diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 02e73d2ce..01d937f2e 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -28,7 +28,7 @@ BinaryCacheStore::BinaryCacheStore(std::shared_ptr<Store> localStore,
 
     StringSink sink;
     sink << narVersionMagic1;
-    narMagic = sink.s;
+    narMagic = *sink.s;
 }
 
 void BinaryCacheStore::init()
@@ -200,14 +200,16 @@ Paths BinaryCacheStore::importPaths(bool requireSignature, Source & source,
 struct TeeSource : Source
 {
     Source & readSource;
-    std::string data;
-    TeeSource(Source & readSource) : readSource(readSource)
+    ref<std::string> data;
+    TeeSource(Source & readSource)
+        : readSource(readSource)
+        , data(make_ref<std::string>())
     {
     }
     size_t read(unsigned char * data, size_t len)
     {
         size_t n = readSource.read(data, len);
-        this->data.append((char *) data, n);
+        this->data->append((char *) data, n);
         return n;
     }
 };
@@ -257,7 +259,7 @@ Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
     Hash h;
     if (recursive) {
         dumpPath(srcPath, sink, filter);
-        h = hashString(hashAlgo, sink.s);
+        h = hashString(hashAlgo, *sink.s);
     } else {
         auto s = readFile(srcPath);
         dumpString(s, sink);
@@ -268,7 +270,7 @@ Path BinaryCacheStore::addToStore(const string & name, const Path & srcPath,
     info.path = makeFixedOutputPath(recursive, hashAlgo, h, name);
 
     if (repair || !isValidPath(info.path))
-        addToCache(info, sink.s);
+        addToCache(info, *sink.s);
 
     return info.path;
 }
@@ -283,7 +285,7 @@ Path BinaryCacheStore::addTextToStore(const string & name, const string & s,
     if (repair || !isValidPath(info.path)) {
         StringSink sink;
         dumpString(s, sink);
-        addToCache(info, sink.s);
+        addToCache(info, *sink.s);
     }
 
     return info.path;
@@ -313,7 +315,7 @@ void BinaryCacheStore::buildPaths(const PathSet & paths, BuildMode buildMode)
         StringSink sink;
         dumpPath(storePath, sink);
 
-        addToCache(info, sink.s);
+        addToCache(info, *sink.s);
     }
 }
 
@@ -352,8 +354,7 @@ struct BinaryCacheStoreAccessor : public FSAccessor
         StringSink sink;
         store->exportPath(storePath, false, sink);
 
-        // FIXME: gratuitous string copying.
-        auto accessor = makeNarAccessor(make_ref<std::string>(sink.s));
+        auto accessor = makeNarAccessor(sink.s);
         nars.emplace(storePath, accessor);
         return {accessor, restPath};
     }
@@ -412,12 +413,11 @@ Path BinaryCacheStore::importPath(Source & source, std::shared_ptr<FSAccessor> a
     bool haveSignature = readInt(source) == 1;
     assert(!haveSignature);
 
-    addToCache(info, tee.data);
+    addToCache(info, *tee.data);
 
     auto accessor_ = std::dynamic_pointer_cast<BinaryCacheStoreAccessor>(accessor);
     if (accessor_)
-        // FIXME: more gratuitous string copying
-        accessor_->nars.emplace(info.path, makeNarAccessor(make_ref<std::string>(tee.data)));
+        accessor_->nars.emplace(info.path, makeNarAccessor(tee.data));
 
     return info.path;
 }
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 7d124f6f0..ed4e0f659 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2009,7 +2009,7 @@ void DerivationGoal::startBuilder()
                         throw SysError(format("linking ‘%1%’ to ‘%2%’") % p % i);
                     StringSink sink;
                     dumpPath(i, sink);
-                    StringSource source(sink.s);
+                    StringSource source(*sink.s);
                     restorePath(p, source);
                 }
             }
@@ -2666,8 +2666,8 @@ void DerivationGoal::registerOutputs()
             StringSink sink;
             dumpPath(actualPath, sink);
             deletePath(actualPath);
-            sink.s = rewriteHashes(sink.s, rewritesFromTmp);
-            StringSource source(sink.s);
+            sink.s = make_ref<std::string>(rewriteHashes(*sink.s, rewritesFromTmp));
+            StringSource source(*sink.s);
             restorePath(actualPath, source);
 
             rewritten = true;
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 9a5706681..8a2b7bb91 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1415,9 +1415,9 @@ Path LocalStore::addToStore(const string & name, const Path & _srcPath,
     if (recursive)
         dumpPath(srcPath, sink, filter);
     else
-        sink.s = readFile(srcPath);
+        sink.s = make_ref<std::string>(readFile(srcPath));
 
-    return addToStoreFromDump(sink.s, name, recursive, hashAlgo, repair);
+    return addToStoreFromDump(*sink.s, name, recursive, hashAlgo, repair);
 }
 
 
@@ -1442,14 +1442,14 @@ Path LocalStore::addTextToStore(const string & name, const string & s,
 
             StringSink sink;
             dumpString(s, sink);
-            auto hash = hashString(htSHA256, sink.s);
+            auto hash = hashString(htSHA256, *sink.s);
 
             optimisePath(dstPath);
 
             ValidPathInfo info;
             info.path = dstPath;
             info.narHash = hash;
-            info.narSize = sink.s.size();
+            info.narSize = sink.s->size();
             info.references = references;
             registerValidPath(info);
         }
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index a3cd4ff0d..5c45c890f 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -288,11 +288,11 @@ template PathSet readStrings(Source & source);
 void StringSink::operator () (const unsigned char * data, size_t len)
 {
     static bool warned = false;
-    if (!warned && s.size() > threshold) {
+    if (!warned && s->size() > threshold) {
         warnLargeDump();
         warned = true;
     }
-    s.append((const char *) data, len);
+    s->append((const char *) data, len);
 }
 
 
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 0fdc4037b..9ba6391f8 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -110,7 +110,9 @@ private:
 /* A sink that writes data to a string. */
 struct StringSink : Sink
 {
-    string s;
+    ref<std::string> s;
+    StringSink() : s(make_ref<std::string>()) { };
+    StringSink(ref<std::string> s) : s(s) { };
     void operator () (const unsigned char * data, size_t len) override;
 };