Priya Desai
Why Pegs and Wedges Still Rule
There is something deeply satisfying about a timber frame held together by nothing but shaped oak. No bolts, no plates, no hidden straps. Just seasoned pegs driven through draw-bored mortise and tenon joints, and wedges tapped into sawn slots to lock everything tight. It is a system that has served British carpenters for the better part of a thousand years, and it remains the benchmark for authenticity in period property restoration.
The reason is simple: oak moves. It swells and shrinks with the seasons, and a frame that is pinned with rigid metal fixings will eventually fight itself. A peg, by contrast, is cut from the same material as the frame and behaves in sympathy with it. Get the pegs right and the frame will settle, tighten and quieten down over the first few years rather than rattling loose.
Choosing and Seasoning the Peg Timber
Traditionally, pegs are riven from straight-grained oak rather than sawn. Splitting along the grain follows the natural fibres and gives you a peg that will not shear off at a hidden weakness. Look for a clean billet of air-dried oak, ideally with the growth rings running roughly parallel to the length. A froe and a wooden mallet will open it down; a drawknife and shave horse then bring it to shape.
Pegs should be dry — drier than the frame itself. As the peg takes on moisture from the surrounding timber, it swells and grips all the harder. Around 12 to 15 per cent moisture content is a sensible target for interior frames, lower still for exposed work.
- Diameter: typically 25mm to 32mm for a domestic frame, up to 38mm on heavy principal trusses.
- Length: 50mm to 75mm longer than the total thickness of the jointed members.
- Shape: tapered along the last third, with a blunt, slightly domed head and a chamfered tip.
- Grain: straight, riven, no knots, no short grain running out at the point.
A light taper matters more than most beginners expect. The peg should enter easily for two-thirds of its length and then bite firmly as it is driven home. If it needs a sledgehammer from the first blow, the hole is undersized and you risk splitting the tenon.
Draw-Boring: Offsetting the Holes
Draw-boring is the quiet genius of the traditional joint. The hole through the mortised timber is bored first, then the tenon is inserted and the hole through it is bored from the same side, but deliberately offset by 3mm to 5mm towards the shoulder of the tenon. When the peg is driven through, it catches the offset and pulls the tenon hard into the mortise. The joint closes up, the shoulders seat, and the frame tightens as if clamped.
The offset direction matters and it is easy to get wrong. Always offset the tenon hole towards the shoulder — that is, so the peg pulls the tenon deeper into the mortise rather than pushing it out. Mark the shoulders and the face side of every member before you begin boring, and keep the auger square to the face. A brace and bit or a modern auger in a low-speed drill both work; what matters is a clean, perpendicular hole with no tear-out at the entry.
Bore the mortise hole slightly undersized relative to the peg — around 1mm smaller — so the peg has to compress on its way through. Withdraw the tenon, clean the swarf, and dry-fit before you commit to a final assembly.
Cutting Wedges That Actually Hold
Wedges are the other half of the story. Where a tenon passes right through a post, the protruding end is sawn with a slot and two wedges are driven in from opposite sides to splay the tenon and lock it permanently. Cut them from the same riven oak, with a taper of roughly one in eight and a length matching the tenon thickness. Too steep a taper and the wedge backs out; too shallow and it will not bite.
- Cut the tenon slot in line with the grain of the post, never across it.
- Keep the wedges slightly thicker than the slot is wide so they compress as they enter.
- Drive them alternately, a few taps each, so the tenon splays evenly and does not split.
- Leave 20mm or so protruding, then cut flush once the frame is finally settled — or leave it proud as a traditional detail.
For blind mortises, wedges are used differently: the tenon is slotted and wedged before insertion, so the wedge splays inside the mortise as the joint is driven home. This takes practice and a little courage, but it makes an exceptionally stiff connection.
Tools, Tolerances and Preparation
You do not need a workshop full of machinery. A sharp auger, a wooden mallet, a bevel, a marking gauge, a tenon saw, chisels and a drawknife will see you through most of a frame. The critical tolerances are modest by modern standards but must be consistent: mortises a millimetre or two wider than the tenon for the peg to do its work, shoulders cut clean and square, and every peg hole bored true.
Label every joint as you cut it. Chisel a number or letter on the face of each member at the joint and record it in a notebook or on a cutting list. A frame trial-assembled on the ground will come apart and go back together far more readily if every piece is marked, and you will thank yourself when you are balancing on scaffolding in February.
Assembling the Frame Without Metal
Work in bents or trusses where you can, assembling flat on trestles and pegging each joint before lifting. Drive pegs with a mallet rather than a steel hammer — a glancing blow from a hammer will bruise the oak and mushroom the peg head. Listen as you go: a hollow, ringing tap means the peg is not biting; a solid thud means it has found its seat.
Once the bents are standing and braced, the wall plates, purlins and common rafters follow. Peg as you go, checking diagonals with a tape or a set of rods at every stage, because a frame that is out of square at this point will only get worse as it is loaded. Leave the final wedges until the frame has had a week or two to settle, then drive them home and trim.
Done properly, a pegged oak frame will outlast the roof covering, the windows and probably a couple of generations of occupants. That is the quiet promise of traditional British carpentry, and it is one worth keeping alive.
