diff --git a/src/core/create_manifests.ts b/src/core/create_manifests.ts
index 74a85ff..3ff7a95 100644
--- a/src/core/create_manifests.ts
+++ b/src/core/create_manifests.ts
@@ -226,8 +226,8 @@ function generate_server(
 			error
 		};`.replace(/^\t\t/gm, '').trim();
 
-	const build_dir = path.relative(cwd, dest);
-	const src_dir = path.relative(cwd, src);
+	const build_dir = posixify(path.relative(cwd, dest));
+	const src_dir = posixify(path.relative(cwd, src));
 
 	return `// This file is generated by Sapper — do not edit it!\n` + template
 		.replace('__BUILD__DIR__', JSON.stringify(build_dir))
@@ -242,4 +242,4 @@ function get_file(path_to_routes: string, component: PageComponent) {
 	}
 
 	return posixify(`${path_to_routes}/${component.file}`);
-}
\ No newline at end of file
+}