diff --git a/fftw3.sh b/fftw3.sh
index 2efe3b7297..3fda9bc195 100644
--- a/fftw3.sh
+++ b/fftw3.sh
@@ -9,44 +9,25 @@ build_requires:
   - "GCC-Toolchain:(?!osx)"
 ---
 #!/bin/bash -e
-export FFTW3_WITH_AVX # use AVX with float or double. Unset automatically for long double and quad
+# ROOT and O2 need different variants of fftw3, but we cannot configure fftw3
+# to build both at the same time. As a workaround, configure and build one,
+# then wipe out the build directory and configure and build the second one.
 
-if [[ $FFTW3_DOUBLE == "ON" ]]; then # ensure only one option is set and AVX is disabled if necessary
-  FFTW3_FLOAT="OFF"
-  FFTW3_LONG_DOUBLE="OFF"
-elif [[ $FFTW3_LONG_DOUBLE == "ON" ]]; then
-  unset FFTW3_WITH_AVX
-  FFTW3_FLOAT="OFF"
-  FFTW3_DOUBLE="OFF"
-else # keep FLOAT default option as it was
-  FFTW3_FLOAT="ON"
-  FFTW3_DOUBLE="OFF"
-  FFTW3_LONG_DOUBLE="OFF"
-fi
+# First, build fftw3 (double precision), required by ROOT.
+cmake -S "$SOURCEDIR" -B "$BUILDDIR/fftw3"              \
+      -DCMAKE_INSTALL_PREFIX:PATH="$INSTALLROOT"        \
+      -DCMAKE_INSTALL_LIBDIR:PATH=lib
+make -C "$BUILDDIR/fftw3" ${JOBS+-j "$JOBS"}
+make -C "$BUILDDIR/fftw3" install
 
-case $ARCHITECTURE in
-    osx_arm64)
-  cmake $SOURCEDIR                                          \
-        -DCMAKE_INSTALL_PREFIX:PATH="${INSTALLROOT}"        \
-        -DCMAKE_INSTALL_LIBDIR:PATH="lib"                   \
-        -DENABLE_LONG_DOUBLE=${FFTW3_LONG_DOUBLE-OFF}       \
-        -DENABLE_FLOAT=${FFTW3_FLOAT-OFF}
-  ;;
-    *)
-  cmake $SOURCEDIR                                          \
-        -DCMAKE_INSTALL_PREFIX:PATH="${INSTALLROOT}"        \
-        -DCMAKE_INSTALL_LIBDIR:PATH="lib"                   \
-        -DENABLE_LONG_DOUBLE=${FFTW3_LONG_DOUBLE-OFF}       \
-        -DENABLE_FLOAT=${FFTW3_FLOAT-OFF}                   \
-        ${FFTW3_WITH_AVX:+-DENABLE_AVX=ON}
-  ;;
-esac
-
-make ${JOBS+-j $JOBS}
-make install
+# Now reconfigure for fftw3f (single precision float), required by O2.
+cmake -S "$SOURCEDIR" -B "$BUILDDIR/fftw3f"             \
+      -DCMAKE_INSTALL_PREFIX:PATH="$INSTALLROOT"        \
+      -DCMAKE_INSTALL_LIBDIR:PATH=lib                   \
+      -DENABLE_FLOAT=ON
+make -C "$BUILDDIR/fftw3f" ${JOBS+-j "$JOBS"}
+make -C "$BUILDDIR/fftw3f" install
 
 #Modulefile
-MODULEDIR="$INSTALLROOT/etc/modulefiles"
-MODULEFILE="$MODULEDIR/$PKGNAME"
-mkdir -p "$MODULEDIR"
-alibuild-generate-module --bin --lib > $MODULEFILE
+mkdir -p "$INSTALLROOT/etc/modulefiles"
+alibuild-generate-module --bin --lib > "$INSTALLROOT/etc/modulefiles/$PKGNAME"
diff --git a/root.sh b/root.sh
index 7a7c7b44c5..3d8e9f5376 100644
--- a/root.sh
+++ b/root.sh
@@ -184,6 +184,10 @@ cmake $SOURCEDIR
 
 cmake --build . --target install ${JOBS+-j $JOBS}
 
+# Make sure ROOT actually found its build dependencies and didn't disable
+# features we requested. "-Dfail-on-missing=ON" would probably be better.
+[ "$("$INSTALLROOT/bin/root-config" --has-fftw3)" = yes ]
+
 # Add support for ROOT_PLUGIN_PATH envvar for specifying additional plugin search paths
 grep -v '^Unix.*.Root.PluginPath' $INSTALLROOT/etc/system.rootrc > system.rootrc.0
 cat >> system.rootrc.0 <<\EOF