diff --git a/city_model_structure/attributes/surface.py b/city_model_structure/attributes/surface.py
index 4f2b38da..d61d55d4 100644
--- a/city_model_structure/attributes/surface.py
+++ b/city_model_structure/attributes/surface.py
@@ -278,14 +278,12 @@ class Surface:
     """
     if self._normal is None:
       points = self.points
-      accepted_normal_difference = 0.01
       # todo: IF THE FIRST ONE IS 0, START WITH THE NEXT
       point_origin = points[len(points)-2]
       vector_1 = points[len(points)-1] - point_origin
       vector_2 = points[0] - point_origin
       vector_3 = points[1] - point_origin
       cross_product = np.cross(vector_1, vector_2)
-      cross_product_next = np.cross(vector_2, vector_3)
       if np.linalg.norm(cross_product) != 0:
         cross_product = cross_product / np.linalg.norm(cross_product)
         alpha = GeometryHelper.angle_between_vectors(vector_1, vector_2)
@@ -295,45 +293,38 @@ class Surface:
         alpha = 0
       if len(points) == 3:
         return cross_product
-      if np.linalg.norm(cross_product_next) != 0:
-        cross_product_next = cross_product_next / np.linalg.norm(cross_product_next)
-        beta = GeometryHelper.angle_between_vectors(vector_2, vector_3)
-      else:
-        cross_product_next = [0, 0, 0]
-        beta = 0
-      delta_normals = 0
-      for j in range(0, 3):
-        delta_normals += cross_product[j] - cross_product_next[j]
-      if np.abs(delta_normals) < accepted_normal_difference:
-        alpha += beta
-      else:
-        alpha -= beta
+      alpha += self._angle(vector_2, vector_3, cross_product)
       for i in range(0, len(points)-4):
         vector_1 = points[i+1] - point_origin
         vector_2 = points[i+2] - point_origin
-        cross_product_next = np.cross(vector_1, vector_2)
-        if np.linalg.norm(cross_product_next) != 0:
-          cross_product_next = cross_product_next / np.linalg.norm(cross_product_next)
-          beta = GeometryHelper.angle_between_vectors(vector_1, vector_2)
-        else:
-          cross_product_next = [0, 0, 0]
-          beta = 0
-        delta_normals = 0
-        for j in range(0, 3):
-          delta_normals += cross_product[j] - cross_product_next[j]
-        if np.abs(delta_normals) < accepted_normal_difference:
-          alpha += beta
-        else:
-          alpha -= beta
+        alpha += self._angle(vector_1, vector_2, cross_product)
+      vector_1 = points[len(points) - 1] - point_origin
+      vector_2 = points[0] - point_origin
       if alpha < 0:
-        cross_product = np.cross(points[0] - points[len(points) - 2],
-                                 points[len(points) - 1] - points[len(points) - 2])
+        cross_product = np.cross(vector_2, vector_1)
       else:
-        cross_product = np.cross(points[len(points) - 1] - points[len(points) - 2],
-                                 points[0] - points[len(points) - 2])
+        cross_product = np.cross(vector_1, vector_2)
       self._normal = cross_product / np.linalg.norm(cross_product)
     return self._normal
 
+  @staticmethod
+  def _angle(vector_1, vector_2, cross_product):
+    accepted_normal_difference = 0.01
+    cross_product_next = np.cross(vector_1, vector_2)
+    if np.linalg.norm(cross_product_next) != 0:
+      cross_product_next = cross_product_next / np.linalg.norm(cross_product_next)
+      alpha = GeometryHelper.angle_between_vectors(vector_1, vector_2)
+    else:
+      cross_product_next = [0, 0, 0]
+      alpha = 0
+    delta_normals = 0
+    for j in range(0, 3):
+      delta_normals += cross_product[j] - cross_product_next[j]
+    if np.abs(delta_normals) < accepted_normal_difference:
+      return alpha
+    else:
+      return -alpha
+
   @property
   def azimuth(self):
     """