First, I would question the professionalism of your installer. Underlayment is not the same as industrial grade PB, that being said, MDF or plywood would be a much better choice, and yes, I would use 5/8" material rather than the 1/2". Especially if you have Euro cabinets you will run into problems. Spanning the DW area should not be a problem, just make sure they don't put a joint close to that area.
FYI Particle board comes in a number of different densities. This indicates how much wood and how much filler or adhesive is in the sheet. Industrial board is a 45 lb density, commerical board is 42 or less while some underlayment is less than 20 lb density. MDF is typically over 50lb density, and very stable, some are even exterior grade and water resistant.

A contractor that buys based on price at a box store, is, in my opinion, not a professional. Good Luck !

We are also going to be installing 2 cm countertop and the installer told us we could save money by installing the subtop ourselves. He told us to use 5/8" plywood. Then at Home Depot we decided to ask for an estimate there and he basically told us that we have to order a expensive edge (not the standard basic edge) because you will be able to see the 5/8" plywood. So I imagine that 1/2" would be even harder to hide. I guess I have questions now too...so is anyone ever able to order the standard basic eased edge? Does the countertop have to be thinner to not require a subtop?

We will also have 2cm granite (being installed tomorrow). I don't know what is necessary or customary. I can only tell you that my contractor installed 3/4'' plywood. Our dishwasher is at the end of a run, so there is a wall directly to the left of it. My contractor attached a supporting piece of wood to the wall next to the dishwasher and also behind the dishwasher.

When my granite installer came out to template, he did comment on what a good job my contractor had done.

Who is installing your granite? Maybe you could call them and ask them what they recommend?

We used 3/4" plywood for a couple of reasons: We needed clearance for the top of the drawers, etc. We have a thick edge on our granite (dupont with square offset) and needed a thicker plywood due to the edge. We used A grade plywood, just because CDX and others had lots of knots, etc that I was worried about. The cost was not too much more. My granite installer said it was perfect, so I gues we picked the right thing!

Hi Tom,

Thanks for your answer. I don't know the density of the PB, it doesn't mention that on its printed side. I shall definitely opt for plywood then.

FWIW - my contractor assured me his decision wasn't based on price--otherwise he would have gone with OSB--and that this is recommended practise. I just haven't seen it recommended anywhere! :(

Thanks again.

Thanks for all the answers. You all seem to be having a good experiences with your installers!

I don't know my granite installer unfortunately. He is provided by my contractor and therefore I can't check with him. I already have one (small) piece of granite on a corner piece and I've tried it with 1/2" -- the drawers don't hit the edge, but they're pretty close. 5/6" will give me a tiny exposed line of plywood but not much. You can't see it unless you're on your knees so it's worth going 5/8" I would think for the strength benefits.

Kevin - that's great. I really appreciate your advice.
Thanks

Here's what we did under our 2cm granite...
1) they recommended 5/8in plywood, said if we used 3/4 it MIGHT show.
2) DH INSISTED on using 3/4 plywood because he didn't think 5/8 would be strong enough esp with our overhang (we also have corbels)
3) We installed the 3/4 against all advice.
4) granite went in and NOTHING shows. It's fine...
Our edge is ogee straight.

Hi Suzieca - thanks for that. All this anecdotal evidence is very useful.

All, I've gone with 19/32 now... it was all they had a Home Depot. Thanks for all the advice -- much appreciated!