diff --git a/toscaparser/elements/capabilitytype.py b/toscaparser/elements/capabilitytype.py
index dc36dd86..a3afa0e5 100644
--- a/toscaparser/elements/capabilitytype.py
+++ b/toscaparser/elements/capabilitytype.py
@@ -66,10 +66,10 @@ class CapabilityTypeDef(StatefulEntityType):
         if parent_cap:
             parent_cap = parent_cap.type
             while parent_cap != self.TOSCA_TYPEURI_CAPABILITY_ROOT:
-                if parent_cap in self.TOSCA_DEF.keys():
-                    capabilities[parent_cap] = self.TOSCA_DEF[parent_cap]
-                elif custom_def and parent_cap in custom_def.keys():
+                if custom_def and parent_cap in custom_def.keys():
                     capabilities[parent_cap] = custom_def[parent_cap]
+                elif parent_cap in self.TOSCA_DEF.keys():
+                    capabilities[parent_cap] = self.TOSCA_DEF[parent_cap]
                 parent_cap = capabilities[parent_cap]['derived_from']
         return capabilities
 
diff --git a/toscaparser/elements/relationshiptype.py b/toscaparser/elements/relationshiptype.py
index 50ecd4a1..3419804f 100644
--- a/toscaparser/elements/relationshiptype.py
+++ b/toscaparser/elements/relationshiptype.py
@@ -35,11 +35,22 @@ class RelationshipType(StatefulEntityType):
         '''Return a relationship this reletionship is derived from.'''
         prel = self.derived_from(self.defs)
         if prel:
-            return RelationshipType(prel, self.custom_def)
+            return RelationshipType(prel, custom_def=self.custom_def)
 
     @property
     def interfaces(self):
-        return self.get_value(self.INTERFACES)
+        interfaces = self.get_value(self.INTERFACES)
+
+        if self.parent_type is not None:
+            if self.parent_type.interfaces is not None:
+                import copy
+                parent_interfaces = copy.deepcopy(self.parent_type.interfaces)
+
+                if parent_interfaces:
+                    if interfaces:
+                        parent_interfaces.update(interfaces)
+                    interfaces = parent_interfaces
+        return interfaces
 
     @property
     def valid_target_types(self):