Do a 30" and a 15". That way all of the doors are the same size and it's more visually pleasing. It will also give you the maximum support for heavy objects.

This post was edited by hollysprings on Mon, Apr 15, 13 at 10:01

I do have one and it is in my mudroom/laundry. I don't have very heavy things in it, but the custom cabinet builder who is very reputable wouldn't do it if he thought it would be an issue. I asked several times too and he just has thicker shelves. However, If it had been in the kitchen I wouldn't have done it as those things are much heavier.

+1 to the door size issue. This should be your concern, not the integrity of a shelf.

For a long shelf, go with plywood, but again, you don't want a bunch of different width doors up there. Whoever is helping you pick your cabinets should be making suggestions.

I second hollysprings recommendation.