From: The Roofing Industry Educational Institute, Synopsis: Introduction to Roofing

From the time mankind left cave, the first successful roofs were those that shed water.  Sloped roofs could be fabricated out of an readily available raw material including: thatch, bark, sod, animal skins, wood shingles and shakes, slate and soft metals such as lead and copper.  Later on, asphalt, aluminum and concrete shingles or tilles were also used, as well as steel and aluminum roof panels.

Membrane roofing is a relatively recent development.  It requires the use of materials that can be sealed together to resist water pressure.  Bituminous materials which were inherently resistant to water and easy to stick together were introduced in the 1800's and have now been joined by plastics, elastomerics, polymer modified bitumens, low slope metal roofing, and sprayed-in-place polyurethane foam systems.

In a steep roof system, rain-water cascades down the slope, with gravity pulling water away from the head lap.

Even though the shingles are not sealed together and have holes punched through by the roofing nails, the water- shedding feature prevents leakage.

On lower-slopes, this feature is inadequate and waterproof underlayments are necessary.

When slope is inadequate, a sealed membrane must be used.  Must be continuous "skin" with watertight "flashing"

Some examples include Built-Up Roofing, Single Ply, Modified Bitumen, Sprayed Foam, Structural Metal Panels with suitable sealant, Liquid Applied Waterproofing.

Besides keeping the water out of a building, a roof system must:

1. Protect the structure and occupants from fire hazard

2. Be practical to install

3. Be affordable and cost effective

4. May need to be aesthetically pleasing

5. Provide human comfort

6. Resist the flow of air or water vapor

7. Keep water out

8. Resist not only weather elements but rooftop traffic as well

The roofing business is a big business!

The roofing market has attracted major industries, competing for their market share.  It's not unusual to see total different materials considered for a roofing project.

Built-Up Roofing - assembled in place from alternating layers of bitumen (asphalt or coal-tar) and reinforcements

Polymer Modified - mixtures of polymers and bitumen, with coated fabrics produced in the factory and field-joined together.

Polymeric Sheets - which are factory premanufactured, requiring relatively narrow seams to be field joined.

Spray-in-place Foam - chemicals are field-mixed, expanding and polymerizing in place.  Followed with surfacings, usually of the liquid polymeric type.

Metal Panels - may sometimes be used on slopes as low as 1/4" per ft (1:48)

A roof system is subjected to many elements.  Combinations of membranes and surfacing materials are frequently synergistic with the surfacing screening out ultraviolet energy and lowering surface temperature.