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Choosing Roof Designs for New Buildings

A roof’s purpose

The deep truss provides more than 2 feet of space for insulation. The overhang shades the building's south wall.
The deep truss provides more than 2 feet of space for insulation. The overhang shades the building’s south wall.

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

Roof vented above the roof decking. Space also provides a drainage plane.
Roof vented above the roof decking. Space also provides a drainage plane.

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.

Roof ventilation

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.

  1. Remove moisture in the event of condensation or roof leakage.
  2. Keep the roof deck cold during winter to prevent ice damming.
  3. Remove solar heat from the roof or attic.
  4. 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

Rigid foam insulation protects the roof cavity from condensation in winter.
Rigid foam insulation protects the roof cavity from condensation in winter.

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

An OSB air barrier connects from the wall exterior to the ceiling indoors, providing a strong air barrier surrounding the building.
An OSB air barrier connects from the wall exterior to the ceiling indoors, providing a strong air barrier surrounding the building.

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.

7 thoughts on “Choosing Roof Designs for New Buildings

  1. Great article, ventilation is one of the main three components of a roofing systems. I have found the best way to vent a roof is with a 50/50 mixture of intake and exhaust venting. For this I usually prefer to use and mixture between soffit vents and ridge venting.

  2. Good explanation of roof ventilation. I didn’t realize how important it actually was before. Thanks so much for sharing!

  3. Great info. My wife and I will be building a home soon, I will have to refer back to this page! Very helpful, even for me!

    1. We’re glad you found it helpful Adrian. Good luck with your home build!

  4. A very interesting article to read.Thanks a lot for sharing this.Good Work.Keep it Up.

  5. I always forget to consider how air might leak through the roof of a house. Most of the time I associate insulation with walls and I have never worried a whole lot about the roof. I appreciate the reminder that energy can be wasted if you do not get a properly insulated roof, I will definitely be sure that my new roof will be keep the air it. Thank you for such a helpful article!

    1. We appreciate your comments, Brooke! Air leakage from the stack effect is most pronounced at the highest and lowest points of a building, so sealing leakage areas at the ceiling is particularly effective.

      It’s important to keep in mind that most insulation materials aren’t airtight- you should always have a separate air sealing strategy that compliments your insulation plan. This is particularly true where pipes, electrical wires, service chases and complex construction details interrupt primary air barrier materials, like drywall or OSB. The Energy Star Thermal Enclosure Checklist outlines some of the most important leakage areas.

      If you can physically access your attic, consider having a RESNET or BPI professional perform a Blower Door test before installing the insulation. The Blower Door test can reveal hidden leakage areas that are only accessible prior to installing insulation. While the RESNET or BPI professional is operating the Blower Door equipment, find and seal the leakage sites as you identify them, or carefully mark each area for later air-sealing. If you wait until after insulating the attic to find the leakage areas, it may already be too late!

      Our Insulation & Air Sealing Tips page is a great starting point for learning how to best air-seal your buildings, and the Saturn Building Shell Field Guide provides guidance to take your air-sealing game to the next level.

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