Why did my planed counter top cup?

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lostwarden

New User
Ben
Hey everybody, I'm in a bit of a perplexed situation. I have an 19" oak counter top that I recently planed flat as I could with a jack/power planer. To my surprise the board cup backwards in the opposite direction a day later.

It was rough cut lumber. I'd like to think I should get it flat again, but how would I get it not to cup?
 

CrealBilly

Jeff
Senior User
Hey everybody, I'm in a bit of a perplexed situation. I have an 19" oak counter top that I recently planed flat as I could with a jack/power planer. To my surprise the board cup backwards in the opposite direction a day later.

It was rough cut lumber. I'd like to think I should get it flat again, but how would I get it not to cup?
FredP will tell you how to do this but you may not like the answer.

Never do anything during the day that will keep you awake at night
 

ehpoole

Administrator
Ethan
Take a look at your plank's endgrain and you should be able to clearly see why it wants to cup. Only quarter-sawn planks have a high resistance to warping due to the orientation of it's endgrain.

When you look at your endgrain you will likely notice it has a cupped shape as well, so if it gains or loses moisture it will inevitably cup and take on a similar shape to the endgrain. But when you plane a wide or thick plank, and especially if you remove much more from one side than the other, you both expose interior wood to the ambient humidity, which may differ from the moisture content of the board's original interior and you also remove a lot of wood that may have been holding back some of the cupping, releasing some of the internal stresses.

When you plane a wide board it is a good practice to remove thickness equally from both sides to balance internal stresses and to balance internal variations in moisture content. It normally takes a much longer time for the wood at the interior to gain or lose moisture with changes in humidity, so it will naturally lag well behind in adapting to humidity changes in your shop...and then suddenly that interior wood becomes the exterior and it gains or loses moisture rapidly. It is also good practice, particularly if removing much thickness, to not fully plane it to final thickness in a single day. Instead remove perhaps 2/3rds on day one, then let the plank sit for a few days to several weeks until it has finished adapting to your shop environment (during which time cupping is not unusual), then evenly plane thickness off both sides (as best you can since cupping may force more off one side than another) until it is at your desired thickness. This leaves you with some room to accommodate cupping or warping during the planing process.

Fortunately, there is a very good chance that some, but likely not all, of the cupping will reduce over the next few days to several weeks as the plank finishes adapting to changes in moisture content. But this is a very good example of why your designs, when working with wide boards, need to allow for the natural movement of the wood with changes in humidity throughout the year as the forces involved can be many tons of pressure, enough to rip apart projects that try to resist that movement rather than allowing for it.

Thankfully the way wood moves is not a great mystery and is pretty well understood and predictable (provided it is a typical grain pattern, wild grain will always have an aura of mystery). There are many good books and some good web resources that discuss wood and its movement in detail and are well worth reading if you desire to work on large wooden projects. There are also online calculators that will help you calculate how much your wood species is likely to move for a given degree of swing in humidity (such as that between summer and winter).
 

zapdafish

Steve
Senior User
not sure if you finished it or not but you need to finish both sides the same way even if you cant see it. Otherwise moisture will enter and leave one side at a different rate and it wont ever reach a happy medium.
 

CrealBilly

Jeff
Senior User
Or you could... Run some saw kerfs down the length on one side and screw (not glue) cross brace down to work out the cup. Don't glue the cross braces or you run the risk of splitting the top over time. It still needs to move - its solid wood after all.

Never do anything during the day that will keep you awake at night
 

Charlie Buchanan

Charlie
Corporate Member
Ethan's explanation is excellent. I got into the habit of planing in several sessons and letting the boards rest between sessions stickered in the shop--both single boards and glued-up panels. Some cupping is inevitable with flat-sawn lumber but planing both sides equal passes and immediately stickering it for a few days between planing sessions will really help. This is true even if the lumber is thoroughly dry to start with.
 

Jeff

New User
Jeff
I have an 19" oak counter top that I recently planed flat as I could with a jack/power planer. To my surprise the board cup backwards in the opposite direction a day later.
1. Was the rough cut board air-dried or kiln dried to begin with?

2. Was the counter top in your shop when the cupping happened or did you move it into the house (ac, etc) and then it cupped?

3. Definitely finish both sides as suggested to equalize the moisture fluctuations.

Thanks to Ethan for a comprehensive explanation too. :thumbs_up
 
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