We received an inquiry the other day with a question in the comments form that essentially asked whether a metal roof "catches" more heat than a non-metal roof, making the home underneath warmer.
The answer involves two separate considerations: the roof covering material (in this case, metal roofing) and the construction of the roof assembly.
When you ask the question about the heat absorbancy of metal, there are a number of variables to consider:
First, both the color and finish of the metal affects its heat absorbancy. Metal exposed directly to the sun will certainly heat up, but whether or not it heats up more or less than other roofing materials has to do with the color and finish. The Energy Star finishes that most of our metal roof products use are certified to be reflective of sunlight and therefore cooler than the older finishes. And a pale color will absorb less heat than a dark color.
While it's true that a metal roof will absorb heat when directly exposed to the sun, the same is true of any roof covering material. A dark colored roof in either asphalt, ceramic, slate or any other material will absorb heat at roughly the same rate. It's unlikely that using the same color in differing material will result in a significant roof top temperature difference.
Once the sun stops striking a roof directly, the lightest material will lose its heat fastest. The lightest roof available is metal, so it cools quickest. Once the sun stops shining on a metal roof, it quickly reverts to the same temperature as the surrounding air. With heavier materials like slate or concrete, their mass means they will retain heat much longer. The longer they stay hot, the more heat they'll transfer to the building beneath. In terms of roofing materials contributing to heat in a building, once the sun stops shining, metal is the best possible option.
While the roof color and material certainly will affect the amount of heat collected by the roof, the bigger contributor to that heat being conducted into a room below is the type of construction between the roof and the occupied space below.
If the room beneath the roof in question has an attic, then controlling the temperature of the attic with a "cool" roof like metal is important, but even more important is properly ventilating the attic to keep it within a few degrees of ambient air temperature. Properly ventilating the attic gives the insulation in the ceiling the best chance of keeping the room from gaining heat from the ceiling. Once you've made certain your attic is properly ventilated, the next most important consideration is the ceiling insulation. The more there is and better quality it is, the less heat from the attic will be conducted into the room below in the summer (and the less heat conducted the other way in the winter!).
There's a simple test to see if the heat in a room is coming from the ceiling (or just accumulating by convection from other areas of the house). You can use a digital thermometer and take the temperature of the ceiling, walls and floors in various rooms of the house to see which surfaces are contributing the most heat to a given space. Try it - you'll learn a lot.