Significant overstress is normally only allowed for temporary construction or occasional loads.

I know of no code guidance for this issue.  I have developed my own parameters:
New construction -- 5% overstress allowed
Verifying existing construction -- 10% overstress allowed
Justifying my own mistakes after something is already built -- 15% overstress allowed
Remember that loads are in general conservative, that structures are in general redundant, and that our methods of analyzing structures are generally oversimplifications of the actual behavior of structures.
If you are uncomfortable with allowing overstress, look for ways to justify the existing structure (e.g., are there other load paths you are not accounting for?).

DaveAtkins

Just another couple of thoughts when evaluating an existing building structure:

1.  Do you check it using code required loads at the time it was constructed or current code loads?  The same questions could be asked regarding material stresses.

2.  Do you make an assement of the actual loads the building is seeing and check it with those?  If so, would this control over the code requirements? In otherwords, if the ACTUAL loads are less than code specified minimums, would you "pass" an existing building even if it didn't meet code loads?  Would it then be appropriate to post load limits?

This all assumes the structural members are in good condition.  If there is deterioration, then I would assume that some discount to orginal strength would have to be made.

When checking existing structures or portions of structures, I would use a more accurate calculated dead loads.  Original design probably used a dead load that is somewhat more conservative than the weight of the finished structure.

All the calculation procedures to verify the adequacy of existing elements shall be based on current code.

Material strengths shall be based on the older stresses that were allowed at the time of construction.  If it is not available, test some samples to get an estimate or use whatever stresses that are reasonable.

Especially in wood, the current allowable stresses are so low (due to the depletion of older forests and the available lumber today are from younger forests and are not as dense).  When dealing with older wood structures, framing maybe overstressed even without adding new loads if checked with current allowable values.