I am told that warped boards are warped because they have too much
moisture in one side of the board. Is this the concave side (the inside
of the cup) or the convex side.
I am also told that you can dry out the side that has the moisture in it
and it will straighten back out. Any truth to that?
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Cupping , bowing or warped?
Warping is a twisting of the board lengthwise usually pre. use as a result
of poor seasoning or unlevel / stacking or stripping out whereas cupping is
where the board accepts moisture on the (usually) underside on fixed floors.
If the flooring is sealed (poly / whatever) on top, the board will be flat
only when the weather / moisture content of the board is close to what it
was at the time it was sealed. The boards are unsealed underneath so they
shrink and swell as moisture levels / seasons change. Cupping is where the
edges are bowed upwards inside, bowing is where the centre of the board is
raised due to lower moisture.
Hope this helps.
| I am told that warped boards are warped because they have too much
| moisture in one side of the board. Is this the concave side (the inside
| of the cup) or the convex side.
| I am also told that you can dry out the side that has the moisture in it
| and it will straighten back out. Any truth to that?
| Check out my Web Page at:
| Where you will find:
| ******** How My Shop Works ******** 5-21-03
| * ** Build a $20 DC Separator Can Lid. 1-14-03
| ** * DC Relay Box Building Plans. 1-14-03
| * ** The Bad Air Your Breath Everyday.1-14-03
| ** * What is a Real Woodworker? 2-8-03
| * ** Murphy's Woodworking Definitions. 2-8-03
| ** * Murphy's Woodworking Laws. 4-6-03
| * ** What is the true meaning of life? 1-14-03
| ** * Woodworker Shop Signs. 2-8-03
The definitions I have read are:
Cup -- curvature accross the face of the board, accross the grain
Bow -- curvature along the face of the board along the length of the
board, that is, with the grain.
Crook -- curvature along the edge of the board, along the length of
the board also with the grain.
Twist -- a combination of bow and crook with or without cup but
once a board is that screwed up who cares. Aka kindling.
Wood swells as it absorbs moisture and shrinks as it dries. When
a rectangular solid is curved (ignoring for the moment that it is
no longer rectagular) the convex side of the curve is longer than
the concave, so that if the curvature is due to a moisture
gradient in the wood the convex side is moister.
Warp is typically used as a more generic term for any deformation
of the wood, including or perhaps especially bending it in some
way that the deformed shape is stable.
I don't try to unwarp or twist a board, as a general rule. My take on
it is that a board is what it is & once it's settled in, it will likely
stay that way. Incorrect storage can cause a board to warp during the
drying process, otherwise, you just need to cut/mill the board up into
smaller sections that are straight & flat. Even if you can pull a
twist or cup out in the shop doesn't mean it won't come back in
We see a lot more of this now than we used to. Trees that wouldn't
have hit the sawmill in years past are being used. A local sawyer told
me he's getting a lot more 'stand alone' trees, not part of a stand,
but ones that are off by themselves. These tend to have more limbs, so
you don't get the clear lumber in the lengths you used to. Knots often
mean warps & twists. Young trees mean cupping on wider boards as they
use both sides of the tree in the same board, rather than pulling the
entire board from one side. [ if an end grain looks like this ->
((((((o)))))) the board will tend to cup at the 'o' since the wood
there is different than at the edges. It's best to get end grain that
is from one side of the tree, if possible like this -> )))))) ]
All the above is very generalized, of course. I mostly deal with NE
US wood & some readily available commercial lumber. Tropical, exotics
& such are a whole different ballgame.
It has worked for me on occasion. "The Furniture Doctor" recommended
putting them out in the yard, concave side down. Sun dries the convex side,
moisture gets to the concave side. Doesn't say how to make the darn thing
stay flat after you put it in the shop tho.
This is *one* way that a board can end up warped.
Is this the concave side (the inside
Wet side convex, dry side concave.
*if* that is the cause of your warpage, it will likely come back some or
most of the way.
You can still have warping when there is completely even distribution of
moisture. More dense wood reacts to moisture changes less to than less dense
wood. Wood varies in density within a singe board. Generaly, the wood closer
to the center of the tree is more dense. Around knots, all bets are off.
Consider this scenario:
A flatsawn board is dried to a certain moiture content and is brought to
complete equalibrium with its envvironment. It this then milled flat and
That board is then moved to a highly airconditioned environment (lower
humidity level) and allowed to fully acclimate. This board will likely cup
toward the outside of the tree. This is because the less dense wood from the
outside of the tree will shrink more than the wood on the opposite face of
the board when exposed to an equal decrease in moisture.
I hope that helps some.
Moisture expands wood. The convex side is wider than the concave.
Therefore, the convex side is the damp one.
Sometimes to usually. Depends on lots of things. Sometimes you can
spritz the concave side lightly with water, weight or clamp the board
flat, wait a week or two and they'll wind up perfect. Sometimes you can
just wait. Sometimes they will wind up perfect, sometimes there will be
some twist. One thing: don't try to dry them fast with heat (heat
gun)...you will almost surely get a bad result.
It is really no different than resawing a thick board...they will almost
always cup. Logical because unless they are ancient boards the inside
has a higher moisture content. Ditto when thickness planing unless both
sides are planed equally.
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