diff --git a/Substrate/SubstrateCS/Source/AlphaBlockCollection.cs b/Substrate/SubstrateCS/Source/AlphaBlockCollection.cs
index 4b3809d..b01e333 100644
--- a/Substrate/SubstrateCS/Source/AlphaBlockCollection.cs
+++ b/Substrate/SubstrateCS/Source/AlphaBlockCollection.cs
@@ -201,21 +201,8 @@ namespace Substrate
 
             // Update height map
 
-            if (BlockInfo.BlockTable[id] != null) {
-                int tileHeight = _heightMap[x, z];
-                int newOpacity = BlockInfo.BlockTable[id].Opacity;
-
-                if (y > tileHeight && newOpacity > BlockInfo.MIN_OPACITY) {
-                    _heightMap[x, z] = (byte)y;
-                }
-                else if (y == tileHeight && newOpacity == BlockInfo.MIN_OPACITY) {
-                    for (int i = y - 1; i >= 0; i--) {
-                        if (BlockInfo.BlockTable[_blocks[x, i, z]].Opacity > BlockInfo.MIN_OPACITY) {
-                            _heightMap[x, z] = (byte)i;
-                            break;
-                        }
-                    }
-                }
+            if (info1.ObscuresLight != info2.ObscuresLight) {
+                _lightManager.UpdateHeightMap(x, y, z);
             }
 
             // Light consistency
diff --git a/Substrate/SubstrateCS/Source/BlockInfo.cs b/Substrate/SubstrateCS/Source/BlockInfo.cs
index 9dfd323..36038df 100644
--- a/Substrate/SubstrateCS/Source/BlockInfo.cs
+++ b/Substrate/SubstrateCS/Source/BlockInfo.cs
@@ -216,6 +216,11 @@ namespace Substrate
             get { return _transmitLight; }
         }
 
+        public bool ObscuresLight
+        {
+            get { return _opacity > MIN_OPACITY || !_transmitLight; }
+        }
+
         public BlockState State
         {
             get { return _state; }
diff --git a/Substrate/SubstrateCS/Source/BlockLight.cs b/Substrate/SubstrateCS/Source/BlockLight.cs
index f2a2dcb..4f32754 100644
--- a/Substrate/SubstrateCS/Source/BlockLight.cs
+++ b/Substrate/SubstrateCS/Source/BlockLight.cs
@@ -89,6 +89,32 @@ namespace Substrate
             UpdateBlockSkyLight();
         }
 
+        public void UpdateHeightMap (int lx, int ly, int lz)
+        {
+            BlockInfo info = _blockset.GetInfo(lx, ly, lz);
+            int h = Math.Min(ly + 1, _ydim - 1);
+
+            int height = _blockset.GetHeight(lx, lz);
+            if (h < height) {
+                return;
+            }
+
+            if (h == height && !info.ObscuresLight) {
+                for (int i = ly - 1; i >= 0; i--) {
+                    BlockInfo info2 = _blockset.GetInfo(lx, i, lz);
+                    if (info2.ObscuresLight) {
+                        _blockset.SetHeight(lx, lz, Math.Min(i + 1, _ydim - 1));
+                        break;
+                    }
+                }
+                UpdateBlockSkyLight(lx, h, lz);
+            }
+            else if (h > height && info.ObscuresLight) {
+                _blockset.SetHeight(lx, lz, h);
+                UpdateBlockSkyLight(lx, h, lz);
+            }
+        }
+
 
         public void RebuildBlockLight ()
         {
@@ -152,7 +178,7 @@ namespace Substrate
                 for (int z = 0; z < zdim; z++) {
                     for (int y = ydim - 1; y >= 0; y--) {
                         BlockInfo info = _blockset.GetInfo(x, y, z);
-                        if (info.Opacity > BlockInfo.MIN_OPACITY || !info.TransmitsLight) {
+                        if (info.ObscuresLight) {
                             _blockset.SetHeight(x, z, Math.Min(y + 1, ydim - 1));
                             break;
                         }
@@ -303,6 +329,7 @@ namespace Substrate
             IBoundedLitBlockCollection[,] chunkMap = LocalBlockLightMap();
 
             int xdim = _xdim;
+            int ydim = _ydim;
             int zdim = _zdim;
 
             while (_update.Count > 0) {
@@ -348,10 +375,15 @@ namespace Substrate
                     cc.SetBlockLight(x, y, z, light);
 
                     if (info.TransmitsLight) {
+                        if (k.y > 0) {
+                            QueueRelight(new BlockKey(k.x, k.y - 1, k.z));
+                        }
+                        if (k.y < ydim - 1) {
+                            QueueRelight(new BlockKey(k.x, k.y + 1, k.z));
+                        }
+
                         QueueRelight(new BlockKey(k.x - 1, k.y, k.z));
                         QueueRelight(new BlockKey(k.x + 1, k.y, k.z));
-                        QueueRelight(new BlockKey(k.x, k.y - 1, k.z));
-                        QueueRelight(new BlockKey(k.x, k.y + 1, k.z));
                         QueueRelight(new BlockKey(k.x, k.y, k.z - 1));
                         QueueRelight(new BlockKey(k.x, k.y, k.z + 1));
                     }
@@ -364,6 +396,7 @@ namespace Substrate
             IBoundedLitBlockCollection[,] chunkMap = LocalBlockLightMap();
 
             int xdim = _xdim;
+            int ydim = _ydim;
             int zdim = _zdim;
 
             while (_update.Count > 0) {
@@ -415,10 +448,15 @@ namespace Substrate
                     cc.SetSkyLight(x, y, z, light);
 
                     if (info.TransmitsLight) {
+                        if (k.y > 0) {
+                            QueueRelight(new BlockKey(k.x, k.y - 1, k.z));
+                        }
+                        if (k.y < ydim - 1) {
+                            QueueRelight(new BlockKey(k.x, k.y + 1, k.z));
+                        }
+
                         QueueRelight(new BlockKey(k.x - 1, k.y, k.z));
                         QueueRelight(new BlockKey(k.x + 1, k.y, k.z));
-                        QueueRelight(new BlockKey(k.x, k.y - 1, k.z));
-                        QueueRelight(new BlockKey(k.x, k.y + 1, k.z));
                         QueueRelight(new BlockKey(k.x, k.y, k.z - 1));
                         QueueRelight(new BlockKey(k.x, k.y, k.z + 1));
                     }