Although your council leaders have been aware of some issues with the plaster in the sanctuary for some time now, additional moisture incursion during the winter and spring of 2018-19 seem to have made matters much worse.
What's causing this? We're not 100% sure -- we know that the issues we are seeing are confined to a few specific places on the roof, namely the "parapet walls," or the places where slanted sections of roof join up to a stone wall. There are four parapet joints: above the altar, above the Luther window, above the Luther Seal window, and behind the choir and organ. We're seeing damage in all four places. The damage could be due to ice buildup, deterioration of the roofing materials, use of sub-standard caulking for cheap-and-easy repairs on a historic roof, or other factors we don't yet understand.
Haven't we maintained our roof? Yes! GLC had the sanctuary roof entirely replaced in 1995 by a reputable roofer. But we're not seeing any issues with most of the roof: just the flashings and water-tight joints around the edges. That said, your GLC council did forgo annual roof service visits in 2014-16 due to budgetary challenges at the time, which may have allowed conditions to worsen.
So, could the roof collapse? No. The stone walls and roof joists and rafters of our 1914 sanctuary are intact and have not been compromised. What we are seeing are leaks in the outer roof... and their after-effects in the walls below. We need to re-seal the roof so that no water can enter, and then we will rebuild the plaster inside where it has deteriorated and discolored.
See photos below, taken in March 2019.