We are building a 16x24 building to house our wood boilers and I want to include Solar mainly for summer DHW. Just wondering what would be the best roofing material to use beneath the panels and for the rest of the roof?

Dimensions of PV modules along with their weight are very critical for example when installed on roofs, slabs and support structures. Another important fact is that the available mounting space may not be enough for the total number of modules needed for a particular building load depending on the module dimensions.
Also it is critical to note whether the structural metal is of non- ferrous material and how bimetallic corrosion is avoided.

I'm planning a new project right now and all the "panel" guys breathe a sigh of relief when they find that the roofing material will be standing seam metal. There are clamps that enable you to mount PV panels and hot water tube arrays without having to pierce the roofing.

The other good thing is that metal roofing systems are already quite light in weight and you might not have to do anything different with the structure in order to support the extra weight of the panels.

Where do you live? Another benefit of the metal roofing is that it is ideal for rainwater collection.

in my plans is a vertical (or near) solar hot water and oversized so that in the low angle winter it will get the most heat and wont be as likely to over heat in the summer although you could shade them.

Don't you mean "vertical" (or near vertical) instead of "horizontal"?

The more vertical you go, the more low Winter sun angles would be favored and the more (high) Summer sun will be reduced.

lol yea thats what i sad it just looked like i typed horizontal lol

From personal experience with our own house, I can say that given a choice, you should prefer to NOT use concrete/clay tiles. Whether you embed panels into the plane of the roof or stand off, it is definitely trickier to work with, and hence more costly.

I am in Colorado, semi arid so not much rain and collecting it is another issue. There are legal reasons why you can not find water butts in Home Depot.

Concrete/clay tiles do not work well here, we can have a dozen freeze thaw cycles a day. Ii was told quite a few manufacturers will not supply here.

Standing seam it is!

Como,
As I said -- concrete roof tiles are just plain nasty.  Even walking across them takes extra care to avoid cracking.  However, that's what was on the house when we bought it.  If I were building new, I'd go for metal, as well.

There is a wide choice of materials used to roof a house, ranging from thatch - dried grass, to slate - pieces of stone. Modern products like plastic, fiberglass and concrete are available, and some innovative, energy-efficient homes are being roofed with sod. New products are being developed to overcome the shortcomings of older roofing materials, meet the demands of modern building techniques, and conform to increasingly stringent building codes.