On 2/22/2012 1:09 PM, Michael Joel wrote:
Swingman wrote:
On 2/22/2012 12:05 AM, Michael Joel wrote:

Paul Conners wrote:

Just a note - cupping takes place towards the moisture not away
from it.



Just to clarify: the concave side is moist, and the convex side is dry?

Yes.



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":
http://www.fpl.fs.fed.us/documnts/fp...chapter_16.pdf 16-6:

quote
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
the grain
/quote



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?

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.




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.

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.





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?

You are not thinking about all of the surfaces that have actually gotten
wet.