If the spell Shield is used by character with a robe with an existing +1 armor boost, such as is commonly provided to Warders, that character will only receive +3 armor from the curret GSF: Abjuration script. These bonuses obviously do not stack. This makes wearing such a robe disadvantageous to an Abjurer.
The spell should work like Mage Armor currently does and apply an armor bonus based on item type instead of current armor value.
+1 for heavy leather, +2 for light leather, +3 for padded cloth, +4 for normal cloth, etc.