May 16, 2017 by Scott - Saws on Skates
We build our DIY furniture projects with wood, but do we really understand how wood works? If we build without taking into consideration the seasonal movement of wood our projects will crack and split. If we build using proper techniques our projects will become family heirlooms. Today I’m sharing how you can be a better builder, prevent your wood furniture from cracking and ensure your projects will last a lifetime.
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A few weeks ago I shared my 9 Pocket Hole Mistakes You Don’t Want to Make. I received lots of feedback that the information was useful. By the way, thank you for the feedback – I love hearing from you! In addition to discussing possible pocket hole mistakes I also touched on wood being a living thing and that seasonal changes cause movement. From the feedback I could tell I needed to dive a little deeper into the movement of wood, what causes wood to crack and share some DIY furniture construction tips.
Wood is a Living Thing
Wood is a living thing. Wait, did I just say wood is a living thing?! Well, not exactly living, but it is hygroscopic. Wait, did I just say hygroscopic – what’s that? Simply put, hygroscopic means it absorbs moisture from the air. Wood is constantly absorbing and giving off moisture, just as it did when it was a tree growing in the ground.
The grain or fibers of wood run the length of a board. Before a tree becomes a piece of lumber, the tree’s roots would drink water and that water traveled through these fibers. The best way to think of those wood fibers is like a box of drinking straws. When the tree becomes a piece of lumber it still “drinks” water. Instead of drinking water through the roots, water is absorbed through the ends of those drinking straws or the end grain of the wood.
When it’s more humid the drinking straws, ummm wood fibers, absorb moisture, so the wood swells and gets bigger. When it’s less humid the wood releases moisture, so it shrinks and gets smaller. If you build a furniture project when it’s humid, you may notice when the air is drier that some of the joints don’t fit as well as it did when there was more moisture in the air.
Why Wood Cracks
The continuous swelling and shrinking movement of wood can cause some issues. When it’s more humid, the moisture in the air is absorbed by those drinking straws or wood fibers and the wood swells. The swelling occurs the most across the width. This swelling actually has enough force to pull joints apart. Worse yet, if the wood is joined improperly it can cause the wood to crack and split.
When it’s less humid, moisture is released by those drinking straws or wood fibers and the wood shrinks. The shrinking occurs the most across the width of the board. This shrinking actually has enough force to pull joints apart. Worse yet, if the wood is joined improperly it can cause the wood to crack and split.
As DIYers we need to understand and account for this continuous swelling and shrinking movement of wood in order to create furniture projects that will last. Part of accounting for that movement means understanding some ground rules when working with wood.
1. We cannot stop wood from moving. As I mentioned earlier instead of drinking water through the roots, moisture is absorbed through the ends of those drinking straws or the end grain of the wood. There is no way to stop this process. It’s a fact that we have to understand and accept. There are ways to reduce moisture absorption. You could apply a sealer like shellac to the end grain of the wood. Again, this will reduce moisture absorption, but not stop it.
2. We cannot limit the movement of wood. Trying to limit or prevent wood from moving will make wood CRACK! Let’s say you make a frame from 2×4’s and a panel to fit within that frame from 1×4’s. To attach the panel to the frame you use your Kreg Jig to drill pocket holes around the edges of the panel. You apply glue to the edges of the panel, insert it into the frame and attach it to the frame. This sounds OK, right? Wrong! This assembly limits or prevents the panel from moving.
Let’s take a closer look at our example from above. The panel we described above is “trapped” within the frame and doesn’t allow for the normal swelling and shrinking movement of wood. Remember, moisture will be absorbed through the end grain of the panel. As the moisture is absorbed, the 1×4’s of the panel will expand side to side. Because the panel is attached to the frame with pocket screws, the sideways expansion could crack the panel. It could also push the joints of the frame apart leaving a gap between the rails and stiles.
Joints Prone to Cracking
Now that we understand wood movement and why wood cracks, it’s important when building or designing DIY furniture to recognize the types of joints prone to cracking. Here are four popular joints that could crack over time.
A cross-grain joint, is a joint where wood is attached perpendicular to each other. I’m working on a new project that needs a brace for a drawer slide to run front to back on a side panel. If I were to glue and screw a 1×4 brace on the side panel the assembly would move in different directions. The side would move side to side while the brace would move top to bottom. This dissimilar movement could crack the wood over time.
It’s best to avoid a cross-grain joint, but if you can’t, don’t glue the joint. This will at least allow a little movement. For my project, it’s a little more time consuming, but what I’m going to do is glue up a brace from several pieces of wood to match the direction of the wood on the side. This way the side and the brace will move in the same direction.
When we talked about understanding wood above, we used a panel as an example. In that example we showed how this assembly could crack. The best way to handle a panel is to create a floating panel. With a floating panel, a dado or groove would be cut into the frame and the panel would float within this groove. This would allow the panel and frame to shrink and swell independent of each other.
A second, less ideal situation would be to use a plywood panel. Plywood is made by gluing many thin layers of wood cross-grain to each other. Wait, didn’t we just learn cross-grain joints are prone to cracking? You’re right, we did, but this joint is a little different. Because the layers of plywood are so thin the moisture absorption is almost nonexistent. Plywood hardly moves or doesn’t move at all compared to a piece of lumber.
Earlier, I described a frame and panel example. The frame had a panel made with 1x4s and attached to the frame with pocket screws. To reduce the chances of cracking you could replace the 1×4 panel with a plywood panel. The plywood panel would virtually eliminate the side to side movement, but movement would still come from the frame rails. This assembly has a lesser chance of cracking, but it’s still a possibility.
Attaching tops to a furniture project is a bit of a challenge. I haven’t had a table top crack yet, but I’m guilty of improperly attaching a top to the base of a furniture piece.
Let’s first look at the improper way to attach a table top, so we can understand what we should do instead. In my stool project I drilled pocket holes through the apron, then used pocket screws to attach the stool top. This is WRONG! The stool base attached to the stool top prevents the top from expanding and shrinking. Over time the top could crack or the top could have enough force to push or pull the joints of the base apart.
A better option for attaching furniture tops to bases is using table top fasteners. Table top fasteners allow the furniture tops to move freely. To install the table top fastener, first cut a slot in the apron. Then the table top fastener is inserted into the slot and attached to the table top with a screw.
You can DIY your own table top fasteners using a plate joiner or biscuit joiner. First cut a biscuit slot into a piece of wood like a 1×3, then drill a countersink hole and cut the piece. NOTE: The pic below is just a demonstration. It’s actually easier to cut several biscuit slots in the side of board (rather than the end), drill a countersink hole behind each slot and then cut into individual pieces.
Next cut several biscuit slots around the inside aprons of the base. To attach, slide a biscuit into the apron, then place a DIY table top fastener over the biscuit and attach to the top with a screw. Just like the store bought table top fasteners, your DIY table top fasteners allows the tabletop to move freely.
If it isn’t feasible to attach your table top to the base by the aprons you can do what I did for the wine cabinet and apothecary cabinet. I drilled elongated holes in the top braces and attached the tops with washers and screws. The elongated holes allow the top to move freely and the washers keep the screws from falling in the holes.
Breadboard ends dress up the ends of table tops, but this is a cross grain situation and we already know this type of joint is prone to cracking. When joining breadboard ends with pocket holes it’s usually just a matter of time before it will crack.
In order for us to be better DIY builders it’s critical we understand how wood works and take into consideration the seasonal movement of wood. Building with proper techniques will reduce the chances of our furniture from cracking and ensure our projects will last a lifetime.
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