# badness messages when changing from report to book class

When I change the documentclass from report to book I suddenly get a lot of messages like this:

Underfull \vbox (badness 10000) has occurred while \output is active []


Does anyone have an explanation?

PS: Note that I never use \\ to add space between paragraphs.

-
do you perhaps have a lot of manual page breaks? the change of style from report to book would certainly change the page breaks for the first page of a chapter, and this change would "flow downhill". – barbara beeton Jul 31 '12 at 13:56
No I don't use \newpage or \clearpage very often. In particular not where these badnesses seem to occur (the log doesn't give line number but I use \input to include code and thus I can locate at least approximately where these things occur). – lpdbw Jul 31 '12 at 14:01
@lpdbw you do not have to accept infinitely bad breaks, In particular it may be that your page height is not a multiple of baselineskip+topskip in which case it is impossible to get flush bottom on pages with no skip. You can see which page is bad by looking for [33] markers in the log file. See also tex.stackexchange.com/questions/50830/… – David Carlisle Aug 9 '12 at 8:19

Contrary to the report class, book uses \flushbottom which may, e.g., change the spacing between paragraphs in order to align the bottom of the text blocks of opposing pages. This, in turn, may lead to problematic spacing when positioning, e.g., floats. Try to add \raggedbottom (which leaves the default spacing alone) to the preamble of your document and see if the warning disappears.