When I picked up my new Roomster I noticed how much less pad thickness there was on the rear discs compared with the front. I realise that the rears only contribute 20% braking power to the fronts 80% but is it engineered for the rear pads to be replaced simultaneously with the fronts? Have the rears ever reached minimum thickness before the fronts?

Quite often the discs need replacing with the pads, or before.

The days of 3 sets of pads or more before discs were needed if discs were ever needed is not a VW Group feature.

Discs dirt cheap, & that is to VW / Skoda from the suppliers. 

I don't know about Roomsters as such, but it's very common for solid discs to be about 40% of the thickness of ventilated ones.

I meant pads on all four wheels. I name the discs (the surfaces the pads press on) as rotors. Some call pads discs and rotors discs. What I am trying to convey is do the rear pads which have less material last as long as the front pads? Sorry for my terminology

Ah, so you're a rotor and hat man then?

 

As to the life of rear pads, they actually do a lot less work than fronts (not unexpected since transient weight distribution heads towards 90% front under heavy braking).