diff --git a/README.md b/README.md
index 52ffeafb38..8267992c75 100644
--- a/README.md
+++ b/README.md
@@ -256,6 +256,8 @@ First clone and build `matrix-js-sdk`:
 1. `git clone https://github.com/matrix-org/matrix-js-sdk.git`
 1. `pushd matrix-js-sdk`
 1. `git checkout develop`
+1. `npm install`
+1. `npm install source-map-loader`  # because webpack is made of fail (https://github.com/webpack/webpack/issues/1472)
 1. `popd`
 
 Then similarly with `matrix-react-sdk`:
diff --git a/package.json b/package.json
index a3f6985ca9..a3c0e6845e 100644
--- a/package.json
+++ b/package.json
@@ -36,22 +36,22 @@
     "build:bundle": "cross-env NODE_ENV=production webpack -p --progress --bail",
     "build:bundle:dev": "webpack --optimize-occurence-order --progress --bail",
     "build:electron": "npm run clean && npm run build && npm run install:electron && build -wml --ia32 --x64",
-    "build:react-sdk": "node scripts/build-watch-sdk.js build react",
-    "build:js-sdk": "node scripts//build-watch-sdk.js build js",
+    "build:react-sdk": "node scripts/npm-sub.js matrix-react-sdk run start:init",
+    "build:js-sdk": "node scripts/npm-sub.js matrix-js-sdk run start:init",
     "build": "npm run build:js-sdk && npm run build:react-sdk && npm run reskindex && npm run build:res && npm run build:bundle",
     "build:dev": "npm run build:js-sdk && npm run build:react-sdk && npm run reskindex && npm run build:res && npm run build:bundle:dev",
     "dist": "scripts/package.sh",
     "install:electron": "install-app-deps",
     "electron": "npm run install:electron && electron .",
     "start:res": "node scripts/copy-res.js -w",
-    "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\"",
+    "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/npm-sub.js matrix-js-sdk run start:watch",
+    "start:js-sdk:prod": "cross-env NODE_ENV=production node scripts/npm-sub.js matrix-js-sdk run start:watch",
+    "start:react-sdk": "node scripts/npm-sub.js matrix-react-sdk run start:all",
+    "start:react-sdk:prod": "cross-env NODE_ENV=production node scripts/npm-sub.js matrix-react-sdk run start:all",
+    "start": "npm run build:js-sdk && npm run build:react-sdk && 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": "npm run build:js-sdk && npm run build:react-sdk && 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",
diff --git a/scripts/block-on-sdk-build.js b/scripts/block-on-sdk-build.js
deleted file mode 100644
index eb3597292d..0000000000
--- a/scripts/block-on-sdk-build.js
+++ /dev/null
@@ -1,46 +0,0 @@
-const path = require('path');
-const chokidar = require('chokidar');
-
-
-// This script waits for a signal that an underlying SDK (js or react) has finished
-// enough of the build process to be safe to rely on. In riot-web's case, this means
-// that the underlying SDK has finished an initial build and is getting ready to watch
-// for changes. This is done through use of a canary file that is deleted when it is
-// safe to continue (see build-watch-sdk.js for why we listen for a delete event).
-
-// Why do we block in the first place? Because if riot-web starts it's initial
-// build (via webpack-dev-server) and the react-sdk or js-sdk are halfway through
-// their initial builds, then riot-web's initial build fails out of the box. This
-// can sometimes be corrected by waiting for the SDK build to complete and triggering
-// a file change, thereby causing a cascading build, however it isn't great if the
-// initial build of riot-web fails out of the box. We block at the js-sdk first so
-// that the react-sdk build doesn't fall victim to the same problem, which also
-// slows down the riot-web build. After the js-sdk completes, we start the react-sdk
-// build which riot-web is waiting for. When complete, riot-web starts building as
-// per normal.
-
-
-function waitForCanary(canaryName) {
-    return new Promise((resolve, reject) => {
-        const filename = path.resolve(path.join(".tmp", canaryName + ".canary"));
-
-        // See build-watch-sdk.js for why we listen for 'unlink' specifically.
-        const watcher = chokidar.watch(filename).on('unlink', (path) => {
-            console.log("[block-on-build] Received signal to start watching for builds");
-            watcher.close();
-            resolve();
-        });
-    });
-}
-
-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);
-waitForCanary(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
deleted file mode 100644
index 44607f6b7b..0000000000
--- a/scripts/build-watch-sdk.js
+++ /dev/null
@@ -1,99 +0,0 @@
-const fs = require('fs');
-const path = require('path');
-const child_process = require('child_process');
-
-
-const task = process.argv[2];
-if (task !== "build" && task !== "watch") {
-    console.error("Expected a task of 'build' or 'watch'");
-    process.exit(1);
-}
-
-const sdkName = process.argv[3];
-if (!sdkName) {
-    console.error("Missing SDK name");
-    process.exit(1);
-}
-
-const sdkPath = path.dirname(require.resolve(`matrix-${sdkName}-sdk/package.json`));
-
-// Note: we intentionally create then delete the canary 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.
-// We also need to ensure the create and delete events are not too close
-// together, otherwise the filesystem may not fire the watcher. Therefore
-// we create the canary as early as possible and delete it as late as possible.
-prepareCanarySignal(sdkName);
-
-// We only want to build the SDK if it looks like it was `npm link`ed
-if (fs.existsSync(path.join(sdkPath, '.git'))) {
-    // Install the develop dependencies just in case they were forgotten by the developer.
-    console.log("Installing develop dependencies");
-    const devEnv = Object.assign({}, process.env, {NODE_ENV: "development"});
-    child_process.execSync("npm install --only=dev", {
-        env: devEnv,
-        cwd: sdkPath,
-    });
-
-    // Because webpack is made of fail
-    if (sdkName === "js") {
-        console.log("Installing source-map-loader");
-        child_process.execSync("npm install source-map-loader", {
-            env: devEnv,
-            cwd: sdkPath,
-        });
-    }
-
-    // Prepare an initial build of the SDK
-    child_process.execSync("npm run start:init", {
-        env: process.env,
-        cwd: sdkPath,
-    });
-
-    // Send a signal to unblock the build for other processes. Used by block-on-sdk-build.js
-    console.log("Sending signal that other processes may unblock");
-    triggerCanarySignal(sdkName);
-
-    // Actually start the watcher process for the SDK (without an initial build)
-    console.log("Performing task: " + task);
-    const watchTask = sdkName === 'js' ? "start:watch" : "start:all";
-    const buildTask = "build";
-    child_process.execSync(`npm run ${task === "build" ? buildTask : watchTask}`, {
-        env: process.env,
-        cwd: sdkPath,
-    });
-} else triggerCanarySignal(sdkName);
-
-function triggerCanarySignal(sdkName) {
-    fs.unlinkSync(getCanaryPath(sdkName));
-}
-
-function prepareCanarySignal(sdkName) {
-    const canaryPath = getCanaryPath(sdkName);
-    const canaryDir = path.dirname(canaryPath);
-
-    try {
-        console.log("Creating canary temp path...");
-        fs.mkdirSync(canaryDir);
-    } catch (e) {
-        if (e.code !== 'EEXIST') {
-            console.error(e);
-            throw "Failed to create temporary directory";
-        }
-    }
-
-    try {
-        console.log("Creating canary file: " + canaryPath);
-        fs.closeSync(fs.openSync(canaryPath, 'w'));
-    } catch (e) {
-        if (e.code !== 'EEXIST') {
-            console.error(e);
-            throw "Failed to create canary file";
-        }
-    }
-}
-
-function getCanaryPath(sdkName) {
-    return path.join(path.resolve(".tmp"), sdkName + ".canary");
-}
\ No newline at end of file
diff --git a/scripts/npm-sub.js b/scripts/npm-sub.js
new file mode 100644
index 0000000000..e0ecbeda53
--- /dev/null
+++ b/scripts/npm-sub.js
@@ -0,0 +1,21 @@
+const path = require('path');
+const child_process = require('child_process');
+
+const moduleName = process.argv[2];
+if (!moduleName) {
+    console.error("Expected module name");
+    process.exit(1);
+}
+
+const argString = process.argv.length > 3 ? process.argv.slice(3).join(" ") : "";
+if (!argString) {
+    console.error("Expected an npm argument string to use");
+    process.exit(1);
+}
+
+const modulePath = path.dirname(require.resolve(`${moduleName}/package.json`));
+
+child_process.execSync("npm " + argString, {
+    env: process.env,
+    cwd: modulePath,
+});