diff --git a/src/main/java/world/bentobox/greenhouses/greenhouse/Roof.java b/src/main/java/world/bentobox/greenhouses/greenhouse/Roof.java
index 5358e3b..2f6b053 100644
--- a/src/main/java/world/bentobox/greenhouses/greenhouse/Roof.java
+++ b/src/main/java/world/bentobox/greenhouses/greenhouse/Roof.java
@@ -1,6 +1,7 @@
 package world.bentobox.greenhouses.greenhouse;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -16,15 +17,9 @@ import org.bukkit.util.Vector;
  *
  */
 public class Roof extends MinMaxXZ {
-    private final Location location;
-    private int height;
-    private boolean roofFound;
-
-    /**
-     * @return a list of valid roof blocks
-     */
-    public static List<Material> getRoofBlocks() {
-        return Arrays.stream(Material.values())
+    public static final List<Material> ROOF_BLOCKS;
+    static {
+        List<Material> r = Arrays.stream(Material.values())
                 .filter(Material::isBlock) // Blocks only, no items
                 .filter(m -> !m.name().contains("DOOR")) // No doors
                 .filter(m -> m.name().contains("TRAPDOOR") // All trapdoors
@@ -32,7 +27,11 @@ public class Roof extends MinMaxXZ {
                         || m.equals(Material.HOPPER) // Hoppers
                         || m.equals(Material.GLOWSTONE)) // Glowstone
                 .collect(Collectors.toList());
+        ROOF_BLOCKS = Collections.unmodifiableList(r);
     }
+    private final Location location;
+    private int height;
+    private boolean roofFound;
 
     /**
      * Finds a roof from a starting location under the roof and characterizes it
@@ -60,10 +59,10 @@ public class Roof extends MinMaxXZ {
                     if (!((x > loc.getBlockX() - radius && x < loc.getBlockX() + radius)
                             && (z > loc.getBlockZ() - radius && z < loc.getBlockZ() + radius))) {
                         Block b = world.getBlockAt(x, roofY, z);
-                        if (!Walls.isWallBlock(b.getType())) {
+                        if (!Walls.WALL_BLOCKS.contains(b.getType())) {
                             // Look up
                             for (int y = roofY; y < world.getMaxHeight(); y++) {
-                                if (getRoofBlocks().contains(world.getBlockAt(x,y,z).getType())) {
+                                if (ROOF_BLOCKS.contains(world.getBlockAt(x,y,z).getType())) {
                                     roofFound = true;
                                     loc = new Location(world,x,y,z);
                                     break;
@@ -125,7 +124,7 @@ public class Roof extends MinMaxXZ {
         Location maxz = height.toLocation(world);
         Location minz = height.toLocation(world);
         int limit = 0;
-        while (getRoofBlocks()
+        while (ROOF_BLOCKS
                 .contains(world.getBlockAt(maxx).getType()) && limit < 100) {
             limit++;
             maxx.add(new Vector(1,0,0));
@@ -134,7 +133,7 @@ public class Roof extends MinMaxXZ {
             maxX = maxx.getBlockX()-1;
         }
 
-        while (getRoofBlocks().contains(world.getBlockAt(minx).getType()) && limit < 200) {
+        while (ROOF_BLOCKS.contains(world.getBlockAt(minx).getType()) && limit < 200) {
             limit++;
             minx.subtract(new Vector(1,0,0));
         }
@@ -142,7 +141,7 @@ public class Roof extends MinMaxXZ {
             minX = minx.getBlockX() + 1;
         }
 
-        while (getRoofBlocks().contains(world.getBlockAt(maxz).getType()) && limit < 300) {
+        while (ROOF_BLOCKS.contains(world.getBlockAt(maxz).getType()) && limit < 300) {
             limit++;
             maxz.add(new Vector(0,0,1));
         }
@@ -150,7 +149,7 @@ public class Roof extends MinMaxXZ {
             maxZ = maxz.getBlockZ() - 1;
         }
 
-        while (getRoofBlocks().contains(world.getBlockAt(minz).getType()) && limit < 400) {
+        while (ROOF_BLOCKS.contains(world.getBlockAt(minz).getType()) && limit < 400) {
             limit++;
             minz.subtract(new Vector(0,0,1));
         }
diff --git a/src/main/java/world/bentobox/greenhouses/greenhouse/Walls.java b/src/main/java/world/bentobox/greenhouses/greenhouse/Walls.java
index 825b347..1fe6598 100644
--- a/src/main/java/world/bentobox/greenhouses/greenhouse/Walls.java
+++ b/src/main/java/world/bentobox/greenhouses/greenhouse/Walls.java
@@ -1,6 +1,7 @@
 package world.bentobox.greenhouses.greenhouse;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -10,15 +11,9 @@ import org.bukkit.World;
 import org.bukkit.block.BlockFace;
 
 public class Walls extends MinMaxXZ {
-    private int floor;
-
-    private static final List<BlockFace> ORDINALS = Arrays.asList(BlockFace.NORTH, BlockFace.SOUTH, BlockFace.EAST, BlockFace.WEST);
-
-    /**
-     * @return list of valid wall blocks
-     */
-    public static List<Material> getWallBlocks() {
-        return Arrays.stream(Material.values())
+    public static final List<Material> WALL_BLOCKS;
+    static {
+        List<Material> w = Arrays.stream(Material.values())
                 .filter(Material::isBlock) // Blocks only, no items
                 .filter(m -> !m.name().contains("TRAPDOOR")) // No trap doors
                 .filter(m -> m.name().contains("DOOR") // All doors
@@ -26,8 +21,13 @@ public class Walls extends MinMaxXZ {
                         || m.equals(Material.HOPPER) // Hoppers
                         || m.equals(Material.GLOWSTONE)) // Glowstone
                 .collect(Collectors.toList());
+        WALL_BLOCKS = Collections.unmodifiableList(w);
     }
 
+    private int floor;
+
+    private static final List<BlockFace> ORDINALS = Arrays.asList(BlockFace.NORTH, BlockFace.SOUTH, BlockFace.EAST, BlockFace.WEST);
+
     public Walls(Roof roof) {
         // The player is under the roof
         // Assume the player is inside the greenhouse they are trying to create
@@ -64,25 +64,25 @@ public class Walls extends MinMaxXZ {
                                 switch (bf) {
                                 case EAST:
                                     // positive x
-                                    if (getWallBlocks().contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
+                                    if (WALL_BLOCKS.contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
                                         stopMaxX = true;
                                     }
                                     break;
                                 case WEST:
                                     // negative x
-                                    if (getWallBlocks().contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
+                                    if (WALL_BLOCKS.contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
                                         stopMinX = true;
                                     }
                                     break;
                                 case NORTH:
                                     // negative Z
-                                    if (getWallBlocks().contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
+                                    if (WALL_BLOCKS.contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
                                         stopMinZ = true;
                                     }
                                     break;
                                 case SOUTH:
                                     // positive Z
-                                    if (getWallBlocks().contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
+                                    if (WALL_BLOCKS.contains(world.getBlockAt(x, y, z).getRelative(bf).getType())) {
                                         stopMaxZ = true;
                                     }
                                     break;
@@ -136,7 +136,7 @@ public class Walls extends MinMaxXZ {
             wallBlockCount = 0;
             for (int x = minX; x <= maxX; x++) {
                 for (int z = minZ; z <= maxZ; z++) {
-                    if (getWallBlocks().contains(world.getBlockAt(x, y, z).getType())) {
+                    if (WALL_BLOCKS.contains(world.getBlockAt(x, y, z).getType())) {
                         wallBlockCount++;
                     }
                 }
@@ -154,10 +154,6 @@ public class Walls extends MinMaxXZ {
         return floor;
     }
 
-    public static boolean isWallBlock(Material blockType) {
-        return getWallBlocks().contains(blockType);
-    }
-
     /**
      * @return width of the space
      */
diff --git a/src/main/java/world/bentobox/greenhouses/managers/GreenhouseFinder.java b/src/main/java/world/bentobox/greenhouses/managers/GreenhouseFinder.java
index 60d0058..0b9a69f 100644
--- a/src/main/java/world/bentobox/greenhouses/managers/GreenhouseFinder.java
+++ b/src/main/java/world/bentobox/greenhouses/managers/GreenhouseFinder.java
@@ -80,8 +80,8 @@ public class GreenhouseFinder {
                     } else {
                         // Check just the walls
                         if (y == roof.getHeight() || x == minX || x == maxX || z == minZ || z== maxZ) {
-                            if ((y != roof.getHeight() && !Walls.getWallBlocks().contains(blockType))
-                                    || (y == roof.getHeight() && !Roof.getRoofBlocks().contains(blockType))) {
+                            if ((y != roof.getHeight() && !Walls.WALL_BLOCKS.contains(blockType))
+                                    || (y == roof.getHeight() && !Roof.ROOF_BLOCKS.contains(blockType))) {
                                 //logger(2,"DEBUG: bad block found at  " + x + "," + y+ "," + z + " " + blockType);
                                 if (blockType == Material.AIR) {
                                     airHole = true;