Planks (a light wood?) that make up the top of a small table are weather
warped. "Cupped" upward.
This isn't meant to be a centerpiece of the home, but it would be nice to
make it presentable again.
What's the usual repair recommended for such a condition?
Tools available: the usual drills and hand tools; table saw; router; belt
sander; palm sander; paint gun and compressor.
Replace the boards. And put them so if they cup they will cup with the
center up. Fasten in the center and keep painted/varnished.
Well, you could ....
a) plane down the boards to make them flat on the surface
b) if thin/narrow enough, bend them down and fasten
c) if not thin enough, remove and rip one or more kerfs on underside, put
glue in kerfs, bend flat.
Depends on the amount of cup and the edges of the table. My wife
inherited an attractive elm claw foot from her mom. It had a similar
problem and one of the glue joints had failed too. I removed it from
the legs, removed the edge trim, sawed through the failed glue joint
and glued it back together. Before gluing I took a thin pass through
a thickness planer to level the top out again. Came out looking
good. If you don't have a planer, search around for a friend or
school that does.
Very hard to say how I would address it without seeing a picture. Can
you post a pic somwhere? Be careful if you plan to try and address it
with a planer because depending on the defect the planer rollers can
sometimes just hold it flat while it planes it, then aftewards it
rools right back into it's curl and you have a nice flat but curled
I like to start this type of conversation by saying, "you're not happy
with it the way it is now, are you?" or "it doesn't work now, does it?".
I guess we can't hurt too bad.
I would remove the top. On a nice spring day I would lay it on the
grass with the humped side up. Let it sit there a day preferably with
some sunshine. The piece has probably gotten wet at some point and the
top dried quicker and pulled the warp. I am suggesting you try to
equalize the moisture throughout the piece. I'm thinking you will see
some substantial improvement after a day of absorbing ground moisture on
the shrunken side and air/sun drying on the top side. I would them give
it lots of weight on a flat table surface, not on the concrete patio or
garage floor. After a week, see what you have.
Table top looks like a U (not as drastic I hope). Top got wet while the
underside remained drier. When it dried out the compressed fibers
(caused be the dry side restricting even swelling of the wet side)
distort it... according to the books :)
As mentioned, a possible sollution is to wet it all down lay it on
stickers and put weight on it to dry.
The "proper" method for an antique is to router out the underside and
replace with hardboard. That leaves the original top surface while
providing a flat base for it. Something I probably would not have the
nerve to do.
Not necessarily ... and AAMOF, and depending upon the grain and the way
the wood was cut off the tree, it more likely to be just the opposite of
what you stated ... the concave side being the dry side, and the convex
the moist. :)
There are indeed some exception with certain grain patterns due to the
way the wood was cut from the log (plain sawn, rift sawn, etc.), or if
laminated, but mostly wood cups and checks parallel to the grain and to
the drier side.
Excellent example is the cupping of hardwood floors, which "cup"
(concave side up) when wet from the underneath ... dry on the top, wet
on bottom ... one of the main reasons why a moisture barrier is always
used _beneath_ a hardwood floor.
Another hole in your balloon ... leaving a board to dry out in the sun,
The concave side is drier from exposure to the sun and heat, and, once
again, the board generally cups toward the dry side.
Try this by laying a board on wet grass before the morning sun hits on
sunny day, check it out by noon. :)
Here is indeed, "according to the books":
Wood warmed by the sun experiences a virtual RH far below the ambient
RH. The surface dries faster than the rest of the lumber. This is why
cupping and checking often occur on decking boards; the top surface is
much drier than the rest of the board. Shrinkage of the top surface
commensurate with this dryness causes cupping and checking parallel to
Sorry but I believe what I said to still be right.
While grain of course will have effects the cupping will take place
towards the moist side (or away from the compression pressure might also
be a way to say it). Take a look at porches or decks, do you seen any
cupped down? Fave you ever found a table top that cupped down?
Your wood floor - is it cupping because it is moist underneath or
because of the mopping and drying taking place above? as for the board
on the lawn - don't know, would have to try it.
Your quote - Is it indeed the sun causing the surface to dry faster or
is it in fact that the problem is not visual until the surface dries?
I will explain it as one of the books due (please recall I said this was
according to the books - not neccessarily the truth - I believe much of
the things "known" are not the truth of what is happening, but only what
we as humans can guess at/or comprehend with our limited knowledge).
Wood cells normally want to swell and then shrink back to their normal
shape and size. When something restricts this the swelling continues on
uncompressed sides (i.e. the bottom of the board, or anywhere else it is
not wet/or drier). This forces the cells into more of an oval shape
instead of their original shape. When they shrink from drying this shape
is not shanged so the compressed sides of the cell (we will call it the
width) ends up smaller than it used to be. As this is repeated we end up
I can never recall seeing a board that was cupped in any direction
except on the side effected by changes in moisture.
Did you bother to even read the supporting evidence I provided
disproving your unequivocal contention, first above?
It is indeed "not necessarily the truth", as I clearly outlined above,
complete with a cite as to the an explanation for why your statement is
not necessarily the case.
By all means, offer a cite that supports your contention to the contrary.
Yes, and as a result, that attitude arguably insures that your knowledge
in this regard will remain limited.
The above is either so poorly worded as to be useless in defining your
contention, or the result of a total misunderstanding of the effects of
moisture and humidity on the dimensional instability of wood.
It's really impossible to tell which?
Again, too loosely worded to be of much use in defining the problem ...
and your admission that you have "never recalled" seeing that, is
certainly no proof of your argument.
Yes, it is indeed this _differential_ in moisture content between
opposite faces that causes the phenomenon of cupping, but not
necessarily for the reason you unequivocally stated first above.
Simply put, cupping is most often the result of the opposite (convex)
side being moist, and the cupped (concave) side being drier ... the
exact opposite of your unequivocal statement.
IOW, your understanding is mostly backwards of the actual effect of
moisture on wood, with the exceptions I noted above. You will certainly
want to do your homework, and provide some supporting evidence to the
contrary, if you want to continue a reasonable discussion of the issue.
Ok. Whatever you would like to make it. I have no need to argue.
I think my sentences were clear enough.
You also call into question whether or not I even undestand moisture -
and that I will not be able to learn (if I don't).
I do believe for some reason you feel offended and seem to have a chip -
but that is fine too.
You have decided what you will take for the answer - nothing I say or do
will change it.
Have you ever seen deck boards or a table top that was not attached?
Given restrictions a board will bow in the direction that is not
restricted. Given no restrictions the surface absorbing moisture will
expand and that will not result in a concaved surface.
Absolutely the underneath side. You mop it and water goes down inside
the cracks to lower and side sections of of the board. Those surfaces
stay wet longer than the top surface. Then the edges expand and they cut
up but the surface has long since dried because of exposure.
You are not thinking about all of the surfaces that have actually gotten
Just 2 mor cents...
Grain of course plays a part is how the reaction takes place, but as you
said "but mostly wood cups and checks parallel to the grain and to the
As my previous post metnions - never seen a floor/table/deck cup down.
But I don't want to argue it. As said - I am simply repeating it from
information I have read. I doubt anyone knows for sure the real reasons
for it all.
You don't want to argue because you were mistaken, charitably, either in
your wording, or your understanding.
And you are very mistaken in your contention that you "doubt anyone
knows for sure" ... a first semester, college level, Botany 101 class
would prove to you that this is well understood, and well documented
with empirical evidence.
For your future benefit, and so this ends on an instructive note, you
will certainly want to explore the definitive work on the matter:
(Amazon.com product link shortened)
Every woodworker can learn something from reading Hoadley, guaranteed.
See - that is why I get pulled back in. Because the logic is so illogical.
If I said I don't believe they really know what is happening - you say
take a "first semester, college level, Botany 101 class
would prove" - The only thing it would prove is they believe it. Very
circular thinking (as the "scientists" would say).
That will be the last think I desire to say on the subject to you. You
may continue to deride me if you like.
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