Flat roofs still need a small incline to let water run off. To facilitate this, built up roofs often use a very hydrophobic roofing covering that water flows over easily. This should be included in your Denver roofing estimate. The built up roof attempts to make the roofing material one cohesive part to minimize the chance of water infiltration. This is often done by applying bitumen or coal tar. Although this leaves a consistent surface that seems attractive to humans, human traffic on this type of roof often causes damage. The whole roof is cheap, but fixing leaks can be more problematic. For example when adding HVAC vents and the like, the roof needs to be re-sealed around the penetration. It is expensive to heat up more coal tar for this small application.
Because of the risk of leakage, it is recommended that you add a wood deck above the built up surface if you plan on using the outside. Coal tar built up roofs use bitumen or coal tar which is a byproduct of steel production. It is very waterproof and because it softens a little in the hot sun, it can actually re-pool and heal itself. This can be bad for people trying to walk around on it. Coal tar will usually last around 50 years, however it is dangerous because of the fumes put off when it is heated.
Asphalt is another product that would have a completely different Denver roofing estimate. Although tar and asphalt look similar, and both are hydrocarbon byproducts they are very different. The problem with asphalt is that it ages in heat. Being black and on a rooftop is not the ideal place for that type of material. Over time the asphalt loses plasticity and becomes hard and cracked. As this happens it needs recoating to stave off leaks. So the costs of a Denver roofing estimate should include this maintenance. One way roofers try and increase the age of asphalt roofs is to use multiple layers with roofing materials in between. The most common mix in is gravel. There are a few specialty roofing materials like rubber that last a long time. However the cost is higher and not all construction companies are familiar with these technologies.