diff --git a/app.py b/app.py
index 6eff416..86b8df9 100644
--- a/app.py
+++ b/app.py
@@ -32,7 +32,13 @@ FRONTEND_DIR = os.path.join(os.path.dirname(__file__), "..", "frontend")
 # live at https://<host>/thermograph/ behind a shared domain. Override with the
 # THERMOGRAPH_BASE env var. The frontend uses base-relative URLs, so it follows
 # this automatically without hardcoding the prefix.
-BASE = "/" + os.environ.get("THERMOGRAPH_BASE", "/thermograph").strip("/")
+#
+# THERMOGRAPH_BASE=/ (or empty) means the app owns the whole domain: BASE becomes
+# "" so every route sits at the domain root ("/", "/calendar", "/api/v2/…") with
+# no prefix and no leading double slash. A non-empty value keeps the sub-path form
+# ("/thermograph").
+_BASE = os.environ.get("THERMOGRAPH_BASE", "/thermograph").strip("/")
+BASE = f"/{_BASE}" if _BASE else ""
 
 
 def _weather_fetch_error(e) -> HTTPException:
@@ -592,10 +598,13 @@ def _page(name):
 
 
 # The un-slashed base redirects to BASE/ so the frontend's relative asset URLs
-# resolve correctly. The app stays scoped to BASE and never claims "/", leaving
-# the domain root free for another app (e.g. a portfolio) to own via the proxy.
+# resolve correctly. Under a sub-path this keeps the app scoped to BASE (never
+# claiming "/", so the domain root stays free for another app via the proxy). When
+# BASE is "" the app owns the root: "/" is already the index route below, so there
+# is no bare base to redirect (and "" is not a valid route path).
 # Pages also answer HEAD — link-preview crawlers probe with it before fetching.
-app.add_api_route(BASE, lambda: RedirectResponse(url=f"{BASE}/"), methods=["GET", "HEAD"], include_in_schema=False)
+if BASE:
+    app.add_api_route(BASE, lambda: RedirectResponse(url=f"{BASE}/"), methods=["GET", "HEAD"], include_in_schema=False)
 app.add_api_route(f"{BASE}/", _page("index.html"), methods=["GET", "HEAD"], include_in_schema=False)
 app.add_api_route(f"{BASE}/calendar", _page("calendar.html"), methods=["GET", "HEAD"], include_in_schema=False)
 app.add_api_route(f"{BASE}/day", _page("day.html"), methods=["GET", "HEAD"], include_in_schema=False)
@@ -604,5 +613,7 @@ app.add_api_route(f"{BASE}/legend", _page("legend.html"), methods=["GET", "HEAD"
 app.add_api_route(f"{BASE}/alerts", _page("subscriptions.html"), methods=["GET", "HEAD"], include_in_schema=False)
 
 # Everything else under BASE (app.js, style.css, nav.js, …) is a static asset.
-# Registered last so the explicit page routes above win.
-app.mount(BASE, StaticFiles(directory=FRONTEND_DIR), name="static")
+# Registered last so the explicit page routes above win. Mounts must start with
+# "/", so at the root (BASE == "") mount at "/" — the earlier routes still win
+# because Starlette matches in registration order.
+app.mount(BASE or "/", StaticFiles(directory=FRONTEND_DIR), name="static")