I plan on building a snake rack very soon but i need to know a few things first
is it ok to us plywood
is backwall or belly heat better
and is it ok to stain or paint the wood or will it hurt the snake
Thank you

Well, you may want to provide a bit more information on what kind of rack your going to build?
A standard Sterlite Box rack?
also, what snakes it will house, and what exactly you mean by backwall heat.

1. is it ok to use plywood
Yes, however, the plywood must be sealed.
2. is backwall or belly heat better
I think most snakes prefer above heating, like you would get from lights, a ceramic heat emitter, or radient heat panels.
AS for backwall or belly, if by backwall/bottom you mean sticking the heat pad on the side/bottom of the box, i would probaly go with bottom.
3. and is it ok to stain or paint the wood or will it hurt the snake
Use only water based stains and sealers.

Heres a link to a good site with tips on building different enclosures and racks.
Herpers toolbox

__________________
"Nobody can do Everything, but Everybody can do Something."

Depending on the size of rack you'll need. Plywood reinforced with 2 x 2's is a good choice.
Putting flexwatt underneath the shelves is probably the best solution. Since heat rises it will envelop the tanks in a more efficient manner.
If you do this you will need rheostats to control the different shelf heaters. As the lower shelves will require more heat than the top ones.
Unless you desire to make the rack more appealing, there really is no need to stain or seal it. If you do it's best not to seal or stain where the heating elements will be applied.
Here's a link to one design made of melamine. Another good material if you have the means to cut it.
12 Unit Breeding Rack Plans - Reptile Information - Caresheets Database

__________________
Mike
"Never worry about numbers. Help one person at a time, and always start with the person nearest you."

I did'nt really think about the sterlite box rack not needing the wood sealed, im thinking more of like a stacking wooden enclosure.
Kriminaals right, you would'nt need to seal the wood so long as you don't have to worry about it getting wet.

__________________
"Nobody can do Everything, but Everybody can do Something."