diff --git a/src/lib/sim/BuildingSystem.cpp b/src/lib/sim/BuildingSystem.cpp
index b5f16f7..1e77834 100644
--- a/src/lib/sim/BuildingSystem.cpp
+++ b/src/lib/sim/BuildingSystem.cpp
@@ -40,8 +40,8 @@ BuildingSystem::BuildingSystem(const GameConfig& config,
     , m_spawnShip(std::move(spawnShip))
     , m_isItemUnlocked(std::move(isItemUnlocked))
     , m_rng(rng)
-    , m_asteroidWidth_tiles(config.world.regions.asteroidWidth_tiles)
 {
+    m_state.asteroidWidth_tiles = config.world.regions.asteroidWidth_tiles;
 }
 
 // ---------------------------------------------------------------------------
@@ -340,7 +340,7 @@ bool BuildingSystem::bodyCellsWithinWorldBounds(const std::vector<QPoint>& bodyC
                                                 QPoint anchor) const
 {
     const int heightTiles = m_config.world.heightTiles;
-    const int leftEdgeX   = -m_asteroidWidth_tiles;
+    const int leftEdgeX   = -m_state.asteroidWidth_tiles;
     for (const QPoint& cell : bodyCells)
     {
         const QPoint worldCell = anchor + cell;
diff --git a/src/lib/sim/BuildingSystem.h b/src/lib/sim/BuildingSystem.h
index 89a7a99..469c841 100644
--- a/src/lib/sim/BuildingSystem.h
+++ b/src/lib/sim/BuildingSystem.h
@@ -77,7 +77,7 @@ public:
     // Sets the current buildable asteroid width in tiles. Grows the left
     // placement bound as the player unlocks asteroid expansions (REQ-EXP-UNLOCK).
     // Defaults to world.regions.asteroid_width_tiles at construction.
-    void setAsteroidWidth_tiles(int widthTiles) { m_asteroidWidth_tiles = widthTiles; }
+    void setAsteroidWidth_tiles(int widthTiles) { m_state.asteroidWidth_tiles = widthTiles; }
 
     // Mark a building or construction site for demolition (REQ-BLD-DECONSTRUCT).
     // A construction site is removed instantly and the full cost is returned.
@@ -298,5 +298,4 @@ private:
                        const std::optional<ShipLayoutConfig>&)> m_spawnShip;
     std::function<bool(const std::string&)>        m_isItemUnlocked;
     std::mt19937&                                  m_rng;
-    int                                            m_asteroidWidth_tiles;
 };
diff --git a/src/lib/sim/FactoryState.h b/src/lib/sim/FactoryState.h
index 62e5966..46f2d1f 100644
--- a/src/lib/sim/FactoryState.h
+++ b/src/lib/sim/FactoryState.h
@@ -45,4 +45,10 @@ struct FactoryState
     // The authority on which building owns which tile; every placement and removal
     // path claims and releases its body cells here.
     BuildingGrid                    grid;
+
+    // Current buildable asteroid width, the left bound for placement. Grows as the
+    // player buys expansions (REQ-EXP-UNLOCK). Deliberately not checksummed: it is
+    // derived from config and Simulation's expansion count, which is folded already.
+    // Seeded from config by BuildingSystem's constructor.
+    int                             asteroidWidth_tiles = 0;
 };