TL;DR – You do not have to alternate the growth rings when you build stuff. Failing to alternate the rings is not why your table top warped. Your top warped because of moisture exchange and lack of proper joinery methods.
I’ve been seeing a steady stream of posts on subreddits and facebook groups where people are asking why their top has warped (Yes, please continue to ask questions. That is why we are all here!). The common response is some variation of “you should have alternated the growth rings” or “cut wide boards into narrow boards”. Neither of these are the cause of the top warping. Now, if you feel like this is personal, do not worry, this post is not directed at you. If you alternate the growth rings feel free to keep doing so (you’re not required to, but you can keep doing so ;)). What I am arguing is that you do not need to alternate the growth rings and failing to do so is not why tops warp.
The reason the top warped was because of moisture exchange often combined with inadequate use of proper joinery. Alternating the growth rings and cutting up wide boards do not prevent a top from warping. That is a myth. Your shop teacher, like mine, was wrong. The orientation of the growth rings will influence what the top will look like after it warps, but the orientation of the growth rings does not prevent nor cause warp.
When the boards are glued together with the growth rings in the same direction the top will develop a cup or a crown along the entire width of that piece if it warps. If you alternate the growth rings the top will look like a washboard if it warps.
There are advantages and disadvantages to using both orientations. If you join boards with the growth rings in the same direction you have two things going for you. First, you can minimize the appearance of sap wood when they are oriented heart side up. Second, if it warps (heart side up) it will rise in the middle. In this orientation the edges will stay tight to the finish piece, in the case of it being a top, and you only need a single point secured to the middle to hold things flat. This can also be a downside in a situation where the board has nothing to help keep it flat, like in a multi-piece cutting board. On something like a cutting board, if it warps it will end up having a hump in the middle that will be noticeable when sitting on a counter.
If you alternate the growth rings, it will require more points to hold the top flat to prevent the washboard look. Additionally, there will be more effort in getting color and grain matches due to the prevalence of sap wood appearing on the show side of the piece. However, in something like a multi-piece cutting board, where there is nothing to keep the piece flat, you can use the potential for washboarding to your advantage. If it washboards, it will no longer be flat, but compared to the previous example, it will appear flatter when sitting on a counter.
With all of that being said, when it comes to tops, panels, boxes, chairs, etc keeping things flat is done with joinery. Breadboard ends, battens, apron buttons, etc are what keeps everything flat. Alternating or not alternating the growth rings does not mean things will or will not be flat. How things look should be the priority. Pick the best side of the board to be the show face. Orient the boards that get the effect you are after. Growth ring orientation is secondary. Looks should be the primary focus.
For example, when I built this chair (pictured above), the seat was made from two pieces. I joined them together with the growth rings going in the same direction and oriented heart side up. Of the two sides, the heart side looked better. Of these two boards, one was primarily green with a bit of white at the edge and the other was primarily white. Joining them together with the growth rings in the same direction let me color match them fairly easily. Additionally, I could use the way wood moves to my advantage. If the seat was to warp it would get a cup along the width of the entire seat. With the heart side up, if it moves, the seat will rise in the middle and the edges will curl down. However, the legs and the stretchers will resist this. If the seat tries to cup it will be pushing into the undercarriage so the seat remains flat. The better choice here, in my opinion, for both looks and structure, was to join the boards with the growth rings going in the same direction.
Again, alternating growth rings is NOT required. Do not limit yourself by rigidly sticking to alternating the growth rings. This is a “rule” that is okay to break.
Similarly, you do not need to cut wide boards into narrow boards. Joinery will keep these boards flat as well. If you have a beautiful piece of 12″ wide stock, why cut it into three 4″ pieces just to glue it back together? The only thing you are doing is cutting up perfectly good wide stock. If you are cutting up wide boards it should be to remove defects not in attempt to keep things flat after a piece is built. The one time you may want to slice up and rejoin a wide board is in the milling process. If you have a rough sawn board that is extremely cupped, twisted, etc that may require ripping and rejoining. In that case, where the defect is severe enough that you cannot get it out by planning and jointing then ripping it and regluing it could remove enough of that tension that you can get it flat. However, that is a very different situation from cutting up a perfectly good board so it stays flat in a finished piece. If a board is flat in the milling stage then joinery will keep it flat when it is in your piece. There is no need to slice up wide boards unless you are trying to remove a defect or get it to the correct size.
With all of this being said, if you experience warp it is due to moisture exchange and exacerbated by the lack of proper joinery methods. First, all wood moves. Even in climate controlled homes and even if they are coated in thick layers of finish. This is why drawers can stick during the wet times of the year and move freely during the dry times. Or why a you may see a bare spot in the finish on the edge of a cabinet door during the dry season. Wood is always trying to be at equilibrium with the surrounding environment. That is what causes wood to warp. You can mitigate this through acclimation and joinery.
Acclimation happens in the building phase. I will focus on kiln dried lumber. You can build furniture with green lumber (fresh from the tree), but the steps to do so can be a bit different and I suspect that a majority of use are building things with kiln dried lumber. The first thing you need to do is let the lumber you buy acclimate to your shop. Bring it in, stack it and sticker it (see below). This allows for air to get to all sides of the boards. This helps to let the moisture release evenly from both faces. If it can escape evenly then the board may warp less. Then just let it sit. Get yourself a moister meter and take a reading. It doesn’t really matter what the number is. Come back a few days to a week later and take another reading. Then repeat as necessary until that number stabilizes. Once you are getting a consistent reading that board has acclimated to your shop environment. If you are buying furniture grade lumber from a hardwood dealer then this process likely will not take a long time. If you are buying 2x material from a home center then this process will most often take a while. Construction grade lumber is kiln dried, but it is not dried as much as furniture grade lumber so it is going to take longer to dump its excess moister. If a completed piece of furniture is going to warp it happens, usually, because the boards still had moister to get rid of when it was built.
The photos at the bottom of this paragraph show what happens to construction lumber when it gets rid of its excess moisture. This is a 2×12 board that was sitting around for about a week after it was purchased. It developed a severe twist. It was flat when I bought it, but it twisted as it acclimated to the shop environment. If that had been in a finished piece it would have caused a major problem. These photos are also a good example when wide boards should be cut down. The two boards underneath under the twisted one could be made flat through jointing and planing. However, they should be ripped and rejoined to remove the pith. Wood likes to move a lot around the pith of the tree. If it is removed though, you end up with incredibly stable quartersawn lumber. Again, these should be ripped to remove a defect. If they were clear with minimal warping ripping and regluing is unnecessary.
After acclimation, there is the matter of making and then keeping everything flat. The next part happens when you are milling the lumber. Try to remove equal amounts of material from each side of the board. Even on an acclimated board, the inner part of the board likely will have more moisture than the outside. When you mill the lumber you will start exposing the wetter core. By taking equal amounts from each side you reduce the likely hood of the board warping. Once a piece is milled your joinery will keep it flat. If you cannot get a part joined then stack and sticker it during the building processes. If it sits on the bench, it will take and give off moisture primarily from one side. You want an even exchange to help keep it flat. On something like a wide panel you can wrap it in plastic. If you slow the moister exchange then it makes it harder for a part to warp before you can get it secured in the finished piece. .
To keep all of your hard work flat you need to join it to something else is such away that it lets wood expand and contract with seasonal changes, but restrains it from moving in a way that you do not want it to. The method that is used essentially comes down to the design of the final piece. On a table it could be breadboard ends, battens, or apron buttons. On a dresser top it may be screws in elongated holes. In a door it could be a panel in a frame. Joinery is what will keep things flat. You may have a perfectly flat top now, but if you do not attach it to something it, in all likelihood, will no longer be flat in the future. For example, a top made from boards edge glued together or from a single slab and then attached to some hair pin legs likely will warp sometime down the road. There is nothing to keep it flat as time goes on. That should not dissuade you from building things in this style. The addition of a couple of hidden battens is a minimal amount of additional effort that is going to extend the life of your piece. You get the same look without the potential issues.
In summary, growth ring orientation and cutting wide boards into smaller ones is not the solution to or cause of a piece of furniture warping. These are “rules” that can be broken. If you are concerned about warping in furniture, pay attention to moister exchange and the application of joinery. This is what will keep furniture flat.
And because you should not just believe some jackass on the internet here is some further reading:
(Edit for clarity- This is not a works cited. Instead, these are links that contain information that I gloss over. Look at all of the furniture you can and try things for yourself. Don’t just take my word for it. )
Avoiding Cupped Panels – The Wood Whisperer https://thewoodwhisperer.com/articles/avoiding-cupped-panels/
Glue-ups and Grain Direction – The English Woodworker https://www.theenglishwoodworker.com/glue-ups-grain-direction/
Text Book Mistakes – Tage Frid, Fine Woodworking Magazine https://www.finewoodworking.com/1976/04/01/textbook-mistakes
When Good Wood Goes Bad – Wood Magazine https://www.woodmagazine.com/wood-supplies/lumber/when-good-wood-goes-bad\
Calculating For Wood Movement – Ed Pirnik, Fine Woodworking Magazine https://www.finewoodworking.com/2013/08/29/calculating-for-wood-movement
How Calculate Wood Shrinkage and Expansion – Popular Woodworking Magazine https://www.popularwoodworking.com/wp-content/uploads/2010/10/WoodMovement.pdf
Keeping Plank Doors Flat – Christian Becksvoort, Fine Woodworking Magazine https://www.finewoodworking.com/membership/pdf/23131/011145078.pdf
Matching For Color, Grain Pattern, and Figure – Stephan Woodworking https://www.stephanwoodworking.com/MatchingForColorGrainPatternAndFigure.htm
Matching Wood Grain – Wood Magazine https://www.woodmagazine.com/woodworking-tips/techniques/intermediate/matching-wood-grain
Michigan Woodworker, p4 https://michiganwoodworkersguild.com/wp-content/uploads/newsletters/JANUARY-2021-NEWSLETTER.pdf
Use Wide Boards Best Side Up – Christian Becksvoort https://www.finewoodworking.com/2010/09/30/use-wide-boards-best-side-up
PS – I have no affiliation with the links listed above.
Eric Meyer - Maker 