Trunnel

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Oak treenails for pinning a wooden structure together. A used one (front center) demonstrates permanent deformation from the forces that bore on it

A treenail, also trenail, trennel, or trunnel, is a wooden peg, pin, or dowel used to fasten pieces of wood together, especially in timber frames, covered bridges, wooden ship- and boat-building.[1] It is driven into a hole bored through two (or more) pieces of structural wood (mortise and tenon).

History and general use

Protruding treenails used in timber framing at Blaubeuren Abbey, Germany. c.1478

Treenails are extremely economical and readily available, making them a common fastener in early wooden construction,[2] with use of wood as a tenon traced back over 7,000 years.[3]

Due to their strength and rot resistance, black locust is a favorite treenail material in North American shipbuilding[4][5] and English oak in European,[6][7] while red oak is typically found in buildings. Traditionally treenails and pegs were made by splitting bolts of wood with a froe and shaping them with a drawknife on a shaving horse. They can also be made with a tine-former, a hollow metal tube with a flaring flange on one end and a sharp edge on the other, usually mounted by the flange atop a low bench called a driving stool. Each roughly-shaped bolt of wood, slightly longer than the tool, is placed above the sharp end of the pipe and driven with a wooden mallet, which avoids the cutting edge; the next billet drives the previous the rest of the way through, which falls through a hole in the bench.

Modern treenails are typically 1.25–1.5 inches (32–38 mm) in diameter and cut from a single piece of wood. When installed, their grain runs perpendicular to that of the receiving mortises, which adds structural strength. Hand whittled treenails retain rough facets, while those produced mechanically using a tine-former or parsed out of turned billets do not. The mortise is drilled 1⁄16 inch (1.6 mm) smaller than the treenail to create a tight fit and take advantage of friction in the mortise, with those used in shipbuilding swelling tight when wet, both preventing leaks and strengthening their hold. In cases where the treenail is 24 inches (61 cm) or longer, it should be shaped 1⁄8 inch (3.2 mm) smaller in its first half than the second. Its corresponding mortise is drilled clear through with a small auger, followed by a larger on its first half. Tapered treenails are made longer than their mortise, then driven till tight, with excess on either end trimmed off.[8] After trimming they may be split and wedged with a small piece of oak on the large end to hold them tightly in place.[9] As an alternative to the wedge, the treenail can receive a plug or a punch in its center that expands the entire circumference. While this method minimizes leakage into the wooden planking, plugs and punches are more likely to fall out in cold temperatures.

Unlike metal nails, treenails can not be removed (without great effort) or reused. Cycles of swelling and contracting help lock the mortise snugly. Failed treenails must be bored out and replaced with a larger fastener. In addition, treenails have the ability to move over time and retain structural integrity.

Uses

In timber framing

Treenails used in the Brown Bridge in Rutland County, Vermont (2003)

Early mortise and tenon trusses with spans of less than 30 feet (9 m) used treenail fasteners. When used in a truss, the connecting mortises are drilled off center such that when the treenail is inserted it creates a tighter joint. Because of the large number of treenails required in a truss, the treenails can be turned on a lathe with a head and a tapered end, often kept extra-long for the tightest fit. The bottom chord often requires 2–3 pegs and is the weakest part of the truss. Hence the treenail can not prevent failure in spans of over 30 feet (9 m). In cases where significant shrinkage may occur, it may be necessary to use iron U-straps or reinforcements.[10]

In ships

Treenail used to fasten planking to frames, with red lead paint for waterproofing, Qui Nhơn, Vietnam
Building the Naga Pelangi, a wooden junk rigged schooner. Fitting the first plank required aligning many treenails

Ancient shipbuilding used treenails to bind the boat together. They had the advantage of not giving rise to "nail-sickness", a term for decay accelerated and concentrated around metal fasteners. Increased water content causes wood to expand, so that treenails gripped the planks tighter as they absorbed water.[11] However, when the treenail was a different wood species from the planking, it usually caused rot. Treenails and iron nails were most common until the 1780s when copper nails over copper sheathing became more popular.[2] As late as the 1870s, merchant ships used treenails and iron bolts, while higher quality ships used copper and yellow metal bolts and dumps. In the 1870s, treenails were typically used in a ratio of four treenails to one bolt, although sometimes more bolts were used. In later corvettes, the ratio was changed to two treenails to one bolt.[12]

In railroads

Similar wooden treenail fastenings were used as alternatives to metal spikes to secure iron rail-support chairs to wooden sleepers (ties) in early Victorian railroads. Treenails were extensively used constructing railways in North England.[13]

In furniture

Example of treenails cut from a dowel rod. Some are fluted.

Treenails are typically referred to as "dowels" when used in furniture construction,[14] which may or may not involve tapering, and sometimes are found with decorative through-pinning of extended ends. It's common to combine dowels with wood glue for a stronger joint.[14] Woodworkers commonly use small or medium sized wooden mallets rather than metal hammers when inserting a dowel to avoid damaging it.[15] Dowels are sold as dowel-rods, which are long wooden sticks the woodworker cuts into dowels of the desired length.[15]

It's common for furniture dowels to be fluted, which means parallel grooves are cut running along the dowel. These grooves allow for excess wood glue to flow out of the hole when the dowel is hammered in,[15] so that the dowel can fit tightly in the hole without creating an air pocket at the end of the hole, which could prevent the dowel being hammered all the way in. Dowels are typically fluted by hand, but sometimes dowel rods can be bought already fluted.[15]

The use of dowels is popular among woodworkers who want an invisible fastener for their furniture, since dowels can be disguised by orienting its grain with the board's grain, and by smoothing and polishing the dowels surface after its hammered in.[15] They are also a popular choice for outdoor furniture, since they do not rust like nails or screws, and can resist water soaking into the joint if built properly.[15]

It's common to use dowels for non-fastening applications, such as making the pegs of a coat hanger.[14]

References

  1. Edwards, Jay Dearborn, and Nicolas Verton. A Creole lexicon architecture, landscape, people. Baton Rouge: Louisiana State University Press, 2004. Print. 237.
  2. Johnson, W. (1986-01-01). "Historical and present-day references concerning impact on wood". International Journal of Impact Engineering. 4 (3): 161–174. Bibcode:1986IJIE....4..161J. doi:10.1016/0734-743X(86)90003-5.
  3. Radkau, Joachim (2012). Wood: A History. Polity. ISBN 978-0745646886. Archaeologist have found traces of wood nails in the excavation of early Germanic sites.
  4. Specter, Peter (1996). Planking and Fastening. WoodenBoat Books. p. 66. ISBN 9780937822418.
  5. Spectre, Peter H. (1996). Planking and Fastening. WoodenBoat Books. ISBN 9780937822418.
  6. Christy, Wyville (1904). Practical Treatise on the Joints Made and Used by Builders in the Construction of Various Kinds of Engineering and Architectural Works ... C. Lockwood and son. p. 99.
  7. Christy, Wyville J. (1904). "Practical Treatise on the Joints Made and Used by Builders in the Construction of Various Kinds of Engineering and Architectural Works".
  8. Adams, J.R. (2013). A Maritime Archaeology of Ships: Innovation and Social Change in Late Medieval and Early Modern Europe. Oxbow Books. ISBN 978-1842172971.
  9. Curtis, W.H. (1919). The Elements of Wood Ship Construction. New York, NY: McGraw Hill Book Company.
  10. Nelson, Lee H. (1996). "Early Wooden Truss Connections vs. Wood Shrinkage: From Mortise-and-Tenon Joints to Bolted Connections". APT Bulletin. 27 (1/2): 11–23. doi:10.2307/1504495. JSTOR 1504495.
  11. Kettunen, P. O., Wood Structure and Properties. Uetikon-Zuerich: Trans Tech Publications, 2006. 377. Print.
  12. Thearle, Samuel James Pope (1876). Naval Architecture: A Treatise on Laying Off and Building Wood, Iron, and Composite Ships. W. Collins, Sons & Company. p. 232. wood architecture treenails.
  13. The Civil engineer & [and] architect's journal. Kent. 1841.
  14. "Dowel Explained". everything.explained.today. Retrieved 2026-10-29.
  15. Brothers, Baird (2023-07-10). "Dowels are Better Than Screws for Woodworking… and Here's Why". Baird Brothers Blog. Retrieved 2026-10-29.