Although they are callled flat roofs they are not suppose to be dead flat, they need enough pitch to drain the water without puddling. Mofiied Bitumen Membranes are the most common for residental applications. They are applied with a torch or hot mopped with asphalt. They either come with a granual protective layer, or are smooth and receive an aluminum roof coating which can be reapllied as needed.

There are spray foam, IB membrane, duro-last and of course traditional tar&gravel. there are a lot of options nowadays. Do some research and see what's right for you and your budget.

Spray foam is a cover up, it doesn't require a roof removal which may hide substate defects. Tar and gravel is old school and very heavy.

Another option is using 'green' roof.

Thanks for help. Think I need to do some research to see what is the best looking. Can't do the green roof cause of Colorado climate and to heavy.

I'd talk to at least 3 local contractors to see what they say. Different areas of the country have different roofing needs. I'm lucky enough to have an architect neighbor who shared a roofer reference, his was the roofer I chose over the other two. One of the other roofers came to my house without a ladder and told me I needed a new roof without being able to see it!. Ended up just needing a $300 repair for now.