This should give you a good starting off point for cage building: 
http://www.dapper.com.au/grotto.htm
http://www.ratfanclub.org/cagepln2.html
http://www.geocities.com/ratsonthehill/cagecrafts.html


Just be aware that home-made cages are a lot of trouble to build and often don't last very long.  Also, by the time you buy all the necessary and safe materials, you may actually end up spending more than a good wire cage would cost.

If you mean the urine getting through the wood i have a plan i was going to have clear tape, over lacquer (is it rat safe?), over paint (could someone give me rat safe brands), over kilz. 

I agree with everyone about the cage not lasting long and costing more... Two years ago I made my own cage because I couldn't afford one of the size I wanted. It held up for awhile, but it got chewed up very quickly and was difficult to clean. It took a lot more time to build it than I expected too, and it needed frequent patching and modification. I ended up spending WAY more than I had planned to build it (about 120$). A year later I found an even bigger used cage on craigslist for 35$, and this is what I use now. Maybe I just didn't plan enough before building, but I would check out craigslist or ebay before you invest in your own cage construction.

I would like to say That I built a Grotto style cage and I love it!! I reccomend it to everyone!! Not only was it cheaper, it looks nicer than a plain old cage, but it is so easy to clean!! I just covered all exposed wood with shelf liner paper, and it is very easy to wipe down and sanitize. I have 5 rats in mine right now, and they havent chewed on any thing! Course I am a firm beliver that it takes a bored rat to chew on things that werent ment to chew on. My rats have plenty to do! ANd lots of room to play and run and of course climb! We had the bookshelf already, we also had the scrap wood, and the welded wire to make the doors. All we had to buy was Hinges, door handles, latches, and the shelf liner. The total cost for us was only 30$!! Our grotto is big enough to house well over 8 rats!! All we had to do was build two doors, cut holes in shelves for ramps, and instal latches and other hardware. Then of course fill it with ratty toys and supplies! My boys love it! And like I said easy to clean. I use fabric or bedding, so I just remove the fabric pieces throw them in the wash, then wipe down the shelves and Voila! Place fresh fabric down and you are done! here are a few pics of the cage after we built it.

The whole thing


Living area(has changed quite a bit since rats have been added!)


Bottom half


top half


I made it so there was two seperate doors, that way if I needed to, I could block of the access, and seperate the rats.

Hope this helped!

ths is the site, where I got my Ideas!
http://www.dapper.com.au/cagebuild.htm

i would suggest making the shelves out of melamine board, its plastic so it should be very easy to clean. you could also trytto make it so they can slide out to clean.

well it took me a while, but i got it all cleaned up and ready to get started. (YES!!)  and soon i will post pics of the process.

Where can i get melamine board?

You should be able to pick up the Melamite board at any hardware store like Lowes ect...

I found some grotto cage pics on-line. Hopefully they can help you some.






Here is the layout I used for mine.


and this site has instructions
http://www.dapper.com.au/grotto.htm

Hope this helps!

Hey Guys Today I manged to get a little done! I took off the back of the dresser to give the ratties more ventilation, and for me to get even more reachability into the cage. It will have four doors and the bottom will hold a big Tupperware thing filled with litter.  Heres where i need YOUR help I will be Kilzing the outside of the cage and painting it BUT i dont know what color to do.  I have Purple or Blue.  Also, the rats are female if that will help you decide what color. Thanks again for all your help.
Today's Progress:

Paints to choose from:

I like blue! It is more of a gender neutral colour than purple, so if you ever decide to get boy rats, it will work well.

Yes, but you would! Why else would we all buy/make the prettily patterned and coloured hammocks and the like for the cages, instead of a little piece of grey cloth? 

I definitely second the melamine-coated pressboard. I made two chinchilla cages using it and it's SOOO helpful. When the walls get messy they are SO easy to wipe clean. I absolutely love my home-made cages for the chins, although I have no experience with rat home-made cages. You can get the melamine pressboard at home depot and you can get it cut there, so if you bring a ruler and measure it out, just ask them to cut it. If you don't want to make the shelves removable, caulk the cracks where the shelves fit in so nothing gets stuck in there. It will be easy to clean that way. You can even buy sticky linoleum tiles to put on the shelves for extra protection.

We have a grotto style cage made from Melamine boards (purchase at Lowe's).  I adore this cage and so do the boys.  We used adhesive shelf liner to cover the raw edges of the boards to prevent urine from soaking through.  It works very well and the boys have yet to chew through it.  It was expensive to build but we loved it so much we built another one and connected the two with pvc pipes used as tunnels.  I should post pictures.. 

I see above where people talk about grotto style cages being hard to clean.. but we haven't experienced that at all.  On each shelf we have strips of velcro (rough side only) along all edges.  Our shelf liners are made out of acrylic felt, cut to fit.  The felt sticks to the velcro without having to use the fuzzy side of the velcro (IE: no sewing involved.. and no need to worry about the adhesive velcro coming off of the liners in the wash).  The acrylic felt doesn't fray so it can be washed often while leaving the edges unbound.. (I don't sew!).  The walls of the cage wipe clean because of the slick surface.  Very easy and the boys love it!

But personally, I love the felt because of all the colors it comes in..  You can mix and match or have a themed cage.  Right now the boys have lime green felt...