For wood, you need to have something that is not softwood, which makes it very expensive as pine is the cheapest wood. Wood is not preferable at all though, as it will soak the urine and never get clean. Some people put sealants over them, but the rats can still chew it, and scratch it, and once it is chewed or scratched, urine will soak in. I'm not convinced that sealer fully keeps the pee out anyway, as if you've ever had a deck, you'll know that sealer needs to be applied semi frequently in order to make it so the water beads up and doesn't soak in. Plus if they chew enough they can escape. If you must make a homemade cage, I'd strongly suggest using a different material for the frame. I'm not overly fond of them, but some people have made frames from the PVC piping for chinchillas. If you do use wood though, make it a frame and not a bookcase. My problem with the bookcase cages is the lack of ventilation, with only one side open it is pretty much the same as a giant aquarium. Plus most book cases aren't deep enough for rats.
Then there is the issue of moving it. Homemade cages tend to be very very heavy.
There also is the issue of safety. A lot of thought needs to go into these cages. You need to make sure the levels are spaced so that they can't fall far. You need to make sure there is no gaps, such as where the doors are, that a rat could get stuck in, which if you've ever tried to build these, you'll realize it is practically impossible to have it line up perfectly so there is no gap when the door is closed.
I agree with others that after the money, time, and cleaning issues, it makes far more sense to buy a cage. I have to disagree with Dahlas about the Critter Nation being the best cage though. Not everyone likes the Critter Nations better than the Martins (I'm surprised canadians ever have martins though, shipping must be outrageously expensive!! lol). The critter nations are too big to fit in the shower for a scrub down, and if you don't have a level entrance/exit to your house, it can't be wheeled outside. Because of the no tool construction, when trying to lift it to get it outside you have to struggle to not lift it in a way that causes the pieces to come apart. Where the pieces join together, pee will build up, so deep cleans are a necessity. I find it to be a huge PITA to disassemble, carry to the tub or outside, wash, carry back to the rats room, and reassemble. They also come with shallow, chewable plastic pans, so you have to pay an additional $65 (in the US anyway) on top of an already expensive cage, if you want to use bedding, which I greatly prefer because I find it better for smells, it allows the rats to burrow which they love, and it prevents food and such from being kicked out of the cage. These are not issues with the Martins, I had a much much easier time cleaning my RUDD, the new Martins have huge doors so there is easy access (I'd of hated the martins older than 2YO though), they come with deep pans that can't be chewed, and are much easier to move and deep clean. The CN is not all bad, I'm actually using it now because I wanted to give my girls more room, but I do not think the CN is the holy grail of cages, IMO, there is not yet a perfect cage.