The customers of this house had a huge problem with heat. When we did some initial testing on this house we found that from inside the house the sheet rock had a inside temperature of 88 degrees. The AC unit would never catch up during the day, it would run and never turn off once the outside temperature reached 85 degrees or higher.
Upon doing some temperture tests in the attic we found the peak of the attic would reach over 150 degrees. That temperature was taken when the outside air temperature was 96 degrees. I was doing tests on the roof it self on that 96 degree day and was getting readings of between 160-170 degrees on the surface of the shingles. I was wearing tinnis shoes and I was only able to stay on the roof 5 minutes because my feet started burning through my shoes, and I noticed when I started coming off the roof parts of the rubber on the bottom of my shoes had melted.
When you think about tempertures of 170 degrees there is an understanding that you only can do so much to stop that heat from entering and passing through to the roof deck and then into the attic. 
Asphalt roofing is a great conductor of heat and it will store that heat for a few hours even after the out side temperature in had dropped. We found that venting the attic to industry acepted standards is not enough in fact you will see those results on the next test case model that is listed as Asphalt Shingle Roof With Soffit And Ridge Vent . In California they will not even allow a asphalt roof to be installed directly to the deck unless it is a lighter color that is energy star verified. The only other exception is if a ventilation system is installed that allows a air gap under the roofing deck. Therma Vent not only Vents the convective hot air out, but it also reflects by way of a radiant barrier and then stop the direct heat transfer with a thermal break.
This house above is the same house that we installed the Therma Vent on and have it as a test case . The result were even better than the model we created that has the Asphalt Shingle Roof with Therma Vent system.