Possessive of a proper name

It seems to me that Charles is a first name and Roberts is a last name.  Last names are regarded as plurals.

Charles's cat belongs only to Charles.
The Roberts' cat belongs to everyone in the Roberts family.

The rules for making possessives from singular proper nouns and the rules for making possessives from plural nouns are different.

CJ

Quirk's CGEL says about possessives for -s words like this way:

[1] plural -s words. -s' : boys', cats', Davys', Roberts'

[2] singular -s word
　　　　1. pronounced S : -s's : Ross's [rosiz]
　　　　2. pronounced Z : -s'/-s's  either forms are allowed.
                             : Dickens'/*Dickens's [dikinziz/*dikinz], Jones'/Jones's[jounziz/*jounz] 
　　　　　　　　　　　           Jesus/*Jesus's [jeezis], Moses/*Moses's [moziz]
              　　　　            <The sign * implies the minority>
　　　　3. Greek names : -s'  : Socrates' [sokrtiz], Xerxes' [kurkusez]
　　　　4. fixed phrases : -s'  : for goodness' sake [goodniz]

 
paco

Hi:

You have probably already received other answers. 

The basic answer is:  because "Roberts" is plural.  According to convention, Roberts's means that you are referring to one person whose name "Roberts" happens to end in "s".

Mr. Roberts's house

The Roberts' house

Mr. Roberts lives alone, whereas the second case refers to a family.

Does this make any sense.

Eve

I think this is the first time I've ever disagreed with Clive.

If the family last name is Roberts. (George Roberts, Jane Roberts, daughter Judy, and their dog Astro), then the entire family are the Robertses.  Mr. Roberts's house as, as described above, but the Robertses' house for the entire family. Speaking as someone whose last name is Davis, we are the Davises. The Davises' house is our collective house.  Barbara Davis's (singular) children are the cutest on the planet.

I also complete agree with Paco about whether you "say" the extra s. My department is Communications.  If we refer to our conference room, we don't say "communications-ziz" so we don't put the 's on Communications's conference room.

I understand there is disagreement on this.  This is just my view. But since I have to sign my own name, sign our family name on holiday cards, and make both of them possessive every once in a while, it's perhaps more relevant to me than a hypothetical example.