diff --git a/Stackage/Build.hs b/Stackage/Build.hs
index b1c6f597..b2f8b572 100644
--- a/Stackage/Build.hs
+++ b/Stackage/Build.hs
@@ -18,11 +18,20 @@ import           System.IO                  (BufferMode (NoBuffering),
 import           System.Process             (rawSystem, runProcess,
                                              waitForProcess)
 
-defaultBuildSettings :: GhcMajorVersion -> BuildSettings
-defaultBuildSettings version = BuildSettings
+defaultBuildSettings :: Maybe Int -- ^ argument to -j
+                     -> GhcMajorVersion
+                     -> BuildSettings
+defaultBuildSettings cores version = BuildSettings
     { sandboxRoot = "sandbox"
     , expectedFailuresBuild = defaultExpectedFailures version
-    , extraArgs = const ["-fnetwork23"]
+    , extraArgs = \bs -> "-fnetwork23" :
+        case bs of
+            BSTest -> []
+            _ ->
+                case cores of
+                    Nothing -> ["-j"]
+                    Just 1 -> []
+                    Just j -> ["-j", show j]
     , testWorkerThreads = 4
     , buildDocs = True
     , tarballDir = "patching/tarballs"
@@ -64,7 +73,6 @@ build settings' bp = do
                          $ "install"
                          : ("--cabal-lib-version=" ++ libVersion)
                          : "--build-log=logs-tools/$pkg.log"
-                         : "-j"
                          : [tool]
                 hPutStrLn handle ("cabal " ++ unwords (map (\s -> "'" ++ s ++ "'") args))
                 ph <- runCabal args handle
@@ -89,7 +97,6 @@ build settings' bp = do
                  : "--build-log=logs/$pkg.log"
                  : "--max-backjumps=-1"
                  : "--reorder-goals"
-                 : "-j"
                  : packageList
         hPutStrLn handle ("cabal " ++ unwords (map (\s -> "'" ++ s ++ "'") args))
         runCabal args handle
diff --git a/app/stackage.hs b/app/stackage.hs
index af664540..c6075fa1 100644
--- a/app/stackage.hs
+++ b/app/stackage.hs
@@ -47,15 +47,17 @@ data BuildArgs = BuildArgs
     , buildPlanSrc :: FilePath
     , extraArgs'   :: [String] -> [String]
     , noDocs       :: Bool
+    , buildCores   :: Maybe Int
     }
 
 parseBuildArgs :: GhcMajorVersion -> [String] -> IO BuildArgs
 parseBuildArgs version =
     loop BuildArgs
-        { sandbox = sandboxRoot $ defaultBuildSettings version
+        { sandbox = sandboxRoot $ defaultBuildSettings Nothing version
         , buildPlanSrc = defaultBuildPlan
         , extraArgs' = id
         , noDocs = False
+        , buildCores = Nothing
         }
   where
     loop x [] = return x
@@ -63,6 +65,7 @@ parseBuildArgs version =
     loop x ("--build-plan":y:rest) = loop x { buildPlanSrc = y } rest
     loop x ("--arg":y:rest) = loop x { extraArgs' = extraArgs' x . (y:) } rest
     loop x ("--no-docs":rest) = loop x { noDocs = True } rest
+    loop x ("-j":y:rest) = loop x { buildCores = Just $ read y } rest
     loop _ (y:_) = error $ "Did not understand argument: " ++ y
 
 defaultBuildPlan :: FilePath
@@ -73,9 +76,10 @@ withBuildSettings args f = do
     version <- getGhcVersion
     BuildArgs {..} <- parseBuildArgs version args
     bp <- readBuildPlan buildPlanSrc
-    let settings = (defaultBuildSettings version)
+    let bs = defaultBuildSettings buildCores version
+    let settings = bs
             { sandboxRoot = sandbox
-            , extraArgs = extraArgs' . extraArgs (defaultBuildSettings version)
+            , extraArgs = extraArgs' . extraArgs bs
             , buildDocs = not noDocs
             }
     f settings bp