diff --git a/packages/compiler-cli/src/ngtsc/util/src/typescript.ts b/packages/compiler-cli/src/ngtsc/util/src/typescript.ts
index d672cc651b..a63ded5d28 100644
--- a/packages/compiler-cli/src/ngtsc/util/src/typescript.ts
+++ b/packages/compiler-cli/src/ngtsc/util/src/typescript.ts
@@ -78,7 +78,12 @@ export function getRootDirs(host: ts.CompilerHost, options: ts.CompilerOptions):
   } else {
     rootDirs.push(host.getCurrentDirectory());
   }
-  return rootDirs.map(rootDir => AbsoluteFsPath.fromUnchecked(rootDir));
+
+  // In Windows the above might not always return posix separated paths
+  // See:
+  // https://github.com/Microsoft/TypeScript/blob/3f7357d37f66c842d70d835bc925ec2a873ecfec/src/compiler/sys.ts#L650
+  // Also compiler options might be set via an API which doesn't normalize paths
+  return rootDirs.map(rootDir => AbsoluteFsPath.from(rootDir));
 }
 
 export function nodeDebugInfo(node: ts.Node): string {