You just have to add a file named:
_ViewStart.cshtml
Under each area views folder:
/Areas/Area1/Views/_ViewStart.cshtml
And edit the file to point to the root layout like this:
@{
Layout = "~/Views/Shared/_Layout.cshtml";
}
In order for this to work, you do not have to specify a value in the view's layout property, if you do, you would be overriding the global layout
Note: As Tony mentioned, you could edit each view's layout property to point to the root layout, however this is not the recommended way to do it since you would be coupling your views with your layout and change it would be painful
Edit 1
If you would like to use code to set the default view's layout, perhaps you should consider writing a custom view engine.
Try to google about custom RazorViewEngine
and RazorView
This article could be a good start point
http://weblogs.asp.net/imranbaloch/archive/2011/06/27/view-engine-with-dynamic-view-location.aspx
I have not done something like this but I hope I'm pointing you in the right direction