diff --git simworld.cc simworld.cc
index 13ad33734..17f932b7e 100644
--- simworld.cc
+++ simworld.cc
@@ -5155,7 +5155,17 @@ static recursive_mutex_maker_t height_mutex_maker(height_mutex);
 
 void karte_t::plans_finish_rd( sint16 x_min, sint16 x_max, sint16 y_min, sint16 y_max )
 {
-	sint8 min_h = min_height, max_h = max_height;
+#ifdef MULTI_THREAD
+	pthread_mutex_lock(&height_mutex);
+#endif
+
+	sint8 min_h = min_height;
+	sint8 max_h = max_height;
+
+#ifdef MULTI_THREAD
+	pthread_mutex_unlock(&height_mutex);
+#endif
+
 	for(  int y = y_min;  y < y_max;  y++  ) {
 		for(  int x = x_min; x < x_max;  x++  ) {
 			planquadrat_t *plan = access_nocheck(x,y);
@@ -5182,6 +5192,7 @@ void karte_t::plans_finish_rd( sint16 x_min, sint16 x_max, sint16 y_min, sint16
 			}
 		}
 	}
+
 	// update heights
 #ifdef MULTI_THREAD
 	pthread_mutex_lock( &height_mutex );