Purlin Dovetail (桁檩燕尾榫): Purlins Joined End to End
Purlin dovetail · 檩头燕尾榫
Purlins run the length of a Chinese roof in pieces, meeting end to end over each beam.
To stop them pulling apart, one end carries a dovetail that drops into the next.
- Chinese
- 桁檩燕尾榫 · héng lǐn yànwěi sǔn
- Joins
- Purlin to purlin, over a beam head
- Found in
- Timber-frame roofs
- Signature parts
- Dovetail 3/10 D long and wide · D/2 high
- Related
- Sleeved dovetail
How it works
End to end over a beam
The purlin ends meet in the purlin bowl on the head of a beam.
A dovetail
A building-conservation guide gives the dovetail’s length and width as three tenths of the purlin diameter and its height as half the diameter, narrowing towards the root by one tenth to one and a half tenths of its length.
In the model
The beam head and its “elephant-nose” notch are left out; only the two purlin ends are shown.
How it's made
Step 1Over a beam
Purlins meet end to end over a beam.
Step 2Dovetail size
3/10 of the diameter long and wide.
Step 3Height
Half the diameter high.
Step 4Socket
A matching socket, open on top.
Step 5Drop in
Drop it in from above.
Step 6Locked
The purlins cannot pull apart.
Variations
Tie-beam dovetails
Tie beams and lintels into columns use similar dovetails.
Can you CNC this?
Possible, with design or setup changes
The dovetail and socket are cut from above, which suits 3 axes, but on a round purlin the part needs a cradle, and the tenon on the end may need the part upright or a side setup.
General note: a round cutter cannot leave a sharp inside corner, so every recess keeps the tool radius. Round the mating edges or add dogbone reliefs. Sloping faces, undercuts and curves may need special bits, extra setups or more axes.
Free STL downloads (one file per part)
- Purlin with socket 带燕尾口的檩 · henglin-yanwei-purlin-socket.stl
- Purlin with dovetail 带燕尾榫的檩 · henglin-yanwei-purlin-dovetail.stl
Proportions are illustrative, not measured dimensions. The files are unitless: scale them to suit and check fits and allowances before cutting. Each file is a closed (watertight) solid with its mortises and slots already cut, exported in its assembled position, ready to import into CAM or modelling software. Licence: CC BY 4.0 (credit: Chinese Joinery).
Related joints
Dovetail with Sleeve Shoulder
xiùjiān yànwěi sǔnA beam end with a dovetail and a short sleeve shoulder drops into a socket in the column head and can’t be pulled out.
Clasping-Head Tenon
gūtóu sǔnThe beam drops into a slot across the column head and runs on past it; the head beyond “clasps” the column.
Hidden Dowel
àn xiāoTwo lengths meet end to end and a square pin hidden inside the joint keeps them in line: nothing shows outside.
Questions
What is a purlin dovetail?
A dovetail locking two purlin ends together over a beam.
How big is it?
A conservation guide gives 3/10 of the purlin diameter long and wide, half the diameter high.
Why a dovetail?
So the purlins cannot pull apart along the roof.