diff --git a/github_branch.tf b/github_branch.tf
index 57ccf01..2240fc7 100644
--- a/github_branch.tf
+++ b/github_branch.tf
@@ -24,15 +24,22 @@ resource "github_branch_default" "default_main_branch" {
   ]
 }
 
-# For new repos: create files_branch inside the module after repo exists.
-# For existing repos: files_branch is created externally (in terraform-eks-deployment) before the module runs.
+# Create files_branch for both new repos (create_repo=true) and existing repos (create_repo=false).
+# source_branch can be customized via files_branch_source_branch; defaults to the repo default branch.
 resource "github_branch" "files_branch" {
-  count         = var.files_branch != null && var.create_repo ? 1 : 0
-  repository    = local.repo_name
+  count         = var.files_branch != null ? 1 : 0
+  repository    = local.repository_name
   branch        = var.files_branch
-  source_branch = var.github_default_branch
+  source_branch = coalesce(var.files_branch_source_branch, var.github_default_branch)
 
-  depends_on = [github_repository.repo]
+  depends_on = [
+    github_repository.repo,
+    data.github_repository.existing,
+  ]
+
+  lifecycle {
+    ignore_changes = [source_branch, source_sha]
+  }
 }
 
 data "github_user" "pull_request_bypassers" {
diff --git a/variables.tf b/variables.tf
index e915fbf..7b38937 100644
--- a/variables.tf
+++ b/variables.tf
@@ -87,6 +87,12 @@ variable "files_branch" {
   default     = null
 }
 
+variable "files_branch_source_branch" {
+  description = "Source branch to create files_branch from. If null, the repository default branch is used. Only has effect when files_branch is set."
+  type        = string
+  default     = null
+}
+
 variable "github_required_approving_review_count" {
   description = "Number of approvals needed for pull requests"
   type        = number