diff --git a/.gitignore b/.gitignore
index 14030632ca..9c2c394cf0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,3 +19,4 @@ electron/pub
 /config.json.*
 /config.local*.json
 /src/component-index.js
+/.tmp
diff --git a/package-lock.json b/package-lock.json
index 90d41371fe..e7da86c95e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -384,6 +384,12 @@
       "integrity": "sha1-GdOGodntxufByF04iu28xW0zYC0=",
       "dev": true
     },
+    "async-lock": {
+      "version": "1.1.3",
+      "resolved": "https://registry.npmjs.org/async-lock/-/async-lock-1.1.3.tgz",
+      "integrity": "sha512-nxlfFLGfCJ1r7p9zhR5OuL6jYkDd9P7FqSitfLji+C1NdyhCz4+rWW3kiPiyPASHhN7VlsKEvRWWbnME9lYngw==",
+      "dev": true
+    },
     "asynckit": {
       "version": "0.4.0",
       "resolved": "https://registry.npmjs.org/asynckit/-/asynckit-0.4.0.tgz",
diff --git a/package.json b/package.json
index 5503a72abe..333966ce30 100644
--- a/package.json
+++ b/package.json
@@ -44,14 +44,14 @@
     "install:electron": "install-app-deps",
     "electron": "npm run install:electron && electron .",
     "start:res": "node scripts/copy-res.js -w",
-    "start:js": "webpack-dev-server --output-filename=bundles/_dev_/[name].js --output-chunk-file=bundles/_dev_/[name].js -w --progress",
-    "start:js:prod": "cross-env NODE_ENV=production webpack-dev-server -w --progress",
-    "start:js-sdk": "node scripts//build-watch-sdk.js watch js",
-    "start:js-sdk:prod": "cross-env NODE_ENV=production node scripts//build-watch-sdk.js watch js",
-    "start:react-sdk": "node scripts//build-watch-sdk.js watch react",
-    "start:react-sdk:prod": "cross-env NODE_ENV=production node scripts//build-watch-sdk.js watch react",
-    "start": "concurrently --kill-others -n js-sdk,react-sdk,reskindex,res,js \"npm run start:js-sdk\" \"npm run start:react-sdk\" \"npm run reskindex:watch\" \"npm run start:res\" \"npm run start:js\"",
-    "start:prod": "concurrently --kill-others -n js-sdk,react-sdk,reskindex,res,js \"npm run start:js-sdk:prod\" \"npm run start:react-sdk:prod\" \"npm run reskindex:watch\" \"npm run start:res\" \"npm run start:js:prod\"",
+    "start:js": "node scripts/block-on-sdk-build.js js && node scripts/block-on-sdk-build.js react && webpack-dev-server --output-filename=bundles/_dev_/[name].js --output-chunk-file=bundles/_dev_/[name].js -w --progress",
+    "start:js:prod": "cross-env NODE_ENV=production node scripts/block-on-sdk-build.js js && node scripts/block-on-sdk-build.js react && webpack-dev-server -w --progress",
+    "start:js-sdk": "node scripts/build-watch-sdk.js watch js",
+    "start:js-sdk:prod": "cross-env NODE_ENV=production node && scripts/build-watch-sdk.js watch js",
+    "start:react-sdk": "node scripts/block-on-sdk-build.js js && node scripts/build-watch-sdk.js watch react",
+    "start:react-sdk:prod": "cross-env NODE_ENV=production node && scripts/block-on-sdk-build.js js && node scripts//build-watch-sdk.js watch react",
+    "start": "concurrently --kill-others-on-fail --prefix \"{time} [{name}]\" -n js-sdk,react-sdk,reskindex,res,riot-js \"npm run start:js-sdk\" \"npm run start:react-sdk\" \"npm run reskindex:watch\" \"npm run start:res\" \"npm run start:js\"",
+    "start:prod": "concurrently --kill-others-on-fail --prefix \"{time} [{name}]\" -n js-sdk,react-sdk,reskindex,res,riot-js \"npm run start:js-sdk:prod\" \"npm run start:react-sdk:prod\" \"npm run reskindex:watch\" \"npm run start:res\" \"npm run start:js:prod\"",
     "lint": "eslint src/",
     "lintall": "eslint src/ test/",
     "clean": "rimraf lib webapp electron_app/dist",
@@ -81,6 +81,7 @@
     "url": "^0.11.0"
   },
   "devDependencies": {
+    "async-lock": "^1.1.3",
     "autoprefixer": "^6.6.0",
     "babel-cli": "^6.5.2",
     "babel-core": "^6.14.0",
diff --git a/scripts/block-on-sdk-build.js b/scripts/block-on-sdk-build.js
new file mode 100644
index 0000000000..cc29275a77
--- /dev/null
+++ b/scripts/block-on-sdk-build.js
@@ -0,0 +1,56 @@
+const path = require('path');
+const chokidar = require('chokidar');
+const AsyncLock = require('async-lock');
+
+
+const WAIT_TIME = 5000; // ms
+
+function waitForCanary(canaryName) {
+    return new Promise((resolve, reject) => {
+        const filename = path.resolve(path.join(".tmp", canaryName));
+
+        // See triggerCanarySignal in build-watch-sdk.js for why we watch for `unlink`
+        const watcher = chokidar.watch(filename).on('unlink', (path) => {
+            console.log("[block-on-build] Received signal to start watching for builds");
+            watcher.close();
+            resolve();
+        });
+    });
+}
+
+function waitOnSdkBuild(sdkName) {
+    // First we wait for a local canary file to be changed
+    return waitForCanary(sdkName).then(() => new Promise((resolve, reject) => {
+        const buildDirectory = path.dirname(require.resolve(`matrix-${sdkName}-sdk`));
+        const lock = new AsyncLock();
+        let timerId = null;
+
+        const watcher = chokidar.watch(buildDirectory).on('all', (event, path) => {
+            lock.acquire("timer", (done) => {
+                if (timerId !== null) {
+                    //console.log("Resetting countdown");
+                    clearTimeout(timerId);
+                }
+                //console.log(`Waiting ${WAIT_TIME}ms for another file update...`);
+                timerId = setTimeout(() => {
+                    console.log("[block-on-build] No updates - unblocking");
+                    watcher.close();
+                    resolve();
+                }, WAIT_TIME);
+                done();
+            }, null, null);
+        });
+    }));
+}
+
+const sdkName = process.argv[2];
+if (!sdkName) {
+    console.error("[block-on-build] No SDK name provided");
+    process.exit(1);
+}
+
+console.log("[block-on-build] Waiting for SDK: " + sdkName);
+waitOnSdkBuild(sdkName).then(() => {
+    console.log("[block-on-build] Unblocked");
+    process.exit(0);
+});
\ No newline at end of file
diff --git a/scripts/build-watch-sdk.js b/scripts/build-watch-sdk.js
index 9a9edd444e..775ea4a502 100644
--- a/scripts/build-watch-sdk.js
+++ b/scripts/build-watch-sdk.js
@@ -36,9 +36,34 @@ if (fs.existsSync(path.join(sdkPath, '.git'))) {
         });
     }
 
+    console.log("Sending signal that other processes may unblock");
+    triggerCanarySignal(sdkName);
+
     console.log("Performing task: " + task);
     child_process.execSync(`npm ${task === "build" ? "run build" : "start"}`, {
         env: process.env,
         cwd: sdkPath,
     });
 }
+
+function triggerCanarySignal(sdkName) {
+    const tmpPath = path.resolve(".tmp");
+
+    try {
+        fs.mkdirSync(tmpPath);
+    } catch (e) {
+        if (e.code !== 'EEXIST') {
+            console.error(e);
+            throw "Failed to create temporary directory";
+        }
+    }
+
+    // Note: we intentionally create then delete the file to work around
+    // a file watching problem where if the file exists on startup it may
+    // fire a "created" event for the file. By having the behaviour be "do
+    // something on delete" we avoid accidentally firing the signal too
+    // early.
+    const canaryPath = path.join(tmpPath, sdkName);
+    fs.closeSync(fs.openSync(canaryPath, 'w'));
+    fs.unlinkSync(canaryPath);
+}
\ No newline at end of file