Bava Batra 162: A Document's Blank Lines and A Sukka's Roofing

Is it true that the last line of a document says nothing new?  Are we that worried about someone adding to a document?  The rabbis look to a baraita to justify this practice.  If two lines are left blank, it is understood that one might easily add new information to that "last line".  

So what if only one and a half lines are left unused?  The baraita is referenced again, and it teaches that if there is one line blank, then the document is valid.  Some of the rabbis agree that one and half lines renders the document void.  

But another baraita suggests that anything less than two lines blank is permitted.  So is it alright to leave one and a half lines blank?  The rabbis then think about who might sign on those extra lines.  If those lines are used to have family members sign the document as 'extra witnesses', the document is still valid.  Relatives cannot be witnesses, and so a relative used as a witness would invalidate the document if that relative were the only witness.

Finally the rabbis compare this line of thought to the halacha regarding sukkot.  If the roof of a suck leaves more than three handbreadths of space between slats, then the sukka is not permitted.  If unfit materials are used as roofing, however, it disqualifies the sukka only if it extends beyond four handbreadths.  Thus unfit material can be used to fill in the three handbreadth space without invalidating the sukka.   This is similar to the case of a document with extra blank lines.  It is valid even if those blank lines are filled with the signatures of unfit witnesses.