6×6 vs 8×8 Cedar Fence Posts: Mortising for Rails

Fence builder asks if 6×6 cedar posts are structurally sound when mortised for two 2×4 rails (offset for pickets), or if 8×8 is needed.

6×6 Is Adequate

A 6×6 post has 5.5" actual dimension. Two mortises (1.75" deep each) centered leave ~2" solid core between them—sufficient for residential fence loads. Offset for pickets is fine; centering is slightly stronger but not required.

Best Practices

• Use through-mortise (open top) or stopped mortise with tight fit.
• No fasteners needed—mortise joint is stronger than screws/nails.
• Cedar is naturally rot-resistant—ideal for ground contact (set on gravel, not concrete).
• Post spacing: 6–8 ft max.
• Embed depth: ⅓ post length (32" for 8 ft post) in gravel/concrete.

When to Use 8×8

• Gate posts (heavy hinge loads).
• Corner/end posts with high wind exposure.
• Aesthetic preference for massive look.

For standard line posts, 6×6 saves money, looks proportional, and performs.

Expert: Mark Harshman | www.prestoexperts.com/expert/markland