diff --git a/deploy/apptainer/qc.sh b/deploy/apptainer/qc.sh
index 588b118..17d87b1 100755
--- a/deploy/apptainer/qc.sh
+++ b/deploy/apptainer/qc.sh
@@ -329,9 +329,8 @@ get_quantcore_image() {  # force(0|1)
         say "   present: virtualenv $QC_VENV (no-build fallback; reinstall: $0 build)"; return 0
       fi
       build_sif || {
-        say ""
-        say "   The image build is not possible on this Spark without root. Falling back to the no-build"
-        say "   setup: a plain Python image plus a virtualenv in $QC_VENV (same libraries, same results)."
+        say "   -> using the no-build setup instead: a plain Python image plus a virtualenv in $QC_VENV"
+        say "      (same libraries, same results)"
         build_venv
       } ;;
     venv)
@@ -348,15 +347,18 @@ get_quantcore_image() {  # force(0|1)
 }
 
 build_sif() {  # build quantcore.sif from the definition; non-zero when this Apptainer cannot build
-  local dst="$QC_IMAGES/quantcore.sif"
+  local dst="$QC_IMAGES/quantcore.sif" log="$QC_HOME/build.log" reason
   mkdir -p "$QC_IMAGES"
-  say "   building $dst from $REPO_DIR (a few minutes)"
+  say "   building $dst from $REPO_DIR (a few minutes; log: $log)"
   prepare_apptainer_dirs
   rm -f "$dst.part"
   # %files paths in the definition are relative to the repository root. Without root Apptainer
-  # builds through --fakeroot (user namespaces); a Spark may not allow that.
-  if ! (cd "$REPO_DIR" && "$APPTAINER" build "$dst.part" deploy/apptainer/quantcore.def); then
+  # builds through --fakeroot (user namespaces); many Sparks do not allow that. The output goes to a
+  # log: on such a Spark Apptainer's red ERROR line is expected, and the caller falls back.
+  if ! (cd "$REPO_DIR" && "$APPTAINER" build "$dst.part" deploy/apptainer/quantcore.def) >"$log" 2>&1; then
     rm -f "$dst.part"
+    reason="$(grep -E 'FATAL|ERROR' "$log" | tail -n 1 | sed -e 's/\x1b\[[0-9;]*m//g' -e 's/^[A-Z]* *: *//')"
+    say "   no image build on this machine${reason:+ ($reason)}"
     return 1
   fi
   mv -f "$dst.part" "$dst"
@@ -391,8 +393,8 @@ cmd_build() {  # rebuild after a code change, in the mode in use
   if [[ "$mode" == venv ]]; then
     build_venv
   elif [[ "$QC_IMAGE" == build ]]; then
-    build_sif || die "the image build failed (see above). Without root it needs Apptainer's --fakeroot. Use the
-       no-build fallback instead: set QC_IMAGE=venv in $CONFIG and run: $0 build"
+    build_sif || die "the image build failed (log: $QC_HOME/build.log). Without root it needs Apptainer's
+       --fakeroot. Use the no-build setup instead: set QC_IMAGE=venv in $CONFIG and run: $0 build"
   else
     die "QC_IMAGE=$QC_IMAGE is a prebuilt image; nothing to build"
   fi