How do I attach kitchen cabinets to metal studs?
With screws, just like you do with wood studs - hopefully whoever build the wall put wood blocking in the metal stud cavity for the screws to bite into.
Unfortunately they didn't. Does this mean I have to tear out the drywall to shore up the metal studs?
I've had cabinet installers tell me that in existing construction they prefer to NOT hit a stud and to use expansion anchors in the drywall instead - their reasoning is that blindly screwing into a stud you have no way of knowing whether you got the screw dead center in the stud or barely on the edge.
I have only installed kitchen cabinets once and then into wooden studs and not metal studs. Trouble was I was always missing the studs even with a stud finder. Still though I think that hanging them by the studs is a good idea when you can. However I remember a trick that Tom Silva did on This Old House and wish that I had seen the same trick before I had put my cabinets up but didn't.
What I am about to tell you will take some time and a great deal of work but will make your work go that much easier especially for the upper cabinets. What Tom Silva did was remove the upper section of drywall and replaced that with plywood of the same thickness. So that means you will need a helper to put that plywood up as it will be heavier than drywall but I think that will hold up better than drywall anchors. Just be sure to mark where the electrical wires are behind the wall on the plywood to avoid getting shocked or damaging the wires. After you install your upper cabinets be sure to screw them together,it just looks better when they are screwed together. Good luck with your project.:)