Are Biscuits Strong Enough?
Article - February 9, 2010
This question comes from David who asks:
I’m just getting started in woodworking. I don’t have many tools, and the ones I do have are Ryobi (fairly cheap) but I like them. Anyway, I saw a video on youtube of this very nice oak chest, but the craftsman used mostly biscuits to join almost everything, I wouldn’t think that biscuits would be strong enough to last a lifetime. Am I wrong? If I wanted to join two boards together, would a tongue and groove be more solid in the long run? What do you think?
And my response:
– Biscuits slots can be a little tight or a little loose, depending on your machine and how steady your hand is.
– The biscuit doesn’t penetrate very far into the joining workpieces: a #20 biscuit will penetrate less than 1/2″.
– The grain of the biscuits runs on a 45 degree angle. Ideally, the grain would run perpendicular to the joint.
Despite their shortcomings, biscuits are still very handy for alignment, especially when doing big tabletop glueups. And I honestly don’t think they are a bad option for light-duty pieces either. But in most cases where biscuits are being used, its the glue that’s doing all the holding. Lets look at the two most common areas where you could use a biscuit, with respect to the grain direction of the adjoining pieces.
First, there’s long-grain to long-grain (a tabletop glueup is a good example of this). Some folks (including me) may use biscuits for alignment or “reinforcement”, but the truth is they do very little to strengthen the joint. In this long-grain situation, its really the glue that’s doing all the work. As you have heard me and others say many times, the glue joint is stronger than the wood itself. So adding a series of shallow biscuits isn’t really going to make the joint much stronger. But if they help you align the boards, I see no harm in using them.
Now the second situation is end-grain to long-grain. Think of a rail meeting a stile or an apron joining a table leg. Whenever end-grain is involved, the joint will be inherently weak using glue alone. So we absolutely need some form of reinforcement. Traditionally, this is the territory of the King of Woodworking Joints: the mortise and tenon. The real magic of the mortise and tenon joint is that it takes this end grain to long-grain union and converts it to long-grain to long-grain, simply by inserting one piece into the other. As a result, the joint has a great deal of glue surface and strength. Make no mistake about it: the biscuit joint is no substitute for the mortise and tenon, or its little brother, Prince Tongue and Groove! All three of the biscuits weaknesses listed above are the reasons for this.
So in case you couldn’t already tell, I would never use biscuits as a primary form of joinery, especially when there are end-grain to long-grain joints involved. But I do suppose for light-duty pieces, they would be acceptable. If the maker of that oak chest used biscuits in a bunch of long-grain joints, it would probably be a reasonably sturdy piece. But if the biscuits were used for end-grain joinery, I wouldn’t recommend moving that chest around too much. If I were you, I would definitely opt for tongue and groove or mortise and tenon joints. Good luck!
Oddly enough, as I wrote this, I noticed Tom Iovino from Tom’s Workbench put up a similary-themed blog post in the community. Check it out!