"Despite overall consistency in profitability measures, Strabag's margins are thin, which (result/outcome/situation) is typical for the industry, and could deteriorate if there are cost overruns or delays on major projects."
Another way of looking at it is that the "which" clause applies to the whole of the preceding clause - not just one word. In cases like that, "is" is always used.
"I need to feed the dogs, which are waiting for their dinner." (refers to 'dogs')
"I need to feed the dogs, which is annoying because I'm engrossed in this TV show." (refers to the whole main clause)