What is a haunched tenon (spatpen)?
A spatpen is a small extra tenon, often about 10 mm high, beside the main tenon of a mortise and tenon joint. It is found mainly in the corner joints of windows, doors and frames, on the outer side of the tenon. Where the main tenon is a good deal narrower than the timber, the spatpen partly fills the space next to it: like a small “lug” on the top of the tenon. In English this is called a haunch, and the joint a haunched tenon.
Function
- Resisting warping. A head or rail tends to twist or cup as moisture levels change. Because the haunch also engages in the timber of the stile, the end of the rail is held over a greater width.
- No open joint. Without a haunch, an open joint or open rebate would appear on the outside of the corner. The haunch fills the groove or rebate, so the corner stays closed.
- Keeping strength in the stile. The main tenon stays some distance from the end of the stile, so the timber above the mortise does not break out (the “bursting” of the wood, hence the Dutch name). The lower haunch still holds the full width.
Application
Haunches are mainly used in a closed mortise and tenon joint, so that there are as few joints and openings as possible for water to be drawn into. How to make one:
- Mark out the tenon. Determine the width of the main tenon; leave enough timber at the end of the stile above the mortise.
- Leave a haunch. On that side, do not saw away the full cheek, but leave a low strip, roughly as deep as the groove or rebate.
- Cut the mortise and recess. Chop the mortise in the stile and a shallow recess for the haunch.
- Fit and glue. The joint must fit tightly. For exterior work, use a waterproof wood glue and paint the end grain well.
Common mistake: making the haunch too high. Too little timber is then left in the stile above the mortise and the stile still splits when the joint is driven together or as the wood moves.
Related terms
- Wood joints
- Mortise and tenon joint
- Window frame
- Head rail
- Frame stile
- Door sill
- Transom panel
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