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I was reading "Illustrated Cabinetmaking" and came across this: "Where a horizontal board tops an upright, the biscuits should be offset". It does not explain why. Does anyone know?

The next sentence says: "Likewise, biscuits used in joining a shelf to an upright should be located below center to increase the shelf's resistance to toploading". Does anyone know what toploading is?

Here's the illustration in the book for both of the above:

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    I have a guess about what the second one is (toploading). It adds more wood sitting on top of the biscuit, thus increasing the shelf's ability to support weight. Of course this assumes the biscuit is stronger than the thickness of the wood. – Adirondack Jim Jun 21 '20 at 16:13
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    I'd love to be proven wrong, but this feels a bit like voodoo to me. – Aloysius Defenestrate Jun 21 '20 at 18:01
  • In the shelf the off-centre position makes some sense, not sure about on top. I can't help but feel positioning centrally gives the best likelihood of resisting all the possible forces the cabinet might experience in use. I very much doubt the author did any testing on this, just a theory they came up with (or exrapolated more widely from a specific case). – Graphus Jun 22 '20 at 6:12
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    Maybe the offset is rooted in the possibility that it's better to know when you've accidentally flipped a shelf before cutting biscuits on the other end... (A biscuit that looks centered probably isn't exactly centered, and this could lead to micro misalignments.) – Aloysius Defenestrate Jun 23 '20 at 3:07
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    @AloysiusDefenestrate I suppose that could be it, but every guide to biscuit joinery I've read was clear that you are to centre the mortise for the biscuit. This can't be harder than machining or carving a mortise and matching tenon. Even if you are a 1/16th off, this is wood; fix it in the finish! – jdv Jun 23 '20 at 18:36
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I have never encountered any rules of thumb about using biscuits, but the diagram makes sense. In the top connection the biscuit will transfer stress resulting from any side to side movement of the cabinet. All that stress is resisted by the small width of the remaining shelf between the biscuit and the outside edge to the left of it. The more wood there the better. Similarly, any weight placed on the lower shelf can only be resisted by the shelf wood above the biscuit.

of course, both connection details could be improved by creating a rabbet joint at the top and inserting the lower shelf into a dado in the vertical member. In this way the entire thickness of the board is being used to address the stresses.

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  • By "side to side", I think you mean racking. If so, then wouldn't forces be on both sides of the biscuit & thus it would be better to put it in the center? – Adirondack Jim Jun 21 '20 at 18:35
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    One side of the connection is compressing and the other side is tension. Think of the joint as a hinge connection. The joint must resist rotating forces which will affect the outside on one side or the other which is trying to sheer off the small width of wood. . – Ashlar♦ Jun 22 '20 at 0:21
  • Adirondack - you are viewing it from the picture on one side (right). On the top biscuit, the wide part is on the outside. Now picture the top and side on the right side of the cabinet. The offset would be with the wide part on the outside. The two top biscuit are acting as a pair, each biscuit preventing racking from different direction. – Programmer66 Jun 23 '20 at 5:02
  • Ashlar, I'm afraid I don't understand what you're saying. I still think you're still referring to racking, but it's the tension/compression part that I don't get. If you think of the biscuit/joint as a hinge, then won't both forces be compression (one side compressing at the top and the other compressing the bottom). So, you have forces on both sides of the wood, so why not center the biscuit? – Adirondack Jim Jun 25 '20 at 15:30
  • On one side the side wall will want to push the the biscuit into the wood top shelf, and on the other it will push the small wood profile out away from the assembly. The only thing resisting this force is the small piece of wood outside of the biscuit on the top shelf. The layers of wood will have a tendency to shear off on the small wood section that remains on the top panel. The more wood that is on the outside, the better it will resist the force. There is plenty of wood on the inside face of the biscuit, not so on the outside. – Ashlar♦ Jun 26 '20 at 23:37
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I think that offsetting the biscuit in the first case just leaves a little extra meat between the biscuit and the end of the board. Imagine slicing 5/16” off the end of a board. The resulting piece would be incredibly weak because the grain runs through its thickness instead of along its length, right? You could snap it in two with hardly any effort. It’s the same situation for the top board in the vicinity of the biscuit when you cut a 1/8” biscuit slot into the face of a board 3/8” from the end. Moving that slot inward 1/8” allows just a bit more long-grain connection between the slot and the end, and makes the wood there a little bit stronger.

Offsetting the biscuit toward the bottom of a shelf seems a little more intuitive: you want more of the shelf’s thickness above the biscuit to provide extra resistance to whatever load is on the shelf. If you offset the biscuit in the other direction, toward the top, then all the weight would be carried by just 1/8” or so of wood, and that doesn’t seem good.

All that said, it’s hard to know how much stock to put in these ideas if the author didn’t cite a source or otherwise back up the claims. It might be fun to test these ideas out and see whether they really make a difference.

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  • Paging Matthias Wandel. Matthias Wandel to the white courtesy phone, please. – FreeMan Feb 16 at 16:30
  • @FreeMan Well thank you kind sir! I will take this as a recommend on who to watch on YT. – G Warner Feb 16 at 19:43
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It was an oversight that the author neglected to explain fully in Illustrated Cabinetmaking.

The racking issue aside, it's important to know different types of wood expand and contract at different rates along the grain and thickness (less so at the endgrain). The biscuit will swell up with the glue but as it dries and contracts it will pull the surrounding wood in with it making the imperfection visible. This segment from a video explains this with boards side by side. It's reasonable to expect that is why it is done for outside corners.

Toploading is exactly that. Loading from the top (as in washing machine), putting a downward force upon something, etc. The offset shelf places more wood above the biscuit lessening the chance of tear out (splitting) and the shelf falling.

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  • I just spotted your Comment, re. not cleaning up with water were you being tongue-in-cheek or was that intended as serious? – Graphus Feb 28 at 8:46
  • The reason I asked is that there are wildly divergent opinions on this (as there are on waiting for the glue to dry and then scraping it off FWIW). Using water is a perfectly valid option if done right, and I know numerous pros who clean up glue when wet with a damp rag. It's clear that the guys who campaign against it — and there are many, including some writing for magazine articles! — simply don't know the right way to do it. – Graphus Mar 1 at 17:58
  • @Graphus I edited out that whole paragraph since it really has nothing to do with the original question and deleted my comments to keep the accepted answer clean and concise :) and the glue debate was answered here woodworking.stackexchange.com/questions/1368/… – G Warner Mar 5 at 20:30

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