diff --git a/src/utilities/buildASTSchema.js b/src/utilities/buildASTSchema.js
index bee0c2fa13..07e05834f8 100644
--- a/src/utilities/buildASTSchema.js
+++ b/src/utilities/buildASTSchema.js
@@ -161,7 +161,7 @@ export function buildASTSchema(
         subscription: (typeMap['Subscription']: any),
       };
 
-  const directives = directiveDefs.map(def => astBuilder.buildDirective(def));
+  const directives = astBuilder.buildDirectives(directiveDefs);
 
   // If specified directives were not explicitly declared, add them.
   if (!directives.some(directive => directive.name === 'skip')) {
@@ -238,18 +238,22 @@ export class ASTDefinitionBuilder {
     return this.getNamedType(node);
   }
 
-  buildDirective(directive: DirectiveDefinitionNode): GraphQLDirective {
-    const locations = directive.locations.map(
-      ({ value }) => ((value: any): DirectiveLocationEnum),
-    );
-
-    return new GraphQLDirective({
-      name: directive.name.value,
-      description: getDescription(directive, this._options),
-      locations,
-      isRepeatable: directive.repeatable,
-      args: this.buildArgumentMap(directive.arguments),
-      astNode: directive,
+  buildDirectives(
+    nodes: Array<DirectiveDefinitionNode>,
+  ): Array<GraphQLDirective> {
+    return nodes.map(directive => {
+      const locations = directive.locations.map(
+        ({ value }) => ((value: any): DirectiveLocationEnum),
+      );
+
+      return new GraphQLDirective({
+        name: directive.name.value,
+        description: getDescription(directive, this._options),
+        locations,
+        isRepeatable: directive.repeatable,
+        args: this.buildArgumentMap(directive.arguments),
+        astNode: directive,
+      });
     });
   }
 
diff --git a/src/utilities/extendSchema.js b/src/utilities/extendSchema.js
index 010df4c3fc..07432be2e6 100644
--- a/src/utilities/extendSchema.js
+++ b/src/utilities/extendSchema.js
@@ -178,7 +178,10 @@ export function extendSchema(
   return new GraphQLSchema({
     ...operationTypes,
     types: objectValues(typeMap),
-    directives: getMergedDirectives(),
+    directives: [
+      ...replaceDirectives(schemaConfig.directives),
+      ...astBuilder.buildDirectives(directiveDefs),
+    ],
     astNode: schemaDef || schemaConfig.astNode,
     extensionASTNodes: concatMaybeArrays(
       schemaConfig.extensionASTNodes,
@@ -205,19 +208,18 @@ export function extendSchema(
     return ((typeMap[type.name]: any): T);
   }
 
-  function getMergedDirectives(): Array<GraphQLDirective> {
-    const existingDirectives = schema.getDirectives().map(directive => {
+  function replaceDirectives(
+    directives: $ReadOnlyArray<GraphQLDirective>,
+  ): Array<GraphQLDirective> {
+    devAssert(directives, 'schema must have default directives');
+
+    return directives.map(directive => {
       const config = directive.toConfig();
       return new GraphQLDirective({
         ...config,
         args: mapValue(config.args, extendArg),
       });
     });
-
-    devAssert(existingDirectives, 'schema must have default directives');
-    return existingDirectives.concat(
-      directiveDefs.map(node => astBuilder.buildDirective(node)),
-    );
   }
 
   function extendNamedType(type: GraphQLNamedType): GraphQLNamedType {