function getRootWeb() {
    //Get and load a reference to the root web of the current site collection.
    var ctx = new SP.ClientContext("http://Server/sites/sitecollection/");
    var site = ctx.get_site();
    var rootWeb = site.get_rootWeb();

    ctx.load(rootWeb);
    //Ask SharePoint to pull data for us
    ctx.executeQueryAsync(onQuerySucceed,onQueryFailed);
};

//Function executed on success
 function onQuerySucceed() {
    alert(this.rootWeb.get_title());
};

//Function executed on failure
function onQueryFailed(sender, args) {
    alert('Unable to retrieve data from the SharePoint. Request failed. ' + args.get_message() + 'n' + args.get_stackTrace());
};