diff --git a/spack.yaml b/spack.yaml
index d7afc34505849b039a70ba7da2918dd662a5fdfa..f99325b89d8ed3bc79956e98e27fb580e5c8bc74 100644
--- a/spack.yaml
+++ b/spack.yaml
@@ -29,3 +29,4 @@ spack:
     - pynn-brainscales@1.0-rc2 ^log4cxx@0.10.0 ^googletest@1.11.0:+gmock %gcc@10.3.0
     # demo for codejam12
     - funniest1022 %gcc@10.3.0
+  concretization: together