diff --git a/passes/techmap/flatten.cc b/passes/techmap/flatten.cc
index 619fa4a689d..3425509b15f 100644
--- a/passes/techmap/flatten.cc
+++ b/passes/techmap/flatten.cc
@@ -357,6 +357,9 @@ struct FlattenPass : public Pass {
 
 		FlattenWorker worker;
 
+		if (design->scratchpad.count("flatten.separator"))
+			worker.separator = design->scratchpad_get_string("flatten.separator");
+
 		size_t argidx;
 		for (argidx = 1; argidx < args.size(); argidx++) {
 			if (args[argidx] == "-wb") {