Rather than treat disintegration like a standard magical hand-wave let's add a little science to it. Let's treat disintegration like nuclear fission. For those unfamiliar, fission is when a neutron strikes an atom and causes the atom to split into two smaller nuclei, the excess binding energy that was holding the original atom together is then released. The energy is generally in the form of thermal energy and a blast wave (shock transmitted through the air) and a small portion is gamma radiation. In order for that energy to be appreciable it needs to be sustained in a chain reaction as neutrons released from the fission continue to strike other nuclei in turn causing more fissions and more resulting energy, obviously a denser material is better in this situation. The thing is, only nuclei of certain elements can be fissioned and only by certain types of neutrons, well more like certain speeds of escaping neutrons.
Now that the science lesson is over let's try applying this to disintegration, treating it as a stream of neutrons. Let's say that the disintegrating ray will only fission one type of material, so if it strikes your metal armor it affects only the metal (and that specific type of metal). So with that in mind Disintegrate becomes a useful utility for knocking out constructs and inorganic obstacles as well as turning the heavily armored fighters into glowing half naked fools. That's not all, when the disintegration goes off a large amount of energy can potentially be released. Imagine the havoc you could wreak disintegrating a door that defenders are bracing against, or taking out a support beam and the resulting shockwave tears down the less sturdy parts of the foundation. The amount of energy released depends on the amount of identical material available as well as simple randomness (whether or not a neutron will actually strike another nucleus) which is perfectly represented by dice. So let's give it a shot.
Range depends on thickness, shape and total amount of material
Armor - 1-2d6
Door - 3-7d6
Pillar - 8-10d6
The ray does not affect non-dense (heavy element) material, nor material that is intentionally spaced apart such as a honeycomb pattern tower or a portcullis. It also does not affect material with high water content such as wood, roman concrete and of course water. Certain other materials do not interact with the ray either by absorbing neutrons or disallowing ionizing radiation. Speaking of radiation, what if we treat many magical effects as if they were forms of radiation (electromagnetic, thermal, etc.) Since I missed a few weeks let's add the next one in as well.
Lead ShieldA lead shield is a very mundane item and not very difficult to obtain for the amazing defense it provides. The shield negates any incoming magical radiation (fire, lightning, spectacular otherworldly energy and the like) of 6 dice or less. The problem of course is how immensely heavy the shield is. Only characters with a strength of 16 or higher are capable of wielding it in one hand, a strength of 12 is required to lift it. Even then the shield weighs approximately 104 ibs (nearly 7.5 stone) for a 3ft diameter round shield with a quarter inch thickness.
Note: This shield cannot be disintegrated.