In my 34 years experience in structural engineering......including the last 18 as a consulting engineer.......I have found that (other than when a collapse occurs) it is quite difficult for the vast majority of those involved with buildings to grasp the reasons we "overdesign". Also.........this "overdesign" tends to encourage practices (by contractors, owners and even architects and engineers) that counteract or "use up" the margin of safety, before it is needed. Many contractors (and owners) take the approach that, as long as the building or structural element (component) is standing now....supporting only the weight of the building.........it must be fine. However that is the wrong standard for judging structural capacity. In general, we design and construct buildings to last, literally, a lifetime (or more). We expect the building to support and resist not just "normal" everyday loads........but also extra-ordinary loads, such as, for example, 2 feet of snow on the roof.......a hurricane........crowds of party-goers packed tight on a deck.......or on a walkway in a Kansas City hotel (1981).......and even (at least in the US) earthquakes. In the event of hurricanes and earthquakes......thousands of buildings could be destroyed at the same time.......with severe injury and death (see; Haiti earthquake). On a more down-to-earth level for a particular house......many conditions that we assign to the "defect" category can be lived with.......sometimes for many years........because that defective element never has to be tested by conditions that we design for, based on long experience (over many years that can exceed the life experience of any single person) that shows ......one day........an extra-ordinary loading is bound to eventually occur. The problem is that some of these defects will eventually have to face the test.........yet we can not predict where and when.