Pinning Joints

Part of Woodworking

Wooden pegs transform a friction-fit joint into a mechanically locked connection that needs no glue, no metal, and can last centuries. Pinning is the original fastening system for everything from furniture to timber-frame barns. The techniques are simple but the details matter — a poorly placed peg splits the joint, while a well-placed one makes it indestructible.

Why Pin Joints

Pegged joints offer advantages that glue and metal fasteners cannot match:

• No glue required — the peg provides mechanical locking through wood-on-wood friction and geometry
• Works with green wood — glue fails on wet wood, but a dry peg in a green joint actually tightens as the frame seasons
• Disassembly possible — drive the peg out (or drill it out) and the joint separates for repair or relocation
• No metal needed — critical when nails and screws are unavailable
• Self-tightening — dry pegs in green frames swell to lock; drawbored pegs pull joints tight permanently
• Proven durability — medieval timber frames pegged with oak treenails still stand after 500+ years

Making Wooden Pegs

The peg must be stronger than the surrounding wood, straight-grained, and properly sized. A bad peg is worse than no peg.

Selecting Wood

Species	Suitability	Notes
Oak	Excellent	Dense, strong, splits cleanly for riving
Ash	Excellent	Tough, flexible, resists shearing
Hickory	Excellent	Strongest option, ideal for structural work
Maple	Good	Hard but can be brittle if grain runs out
Birch	Good	Fine-grained, good for furniture pegs
Pine/Spruce	Poor	Too soft, compresses under load

Riving Blanks

Riving (splitting) produces pegs with continuous grain — far stronger than sawn pegs where the grain may run diagonally and create a weak point.

1. Start with a straight-grained billet: No knots, no twist, no interlocked grain
2. Split with a froe or wedge: Halve the billet, halve each half, continue until you have square blanks slightly larger than the finished peg diameter
3. Check grain runout: Hold the blank to the light — grain lines should run straight from end to end. Discard any blank where grain exits the side.

Shaping Pegs

Method 1 — Knife and Dowel Plate

1. Whittle the blank to a rough octagon with a knife
2. Drive through a dowel plate (a steel plate with graduated holes) to achieve final round diameter
3. Taper one end slightly to ease insertion

Method 2 — Knife Only

1. Whittle the blank round by hand
2. Roll on a flat surface to check for high spots
3. Pare high spots until the peg rolls true

Method 3 — Rounder (Dowel Cutter)

A rounder is a block of hardwood or metal with a tapered hole and a cutting edge. Drive the blank through and it emerges as a cylinder. Faster than knife work for production quantities.

Tip

Make pegs slightly longer than needed — you can always trim flush after driving. Making them too short means you cannot seat them fully, and a peg that stops halfway is just a plug, not a fastener.

Drilling Through the Joint

Procedure

1. Assemble the joint fully — shoulder tight against the mortise face
2. Mark the peg hole location: Center it on the tenon width and at least 15 mm from any edge of the mortise
3. Select bit diameter: 1/3 to 1/2 of the tenon width (see sizing table below)
4. Drill straight through both mortise walls and the tenon in one pass
5. Use a guide block clamped to the surface for accuracy
6. Back up the exit side with scrap to prevent blowout

Peg Sizing

Tenon Width	Minimum Peg Diameter	Maximum Peg Diameter
20 mm	6 mm	10 mm
30 mm	10 mm	15 mm
50 mm	16 mm	25 mm
75 mm	20 mm	32 mm
100 mm+	25 mm	38 mm

Warning

Drilling too close to the edge of the tenon or mortise wall will split the wood when the peg is driven. Keep the hole center at least one peg diameter from any edge. For hardwood, 1.5 diameters is safer.

Dry Pegs in Green Wood

This technique exploits differential moisture to create a self-tightening joint.

How It Works

1. Build the frame from green (freshly cut) wood — moisture content 25-40%
2. Make the pegs from fully air-dried wood — moisture content 8-12%
3. Drive the dry pegs into the green frame
4. As the frame dries over weeks and months, the wood around the peg shrinks, clamping down on the peg
5. Meanwhile, the peg absorbs some moisture from the green wood, swelling slightly
6. The result is a permanent compression fit that tightens over time

Requirements

• Peg must be drier than the frame — the bigger the moisture differential, the stronger the lock
• Use species that shrink minimally along the grain (all wood shrinks much more across the grain than along it — the peg’s length barely changes, but its diameter swells)
• Fit the peg snug when driving — it should require firm mallet blows but not splitting force

Tip

Kiln-drying pegs over a fire (carefully, slowly, far from flames) drops their moisture content below 10%. These bone-dry pegs swell dramatically when they absorb moisture from the green frame, creating an extremely tight joint.

Drawboring

Drawboring is the pinnacle of pegged joinery. An intentional offset in the peg hole forces the joint tight without clamps, glue, or waiting.

The Principle

When the peg hole through the tenon is offset slightly toward the shoulder, the peg must bend as it passes through. This bending creates a pulling force that draws the tenon shoulder tight against the mortise face.

Step-by-Step

1. Assemble the joint dry — shoulder seated tight
2. Drill through the mortise walls and tenon in one pass — this creates perfectly aligned holes
3. Disassemble the joint — mark the tenon so you can identify which hole is which
4. Plug or mark the existing hole in the tenon for reference
5. Re-drill the tenon hole only, offset 1.5-2 mm toward the shoulder. The easiest method: mark the new center with an awl 1.5 mm closer to the shoulder, and drill a new hole at that mark. The new hole will partially overlap the old one.
6. Taper the peg tip — a slight taper helps the peg navigate the offset
7. Reassemble the joint (apply glue to the tenon if desired, but it is not required)
8. Drive the peg through — as it enters the offset tenon hole, it pushes the tenon forward, pulling the shoulder tight
9. Listen for the joint to seat — you will hear (and feel) the shoulder close the last fraction of a millimeter
10. Trim flush with a saw, then pare with a chisel

Critical Parameters

Parameter	Value	Why
Offset distance	1.5-2 mm	Less than 1 mm has no effect; more than 3 mm risks splitting the tenon
Offset direction	Toward the shoulder	Pulling the tenon deeper into the mortise
Peg taper	3-5 mm over the first 20 mm	Allows the peg to find the offset hole without jamming
Peg material	Straight-grained hardwood	Must flex without snapping

Warning

Do not exceed 2 mm offset in softwood or thin tenons. The peg must flex as it passes through the misaligned holes. Too much offset snaps the peg or splits the tenon. In hardwood with large tenons, you can push to 2.5 mm.

When to Use Square Pegs

Contrary to the idiom, square pegs in round holes actually work. And they grip better than round pegs.

Why Square Pegs Hold

A square peg driven into a round hole compresses the wood at four corners, creating continuous friction along four lines instead of a single curved surface. The corners dig into the softer hole wall and resist withdrawal.

Making Square Pegs

1. Rive a blank to the correct square size — each side should equal the hole diameter
2. Chamfer all four edges at the tip to help entry
3. Drive with a mallet — the peg deforms slightly as the corners seat

When to Use Them

• Timber framing: Traditional treenails are often square-section
• Joints under withdrawal load: Square pegs resist pulling out better than round ones
• When round pegs spin: A round peg in a loose hole can rotate; a square one cannot

Removing Broken Pegs

Pegs sometimes break during driving or need removal for joint repair.

Methods

1. Drill out: Use a bit slightly smaller than the peg. Center it carefully and drill through the broken peg. The remaining shell pries out with a narrow chisel.
2. Drive through: If the joint allows access from the other side, drive the broken peg through with a smaller dowel and mallet.
3. Burn out: For non-structural joints, heat a metal rod and burn through the peg. Messy and enlarges the hole — re-peg with a larger diameter.
4. Collapse and extract: Score the end grain of the broken peg with a chisel to collapse it inward, then pick out the fragments.

Tip

If you must re-peg a hole that has been enlarged by removal, do not just use a fatter peg. Instead, drill the hole to a clean larger size and use a properly fitted peg for that new diameter.

Treenail Construction for Timber Framing

A treenail (pronounced “trunnel”) is a large wooden peg used in timber framing, shipbuilding, and heavy construction.

Specifications

Property	Specification
Diameter	20-38 mm (typically 25 mm for house framing)
Length	1.5x the thickness of the combined joint members
Species	Oak, locust, or other dense hardwood
Grain	Dead straight, no runout, rived (not sawn)
Moisture	Air-dried to 10-15% for use in green frames

Production Process

1. Rive billets from a straight-grained log section
2. Square to rough dimensions with a hatchet
3. Round with a drawknife on a shaving horse, or drive through a large-diameter rounder
4. Taper one end for easy starting
5. Season for at least 3-6 months in a dry, ventilated space
6. Grade: Reject any treenail with knots, grain runout, checks, or twist

Driving Treenails

1. Pre-drill the hole 1 mm smaller than the treenail diameter
2. Start the tapered end into the hole
3. Drive with a heavy wooden mallet (never a metal hammer — it mushrooms the end)
4. Strike firmly and squarely — angled blows cause the treenail to bend inside the hole
5. Drive until the treenail protrudes equally from both sides (for through joints)
6. Trim and chamfer the protruding ends

Pinning Joints — At a Glance

Wooden pegs provide mechanical locking without glue or metal. Rive blanks from straight-grained hardwood and shape with a dowel plate or knife. Size pegs at 1/3 to 1/2 of tenon width and drill at least one peg diameter from any edge. For self-tightening joints, use dry pegs in green frames. For joints that pull themselves tight, use drawboring with a 1.5-2 mm offset toward the shoulder. Square pegs grip better than round ones in many applications. For timber framing, season oak treenails for months and drive with a wooden mallet. A well-pegged joint outlasts the tree it came from.