So just to confirm rooting in Canada voids your warranty no matter what. In order to get your warranty back you need to unroot and relock your boot loader.
Let's look at this from a practical perspective.
First, rooting technically does void the warranty as you are "modifying" the factory firmware. Almost all warranties state this. Once a warranty is voided, you can't get it back.
Practically a warranty is voided only when the carrier or manufacturer no longer honor it. Rarely have I seen a warranty not honored simply because the device is rooted, or had been rooted.
Let's consider this. Why would you place a warranty claim? Obviously because the device isn't working properly. If the phone won't turn on, then root doesn't matter at all. If the phone turns on but won't boot into Android, it will be possible (very easy, as a matter of fact) to reflash the stock firmware. There are two very excellent toolkit's that will automate a lot of these processes.
WUG's toolkit and
Skipsoft's unified Android Toolkit.
If you can reflash stock and relock the bootloader ... AND the problem persists, then it's obviously hardware and has nothing to do with root. Moreover, there will be no way to tell that a Nexus has ever been rooted, unlike other branded phones that have various methods of trying to monitor this.
Now, let's look at how the warranty process works. You call the carrier or manufacturer, depending on who is providing the warranty service and put in a claim. They ask you to send the phone to them for evaluation. A lot of times they ask you to send only the phone with no sim, SD card or battery. Once they get the phone, they verify the problem and approve or deny the claim. Again, i've never seen a hardware claim denied because of rooting. The first thing they do, if the phone turns on, is to reflash the official stock firmware, including radios and recovery. If it passed diagnostics at this point, they return it to you. If not they will replace it with, most likely, a certified refurb which will be covered by the remainder of your original warranty.
Please understand that when you start to modify your phone you assume all the risk. If you end up with a problem and they decline the claim, then that's pretty much it. However, the risk is minimal, IMO. It's very hard to damage your phone's hardware with root, unless you are going to be flashing bizarre kernels and overclocking the bejeezus out of it.
Hope this helps. (FYI, All my devices are rooted
)