diff --git a/Stackage/Build.hs b/Stackage/Build.hs
index 87094a35..f70ab76a 100644
--- a/Stackage/Build.hs
+++ b/Stackage/Build.hs
@@ -25,6 +25,7 @@ defaultBuildSettings version = BuildSettings
     , extraArgs = const ["-fnetwork23"]
     , testWorkerThreads = 4
     , buildDocs = True
+    , tarballDir = "patching/tarballs"
     }
 
 build :: BuildSettings -> BuildPlan -> IO ()
@@ -72,6 +73,7 @@ build settings' bp = do
 
     putStrLn "Beginning Stackage build"
     ph <- withBinaryFile "build.log" WriteMode $ \handle -> do
+        packageList <- mapM (replaceTarball settings) $ bpPackageList bp
         let args = addCabalArgs settings BSBuild
                  $ "install"
                  : ("--cabal-lib-version=" ++ libVersion)
@@ -79,9 +81,7 @@ build settings' bp = do
                  : "--max-backjumps=-1"
                  : "--reorder-goals"
                  : "-j"
-                 : concat
-                    [ bpPackageList bp
-                    ]
+                 : packageList
         hPutStrLn handle ("cabal " ++ unwords (map (\s -> "'" ++ s ++ "'") args))
         runCabal args handle
     ec <- waitForProcess ph
diff --git a/Stackage/Test.hs b/Stackage/Test.hs
index ece2290c..6040d60b 100644
--- a/Stackage/Test.hs
+++ b/Stackage/Test.hs
@@ -97,7 +97,8 @@ runTestSuite settings testdir (packageName, SelectedPackageInfo {..}) = do
         runGhcPackagePath = runGen True
 
     passed <- handle (\TestException -> return False) $ do
-        getHandle WriteMode  $ run "cabal" ["unpack", package] testdir
+        package' <- replaceTarball settings package
+        getHandle WriteMode  $ run "cabal" ["unpack", package'] testdir
         getHandle AppendMode $ run "cabal" (addCabalArgs settings BSTest ["configure", "--enable-tests"]) dir
         when spiHasTests $ do
             getHandle AppendMode $ run "cabal" ["build"] dir
diff --git a/Stackage/Types.hs b/Stackage/Types.hs
index a2f8d5e1..a3fa9369 100644
--- a/Stackage/Types.hs
+++ b/Stackage/Types.hs
@@ -132,6 +132,8 @@ data BuildSettings = BuildSettings
     -- ^ How many threads to spawn for running test suites.
     , buildDocs             :: Bool
     -- ^ Build docs as part of the test procedure.
+    , tarballDir            :: FilePath
+    -- ^ Directory containing replacement tarballs.
     }
 
 -- | A wrapper around a @Map@ providing a better @Monoid@ instance.
diff --git a/Stackage/Util.hs b/Stackage/Util.hs
index 4ed57ff2..45b20ffa 100644
--- a/Stackage/Util.hs
+++ b/Stackage/Util.hs
@@ -18,11 +18,11 @@ import           Distribution.Version            (thisVersion)
 import           Stackage.Types
 import           System.Directory                (doesDirectoryExist,
                                                   removeDirectoryRecursive)
-import           System.Directory                (getAppUserDataDirectory)
-import           System.Directory                (canonicalizePath,
-                                                  createDirectoryIfMissing)
+import           System.Directory                (getAppUserDataDirectory
+                                                 ,canonicalizePath,
+                                                  createDirectoryIfMissing, doesFileExist)
 import           System.Environment              (getEnvironment)
-import           System.FilePath                 ((</>))
+import           System.FilePath                 ((</>), (<.>))
 
 -- | Allow only packages with permissive licenses.
 allowPermissive :: [String] -- ^ list of explicitly allowed packages
@@ -148,3 +148,16 @@ fixBuildSettings settings' = do
     createDirectoryIfMissing True root'
     root <- canonicalizePath root'
     return settings' { sandboxRoot = root }
+
+-- | Check if a tarball exists in the tarball directory and, if so, use that
+-- instead of the given name.
+replaceTarball :: BuildSettings
+               -> String
+               -> IO String
+replaceTarball bs pkgname = do
+    exists <- doesFileExist fp
+    if exists
+        then canonicalizePath fp
+        else return pkgname
+  where
+    fp = tarballDir bs </> pkgname <.> "tar.gz"