Do They Know About Shrinkage?
Article - July 25, 2011
If you’ve been involved with woodworking in any capacity at all, you have undoubetly heard that wood moves with changes in moisture. And if you delved a little deeper into the topic you found out that the movement primarily occurs across the grain. You might have even discovered that if you don’t allow for wood movement in a project, you could be in for some serious structural problems. But I can tell you from personal experience, its not until you see this stuff first-hand that the reality truly sinks in. If you are unfortunate enough to see this occur on your own projects, my condolences. I bet you’ll never do that again! But if you’re fairly new to the craft, you are just beginning the fabulous journey of learning from your mistakes. So perhaps, I can save you some trouble by covering two examples of wood movement I recently came across.
Solid Top + Frame = No Joy
The first comes in the form of a call for help from Tasha. Her email described a beautiful dining table that continues to crack on her. The project was initially completed in a cold/damp garage and was brought into her home which was heated by wood fire. As many of you no doubt know, a house warmed by wood fire is DRY DRY DRY. So when she first brought her table in, a crack quickly developed. She repaired the table and hoped for the best. Unfortunately, the following Winter rolled around and a new crack became evident. Obviously she wants to fix this table for good and asked for my advice.
As you can see above, the first picture doesn’t give us much information. A crack is a crack. But it’s the second picture that tells the tale! Notice any potential problems with this design? While it certainly looks beautiful, the solid wood frame that trims out the perimeter of the table is actually its “Achilles’ Heel”. The strips running along the long grain of the top are no problem at all, but the pieces that run cross-grain are essentially restricting the movement of the top. So as the top expands and contracts with the changes in humidity, the cross-grain strips are fighting it every step of the way. Essentially, the integrity of the table is compromised and the pressure is released at the weakest link in the chain, which in this case looks like a glue joint.
So the only true fix for this table is to remove the trim and glue the top back together. The top must be allowed to move freely with no major restrictions along its width from either the trim or the base. Now if Tasha absolutely insisted on this design and this exact look, she would have no choice but to make a new top using plywood instead of solid boards. Thanks Tasha for allowing me to use the pictures and best of luck re-doing the table.
The Breadboard End
You might be thinking at this point, “Marc’s a ninny! I know I have seen solid table tops with boards going across the grain at each end of the table!”. And you would be correct, I am indeed a ninny! But the thing you are referring to is known as a “breadboard end” and this is a very special (and clever) woodworking element. It gives you the beauty and stabilization that a cross-grain board can provide, but it also allows the primary wood of the table top to move freely. And that brings us to my second example.
Several months ago I took a class on a Greene & Greene Coffee Table at the William Ng School. The table features stylish breadboard ends. To allow for movement, the breadboard is attached with screws that reside in slotted holes. This holds the board firmly to the end grain of the top, while allowing the top to move across the grain if need be. On the very ends, a decorative ebony spline is added primarily for looks. You can see here what the spline slot actually looks like. The long shallower side receives the spline with glue. The breadboard portion of the slot is cut deeper and the spline receives absolutely no glue on that side to allow for expansion and contraction.
Before leaving Anaheim, Ca, my splines were perfect! I knew the migration from California to Arizona would cause the top to shrink but when you’re taking a 5-day class, you don’t usually have time to make the appropriate calculations and adjustments. So I simply built my project to specifications and hoped for the best. Well, here’s a close up of the spline today. You can see that not only did the top shrink enough to take up the allotted space in the deeper slot, it managed to shrink even further causing the spline to angle out slightly. Normally, these splines sit proud of the surface by about 1/8″. They are NOT supposed to be recessed at all. But hey, the good news is if I ever move back to California, my table is going to be absolutely tip top!
Just for kicks, I decided to use the Woodshop Widget to calculate just how much the table top shrunk. At this point, I can only estimate the relative humidity at that time, so let’s say it was about 70% in Anaheim and 20% in Phoenix. For some help estimating relative humidity, check out this handy chart. And according to the calculator, I should have expected the top to shrink at least .5″. And that, my friends, is what I call SIGNIFICANT wood movement. That certainly explains what I am seeing with my ebony splines, doesn’t it?
Remember, wood is a dynamic medium. A board that is flat today may not be tomorrow. A solid top with a frame will most likely self-destruct. And a coffee table built in Anaheim will most definitely shrink after a trip to the desert. Do they know about shrinkage?!?! (Seinfeld fans will get that one).