diff --git a/library/include/modules/Constructions.h b/library/include/modules/Constructions.h
index 41959b2d7..3831d4bb1 100644
--- a/library/include/modules/Constructions.h
+++ b/library/include/modules/Constructions.h
@@ -59,6 +59,7 @@ DFHACK_EXPORT bool isValid();
 DFHACK_EXPORT uint32_t getCount();
 DFHACK_EXPORT bool copyConstruction (const int32_t index, t_construction &out);
 DFHACK_EXPORT df::construction * getConstruction (const int32_t index);
+DFHACK_EXPORT df::construction * findAtTile(df::coord pos);
 
 DFHACK_EXPORT bool designateNew(df::coord pos, df::construction_type type,
                                 df::item_type item = df::item_type::NONE, int mat_index = -1);
diff --git a/library/modules/Constructions.cpp b/library/modules/Constructions.cpp
index 16c1f1b89..a60059b5c 100644
--- a/library/modules/Constructions.cpp
+++ b/library/modules/Constructions.cpp
@@ -68,6 +68,15 @@ df::construction * Constructions::getConstruction(const int32_t index)
     return world->constructions[index];
 }
 
+df::construction * Constructions::findAtTile(df::coord pos)
+{
+    for (auto it = begin (world->constructions); it != end (world->constructions); ++it) {
+        if ((*it)->pos == pos)
+            return *it;
+    }
+    return NULL;
+}
+
 bool Constructions::copyConstruction(const int32_t index, t_construction &out)
 {
     if (uint32_t(index) >= getCount())
diff --git a/plugins/automaterial.cpp b/plugins/automaterial.cpp
index 56b94483b..83573b9fb 100644
--- a/plugins/automaterial.cpp
+++ b/plugins/automaterial.cpp
@@ -32,6 +32,7 @@
 #include "modules/Constructions.h"
 #include "modules/Buildings.h"
 #include "modules/Maps.h"
+#include "modules/MapCache.h"
 
 #include "TileTypes.h"
 #include "df/job_item.h"
@@ -455,27 +456,17 @@ static bool is_valid_building_site(building_site &site, bool orthogonal_check, b
         }
         else
         {
-            if (shape_basic != tiletype_shape_basic::Floor)
+            if (shape != tiletype_shape::STAIR_DOWN && shape_basic != tiletype_shape_basic::Floor)
                 return false;
 
-            if (material == tiletype_material::CONSTRUCTION)
+            // Can build on top of a wall, but not on other construction
+            auto construction = Constructions::findAtTile(site.pos);
+            if (construction)
             {
-                // Can build on top of a wall, but not on a constructed floor
-                df::coord pos_below = site.pos;
-                pos_below.z--;
-                if (!Maps::isValidTilePos(pos_below))
-                    return false;
-
-                auto ttype = Maps::getTileType(pos_below);
-                if (!ttype)
-                    return false;
-
-                auto shape = tileShape(*ttype);
-                auto shapeBasic = tileShapeBasic(shape);
-                if (tileShapeBasic(shape) != tiletype_shape_basic::Wall)
+                if (construction->flags.bits.top_of_wall==0)
                     return false;
             }
-
+           
             if (material == tiletype_material::FIRE ||
                 material == tiletype_material::POOL ||
                 material == tiletype_material::BROOK ||