The force shield doesn't directly give an armor bonus
dcollins said:Look, clothing such as shirts are not armor.
- They're not listed in the PHB armor table.
- They're not listed in the DMG magic armor tables.
- They're not mentioned in the Craft Arms & Armor feat.
- Armor takes up one slot, shirts/robes take up different slots.
Thus there's nothing in the rules that allows armor enhancements to be put on them. Armor enhancements go on a "suit of armor" (quoted about a hundred times through the DMG magic section). Anything else is a house rule.
www.superdan.net/dndfaq3.html
Robbert Raets said:
Well, mr. Collins I have to ask you: Where is the big difference between a Woolen Shirt and Padded Armor?

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Rulebook featuring "high magic" options, including a host of new spells.