What Thickness Posts and Beams Should an Oak Pergola Have?

When designing an oak pergola, choosing the right post and beam thickness is one of the most important decisions. The correct dimensions ensure the structure is strong, stable, and visually balanced. Oak is exceptionally durable, but like all natural timber, it needs to be sized correctly to perform well and maintain its appearance over time. This guide explains the ideal post, rafter, and beam thickness for oak pergolas — and offers professional recommendations based on real experience in oak framing.

Understanding Oak Pergola Structure

An oak pergola typically consists of three main elements:

• Posts (uprights) — the vertical supports that carry the load.
• Headers or Beams — the horizontal members connecting the posts, bearing most of the weight.
• Rafters — smaller crosspieces on top, adding shade and structure.

Each component needs to be thick enough to support its load, resist weather movement, and stay in proportion with the overall design.

Recommended Thickness for Oak Pergola Posts

The minimum post size for an oak pergola should be 140 × 140 mm, but the standard Timber Solution specification is 145 × 145 mm. Posts smaller than 145 mm are not recommended — they appear under-scaled and may compromise the frame’s stability, especially in tall or freestanding designs. Bigger oak beams create a bold, architectural look suited to open garden spaces.

Recommended Size for Oak Rafters

The minimum rafter size is 60 × 140 mm. This dimension provides good rigidity and visual balance for most oak pergolas. Rafters can be spaced closer together for more shade or positioned wider apart for a lighter design. However, thinner rafters or those with excessive knots are more prone to twisting or bending, particularly in exposed locations.

Recommended Size for Header Beams

The header beam (also known as the ring beam) is one of the most critical components in an oak pergola. For most builds, a 145 × 145 mm beam provides excellent strength and proportion. However, the correct beam thickness depends on the span between posts:

• Up to 3 m: 145 × 145 mm is ideal.
• Over 3 m: thicker or deeper beams may be required to reduce deflection.

In large pergolas, where spans are greater, a thicker beam ensures the structure remains visually straight and stable over time.

Professional Recommendations and Oak Movement

In practice, the thinner beams tend to be the most vulnerable parts of any oak pergola. Like all timbers, oak expands during wet weather and shrinks in dry, hot conditions. These natural movements can slightly affect the pergola’s appearance — causing beams to bow, twist, or curve (sometimes called the banana effect). This is a normal physical process, especially with a dense hardwood like oak. Unfortunately, it’s impossible to predict exactly which beam will move, but based on our experience, twisting or bending usually occurs in:

• Beams with many knots
• Longer or thinner beams
• Beams that are not fully fixed or secured

Oak posts are the most stable components, as both ends are securely fixed.

• At the top, we use a 50 mm thick × 75 mm long tenon joint, locked with a single kiln-dried oak dowel. Read More
• At the bottom, we recommend using post bases or staddle stones for stability and moisture protection. Read More

Headers or ring beams (145 × 145 mm) are also secured to the posts, while rafters on top help prevent the beams from twisting. From experience, joists are typically the most challenging. In most projects, 60 × 140 mm rafters perform perfectly — but in some cases, slight twisting or bending may occur.

At Timber Solution, we’re always happy to replace any problematic beams or suggest practical options like noggins (timber spacers between rafters) to minimise movement. However, noggins aren’t always the most visually appealing solution, and some clients prefer to keep a cleaner aesthetic.

Summary Recommendation. To achieve the best balance of strength, proportion, and longevity, we recommend:

• Posts: 145 × 145 mm (minimum 140 × 140 mm)
• Headers / Beams: 145 × 145 mm, increase for longer spans
• Rafters: 60 × 140 mm minimum

Natural movement is part of oak’s character. By using the right dimensions, solid joinery, and professional fixing methods, you can ensure your oak pergola remains both structurally secure and visually stunning for years to come.