>>Thanks for the replies. So i probably am going to use plywood to build the cage because in my area you have to drive like 50 miles (1-2 hours) to find the closest lumber yard (stupid subburbs).
Hopefully, you'll be visiting a real lumberyard rather than a Home Depot or Lowe's. In any event, take your time and select your plywood carefully. If you buy from a big box home improvement store, watch out for bowed sheets...
>>What thickness plwood do you recomend?
For a cage that large, you want 3/4" thick plywood
>>if the cage was all plywood except the front which has sliding doors would the top of the cage sag? would it sag with like an extra 25 lbs on it?
Don't remember your exact dimensions, but plywood is stiff enough that the cage probably won't sag under its own weight. Of course, some of this depends on your exact construction technique...
Placing weight on the top, front, center of the cage will make it more likely to sag than placing the weight anywhere else. Keep that in mind when setting things up.
>>should i drill pilot holes for the screws if i use plywood?
Yes. This is a MUST! Unless you buy special "self-tapping" screws, that is. Even then, I'd probably pre-drill just for peace of mind.
>>What grade plywood is the best for building herp cages?
Anything that doesn't look like crap... Typically, you'd want one face to have an A (or maybe B) rating and the other a B, C or C-Plugged. Avoid CDX, which is designed for outdoor applications and is full of knots.
>>does plywood hold heat well?
Not especially.
>>would 3 or 4 coats of a polyurethane stain/sealant work well for making it water proof?
If you want waterproof, you should probably consider MDO (medium-density overlay). It's a plywood core, but the outer layer is sort of MDF-like. MDO is designed to be used outdoors to make signs, so it is manufactured with waterproof glues/binders. I've been lead to believe that this also leads to less formaldehyde out-gassing than other engineered sheet goods. The outer layer is designed to have a smooth surface that can be painted with minimal prep work. Downside is it doesn't look anything like wood... Haven't used it myself, but if I wanted a waterproof cage or I was going to paint my building material, I'd give MDO a hard look!