Why Choose Mortise & Tenon Over Metal Fixings?

When building an oak pergola, the jointing method you choose defines its strength and long-term performance. Some builders rely on metal fixings or stainless steel screws. However, traditional mortise and tenon joinery continues to outperform modern methods in both durability and craftsmanship.

The Strength of the Mortise & Tenon Joint

A mortise and tenon joint is a proven connection where a projecting tenon (male part) fits tightly into a mortise (female slot). This technique has been used for centuries because it creates a natural and balanced load path through the timber.

In a high-quality oak pergola, tenons are usually 25 mm thick, and in structural areas, they can reach 50 mm for extra load capacity. At Timber Solution, every tenon penetrates 75 mm into the oak beam, creating a deep and secure connection. As a result, the load spreads evenly, which minimizes stress and prevents premature movement at the joint.

Why We Use Kiln-Dried Oak Dowels

To lock the joint, Timber Solution uses kiln-dried oak dowels rather than metal fixings or glue. This choice is based on the natural behavior of green oak, which gradually shrinks as it dries.

Because kiln-dried dowels do not shrink, the surrounding green oak tightens around them over time. Consequently, the joint becomes even stronger. This creates a self-tightening compression fit that is nearly impossible to remove. The only way to replace a dowel is to re-drill the joint and insert a new one.

This natural locking process results in a bond stronger than any screw or adhesive, while also maintaining the timber’s flexibility and integrity.

Why Not Metal Fixings or Screws?

Some oak framers prefer stainless steel screws to attach braces to posts and eaves plates. At first, this seems secure. However, our yard tests show that as the oak dries and moves, metal fixings cannot adapt to timber shrinkage.

• Beams may twist or misalign.
• Gaps can appear around screw points.
• Joints may offset, affecting overall appearance.

Although the braces stay in place, this approach is not traditional joinery and can reduce the visual and structural quality of the oak frame.

On the images below, you can see the precision of our craftsmanship. Every beam features perfectly fitted mortise and tenon joints secured with oak dowels that protrude slightly through the beams, showcasing the strength and authenticity of traditional joinery.

Facts & Figures: Mortise & Tenon Joinery for Oak Pergolas

• Tenon thickness: 25–50 mm
• Tenon penetration: 75 mm
• Dowels: Kiln-dried oak, typically 16 mm in diameter
• Strength retention: Up to 30% higher than screw-fixed joints after one year
• Lifespan: Over 50 years with minimal maintenance

Conclusion

A well-made mortise and tenon joint offers unmatched strength, flexibility, and timeless design. By combining precisely fitted joints, kiln-dried oak dowels, and skilled craftsmanship, the structure naturally tightens and strengthens over time.

No metal fixing can replicate the enduring quality and traditional integrity of true mortise and tenon joinery.