A roof’s purpose
Before you choose a roof design for a new building, you should understand the purpose of a roof. The primary purpose of roof is to shed rainwater and snow. A secondary purpose — and a less appreciated one — is to provide shade.
A roof’s enemies
Moisture, and to a lesser extent heat, can degrade roofs. A designer must understand the flow of moisture through and around the roof and the vulnerability of roof materials to moisture damage. Intense solar heat can damage roofing materials too.
A roof’s functions
Here is a list of subsidiary functions that a roof must provide, depending on climate and roof structure.
- Support the weight of snow in cold climates.
- Provide a water resistive barrier underneath roofing material to prevent wetting by water that leaks through the roofing material.
- Prevent moisture condensation in the roof cavity or attic.
- Provide a sufficient overhang to shed rainwater away from the foundation and to provide shade.
- Block most solar gain in climates that require air-conditioning for comfort.
- Provide solar control for materials that are sensitive to high temperatures such as foam insulation.
A roof may be ventilated or unventilated. Ventilating the roof or attic is the safest choice. Ventilating air may move underneath the roof deck or on top of the roof deck. Here are four functions of roof ventilation.
- Remove moisture in the event of condensation or roof leakage.
- Keep the roof deck cold during winter to prevent ice damming.
- Remove solar heat from the roof or attic.
- Less common: provide air exchange for a whole-house fan or evaporative cooler.
Especially in wood structures, the roof deck is one of the building’s most important structural elements. The roof deck is also one of the most vulnerable structural elements to damage from moisture and heat. A leading cause of building abandonment is a failed roof structure from moisture deterioration or fire.
Hot roofs and cold roofs
Roofs control the flow of heat and moisture using two different roof designs: cold roofs and hot roofs. Cold roofs are ventilated, and hot roofs aren’t ventilated.
A cold roof’s ventilation air moves against the roof deck’s inner or outer surface 1) to keep the roof deck cold in winter, 2) to remove moisture if the roof deck becomes damp, and 3) to remove some solar heat in summer. Cold roofs are much more common than hot roofs because they are less expensive to build. Cold roof also have a better durability record and lower failure rate compared to hot roofs.
Hot roofs have no ventilation and depend upon rigid insulation above the roof deck to prevent ice damming and moisture condensation in the roof cavity. The International Residential Code (IRC) specifies the required thermal resistance of this exterior insulation depending upon climate. The colder the climate the more insulation you install above the roof deck of a hot roof. You can also install air-impermeable insulation to the bottom of the roof deck (usually spray foam), creating what’s called a cathedralized attic, which is another form of a hot roof.
Air leakage through roofs and ceilings
No building flaw causes more energy waste and moisture problems than air leaks in the ceiling of a building. The ceiling’s interior surface is located under a ventilated attic or under a sloped ceiling attached to the roof rafters or trusses. Planning for an air barrier and correctly installing the ceiling’s air barrier are essential steps to preventing ceiling air leaks.
More about roofs coming soon
I’ll write about all these roof design topics in greater detail in the next five blog posts. Consult our Building Shell Field Guide for information on retrofitting roofs.