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overview of joint

I am fixing a bench and came across the above mortise and tenon joint between the foot and the vertical support (2 actually, one on each support). Quite clearly, someone has messed with this joint in the past cutting down the tenon so that it is about 1-1/4” too short on its length and 1/2” too short on its depth (they also added a screw into the end grain of the tenon, presumably because the joint wasn’t strong enough after they cut down the tenon).

Obviously, I have to fix this in putting the bench back together, but I’m not sure if I should approach it as adding to the tenon or filling the mortise to make them match. Which is the better option, or is there some other way that I haven’t thought of?

mortise depth of mortise depth of tenon

Edit in response to comment asking about tenon thickness: The thickness of one tenon is what I’ve always thought is perfect: dry assembly requires no persuasion, the joint holds together by friction alone against gravity, and yet can be shaken apart with one good shake. The other tenon is 3 sheets of copy paper shy of that point (i.e., by placing three sheets of paper alongside the tenon I can get to the same degree of tightness).

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    Welcome to WSE. How snug is the fit between the mortise and tenon across the thickness of the tenon? IF it is snug you should have plenty of contact between the two pieces for a good glue job.
    – Ashlar ♦
    Commented Jul 5, 2020 at 3:20
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    Tenons shorter than the mortise are not just commonplace, they are the norm. It is far preferable to a mortise that fits the tenon exactly (no room for glue to be pushed into) or worse one that is even a tiny fraction too shallow of course. What matters for strength here is primarilyy the tightness of the tenon is its mortise. If the fit is tight enough that a good glue joint can form then you're fine really, just glue together with PVA. But reinstall the screw (and maybe add one more) for security if you like.
    – Graphus
    Commented Jul 5, 2020 at 7:49
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    *"One tenon is the perfect thickness (dry assembly without persuading, friction holds against gravity, and a single shake suffices to separate the two pieces)."8 That's not actually a perfect fit, that's loose — tenons should require being at least tapped home with the heel of a hand, but more commonly will require a mallet to seat them. For structural integrity I'd glue that with epoxy. The other one is way off obviously. You can add veneer or plane shavings to build out the tenon to thickness, but again you can use epoxy to glue it as it stands. Thicken the epoxy with wood dust if needed.
    – Graphus
    Commented Jul 6, 2020 at 8:40
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    Many of these comments should just be Answers so we can kibitz about them in context.
    – user5572
    Commented Jul 6, 2020 at 18:58
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    I'm a professional woodworker (joiner, making windows and doors). I can confirm that it absolutely is standard to make mortice & tenon joints with very little clearance, such that they require hammering and clamping to get them to fully close. You don't want there to be any gap, really. Certain glues with gap-filling properties like cascamite will tolerate gaps but it's still better to have a tight fit in my understanding.
    – WhatEvil
    Commented Aug 17, 2020 at 4:59

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...I’m not sure if I should approach it as adding to the tenon or filling the mortise to make them match. Which is the better option, or is there some other way that I haven’t thought of?

Adding to the tenon and filling the mortise are the same solution; either way, the glued up joint is the same, with a 1" tenon going into material that fills the 2" mortise. Instead, you need to decide whether you're okay with the existing tenon or want something larger. The alternative to what you've proposed is to remove the existing tenon, cut a mortise into the rail, and use a loose tenon to join the two sides of the joint.

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I would say that shallow depth is a lesser problem and personally would just ignore it. A narrow tenon is a bigger problem because the tenoned piece can cast (it probably won't or it would already). My solution would be to insert two dowels of an appropriate diameter (so they're snug in the mortise) inline with the tenon so that dowels touch the flanks of the mortise. It might be a tricky operation even with a drill press, so I would also consider padding the mortise from the flanks, which would allow positioning dowels away from the piece edges.

The mortised piece looks like a clamp, if that's the case then its only responsibility to keep tenoned piece flat. Dowels should fulfill the purpose, also a single screw in end grain is totally adequate (however still a questionable choice)

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  • "the tenoned piece can cast" What does "cast" mean in this context? That's a use I'm not familiar with.
    – FreeMan
    Commented Sep 15, 2020 at 12:21
  • It's when a part of a rail that doesn't have a tenon goes out of true, it's a synonym to "warped", or "cupped" (though it's not limited to just these types of distortion), a haunch on a tenon is usually there to avoid casting.
    – user9049
    Commented Sep 17, 2020 at 18:48

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