diff --git a/docs/changelog.txt b/docs/changelog.txt
index 4dae12dbfc..7a1b7075a8 100644
--- a/docs/changelog.txt
+++ b/docs/changelog.txt
@@ -115,6 +115,7 @@ Template for new versions:
 - Added ``Maps::describeSurroundings`` to get surroundings classification from savagery and evilness
 - Added ``Maps::getCurrentWeather`` to determine the current weather on the map.
 - Added ``Maps::forEachTile`` with ``Maps::TileFilter``/``Maps::TileActions`` to scan a cuboid of tiles, matching tiles by tiletype, tiletype attributes, and designation/occupancy bits, and counting, rewriting tiletypes, editing designation/occupancy bits, spawning constructions, or invoking a callback on matching tiles
+- Added ``MapExtras::MapCache::propagateVerticalFlags`` to update the light/outside/subterranean flags of tiles below a tile that changed between an open and a solid shape
 - Added ``MiscUtils::print_range`` to print ranges with prefix, separator, and suffix
 - Added ``Units::breathes`` and ``Units::getBreathingState``
 - Deprecated ``World::ReadCurrentWeather`` in favor of ``Maps::getCurrentWeather``
diff --git a/library/include/modules/MapCache.h b/library/include/modules/MapCache.h
index c4502cc4b9..7428536afc 100644
--- a/library/include/modules/MapCache.h
+++ b/library/include/modules/MapCache.h
@@ -433,6 +433,11 @@ class DFHACK_EXPORT MapCache
         return b ? b->Allocate() : false;
     }
 
+    /// Propagate the light, outside, and subterranean flags of the tile at
+    /// pos to the tiles below it. Call after the tile at pos changed
+    /// between an open and a solid shape.
+    void propagateVerticalFlags(DFCoord pos);
+
     /// delete the block from memory
     void discardBlock(Block *block);
 
diff --git a/library/modules/MapCache.cpp b/library/modules/MapCache.cpp
index a14d81288e..8f8f2eb4c7 100644
--- a/library/modules/MapCache.cpp
+++ b/library/modules/MapCache.cpp
@@ -1412,6 +1412,50 @@ void MapExtras::MapCache::discardBlock(Block *block)
     delete block;
 }
 
+void MapExtras::MapCache::propagateVerticalFlags(DFCoord pos)
+{
+    df::tiletype_shape shape = tileShape(tiletypeAt(pos));
+    bool open = shape == tiletype_shape::EMPTY || shape == tiletype_shape::RAMP_TOP;
+
+    df::tile_designation td = designationAt(pos);
+    if (open)
+    {
+        if (!ensureBlockAt(DFCoord(pos.x, pos.y, pos.z+1))) {
+            // only the sky above
+            td.bits.light = true;
+            td.bits.outside = true;
+            td.bits.subterranean = false;
+        }
+    }
+    else
+    {
+        // a solid tile covers the column below; subterranean is a property
+        // of each tile and is preserved
+        td.bits.light = false;
+        td.bits.outside = false;
+    }
+
+    for (int32_t z = pos.z - 1;
+         z >= 0 && ensureBlockAt(DFCoord(pos.x, pos.y, z));
+         --z)
+    {
+        DFCoord pos_below(pos.x, pos.y, z);
+        df::tile_designation td_below = designationAt(pos_below);
+        bool sub = open ? td.bits.subterranean : td_below.bits.subterranean;
+        if (td_below.bits.light == td.bits.light
+                && td_below.bits.outside == td.bits.outside
+                && td_below.bits.subterranean == sub)
+            break;
+        td_below.bits.light = td.bits.light;
+        td_below.bits.outside = td.bits.outside;
+        td_below.bits.subterranean = sub;
+        setDesignationAt(pos_below, td_below);
+        shape = tileShape(tiletypeAt(pos_below));
+        if (shape != tiletype_shape::EMPTY && shape != tiletype_shape::RAMP_TOP)
+            break;
+    }
+}
+
 void MapExtras::MapCache::resetTags()
 {
     for (auto it = blocks.begin(); it != blocks.end(); ++it)