When it is a mono draft ventilation housing.
Or when it is no longer in an upright position.
This particular mono draft ventilation housing used to be held on with lots of bolts, which have sheared in the recent storms, leaving one remaining. Of course this means the whole thing is in a rather perilous state, and we've had to close off reception just in case the rope that is securing the thing gives and it comes crashing through the glass. Because we care about our visitors and don't want them getting a hair full of shards, you can now enter through the shop. However, it does mean the lifts to the café are out of action.
This architecturally interesting structure helps ventilate the building, a bit more now than it did, and it's going to cost thousands of pounds to fix. Even if one of you has a mono draft ventilation housing lying around, we've still got to erect all the scaffolding to get it up there.
Now where did I put my hard hat ?