A trenail — also spelled treenail and known in Dutch as a *deutel* — is a short hardwood peg, usually of oak, driven through a mortise-and-tenon joint to lock it together. It runs across the joint so the tenon cannot slip out of the mortise. The trenail is a fundamental element of traditional carpentry and is still used in restoration work and in handcrafted timber buildings today.
How it works
The trenail passes through both the cheeks of the mortise and the tenon via a single drilled hole. If the holes are offset by one or two millimetres before the peg is driven, the trenail draws the joint together: as it is hammered in, the tenon is pulled tight against the back of the mortise. This technique is known as draw-boring.
A trenailed joint has some unusual properties:
- Self-clamping — works without glue or metal fasteners
- Demountable — the peg can be drilled or driven out to undo the joint
- Works with the timber — moisture-driven swelling and shrinkage keep the joint snug
- Fully reusable — important when historic timber is being reused
Where they are used
Trenails were — and still are — used in nearly every traditional timber structure:
- Bents and roof structures — connecting post, tie beam and brace
- Timber-framed buildings — between studs and rails
- Wooden bridges and locks — particularly when reusing oak
- Wooden ships — historically in nearly all hull and deck framing
- Restoration of listed buildings — required for authentic repairs
Material
A trenail is traditionally made of rived (cleft) oak rather than sawn timber, so that the wood fibres run unbroken along its length and the peg stays strong. The typical shape is:
- Slightly tapered, with a square cross-section
- Length: about one-and-a-half times the diameter of the tenon
- Sometimes pre-treated with oil or tar for added durability
How it differs from other timber connectors
| Element | Function | Difference |
|---|---|---|
| Dowel | Round pin in round holes | Not wedge-shaped; mainly aligns parts |
| Mortise and tenon | The whole joint | Includes the mortise and the tenon, not the peg |
| Trenail | Peg holding the tenon | Added component running across the joint |
| Cotter pin | Metal split pin | Plays a similar locking role in metal assemblies |
Heritage value
For building archaeologists, the presence and position of trenails in a historic structure are valuable evidence of original carpentry practice. In restoration projects the count of trenails per joint, their position and their wood species are always recorded.
Related terms
- Mortise-and-tenon joint
- Dowel
- Timber joint
- Bent
- Roof structure
- Restoration
Want to learn more about construction terms? Visit the knowledge base at Fred’s DIY Plans.