch is wind, pushing sideways at the top of the posts. A post is a lever fixed at the base, so doubling the height roughly doubles the turning force at the footing for the same push. That is why:

Height matters more than span when sizing posts
Footing width matters as much as depth for resisting overturning
Bracing at the top joints is worth more than extra post section

Spacing and beams are one decision

Posts at 8 to 10 feet is the normal range; 12 feet is about the practical limit before the beams need to be noticeably deeper to avoid sagging.

Stretching the spacing to use fewer posts usually costs more in beam timber than it saves in posts. Keep the bays even — an odd short bay at one end reads as a mistake.

Footings

Below the local frost line where ground freezes — this is the governing rule, and it is local
Roughly a third of the above-ground height, or 2 feet, whichever is greater, as a general guide
Wider rather than just deeper, to resist overturning
Check local codes rather than estimating

Set in concrete, or on brackets?

This table answers the question on its own

	Buried in concrete	Galvanised standoff bracket
Rigidity	Highest	Good, with a substantial bracket
Rot risk	Highest — water collects there	Low — post sits clear
Replaceable later	No	Yes
Cost	Lower	Higher

For a structure meant to last, standoff brackets with a decent standoff height are usually the better trade: the post never sits in standing water, and it can be replaced without demolishing the footing.

Timber

Pressure-treated softwood rated for ground contact is the normal and sensible choice. Cedar and redwood look better and cost considerably more, and are not as resistant in direct ground contact as treated timber is.

Whatever you use: treat every cut end with preservative before installation. The factory treatment does not reach the middle of the timber, and a fresh cut sitting in a bracket is an open door.

Common questions

What size post should I use for a pergola?

6x6 for most builds, and 4x4 only for a small, low pergola in a sheltered position. The reason is not that a 4x4 cannot carry the weight — it easily can — but that pergola posts fail by bending and racking under wind leverage rather than by crushing. A 6x6 has well over twice the stiffness of a 4x4 against that bending, and it also gives enough material to cut proper joints at the top without weakening the post.

Is a 4x4 strong enough for a pergola?

For a small structure under about 8 feet tall with spans under about 8 feet, in a sheltered garden, yes. Beyond any of those it becomes marginal quickly. Height is the factor that bites first, because a taller post means the same wind force applies far more leverage at the base. A 4x4 also leaves very little material once you notch it for beams, which is where many of them actually fail.

How far apart should pergola posts be?

Commonly 8 to 10 feet, with 12 feet being about the practical limit before the beams need to be noticeably deeper. Spacing and beam size are one decision rather than two: a wider span needs a deeper beam to avoid sagging, so stretching posts further apart to use fewer of them tends to cost more in timber than it saves. Keep spans even where you can, since an odd short bay looks wrong.

How deep should pergola post footings be?

Below the local frost line is the governing rule where ground freezes, and a common general guide is around a third of the post's above-ground height, or at least 2 feet, whichever is greater. Width matters as much as depth for resisting overturning — a wider footing resists the leverage of a tall post better than a narrow deep one. Local codes set the frost depth and should be checked rather than guessed.

Should pergola posts be set in concrete or on brackets?

Both work and they fail differently. Posts set directly in concrete are the most rigid but sit in a position where water collects around the timber, which is where rot starts even in treated wood. Galvanised standoff brackets bolted to a concrete footing hold the post clear of standing water and let it be replaced later, at the cost of some rigidity unless the bracket is substantial. For a long-lived structure, brackets with a good standoff height are usually the better trade.

What timber should pergola posts be?

Pressure-treated softwood rated for ground contact is the normal choice and the one most people should make; cedar and redwood are naturally durable and look better but cost considerably more and are not as rot-resistant in direct ground contact as treatment is. Whatever the species, the critical detail is that any timber in or near the ground carries a ground-contact rating, and that cut ends are treated with preservative before installation.

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A timber pergola in a garden with climbing foliage turning autumn colours over its rafters

Gear & Reviews · Oct 8, 2026