The palette of practical, durable roof sheathing materials has not changed a great deal in the past century, with rubber membranes and asphalt shingles being the newcomers (and the least environmentally-friendly). It’s not a long list of options to consider, and often price and availability will shrink the list to the point where an owner is choosing between a very limited range of materials.
Making the right choice for roof sheathing material is very important, as the roof plays a critical role in the durability of a building. A building with a weak, leaky roof will have a very short lifespan, regardless of the rest of the materials used.
Many of the conventional roofing options are also reasonably sustainable, making it possible to choose a material that is widely available, affordable, has a proven history and still meets high environmental standards.
More than any other element of a building, roof sheathing requires not just a good choice of material but also a very high quality of workmanship. The best roofing material laid in a poor manner will be leaky. This is one area in particular where quality of installation is every bit as important as quality of material. This factor must be weighed carefully when choosing a particular roofing material, as an owner needs to be able to source not just the material but the right person or thorough instructions for installation. Material warranties and workmanship guarantees are important considerations in the selection process.
Another important consideration when assessing the environmental and cost impacts of a particular roofing choice is the type of strapping or sheathing that is required underneath the sheathing. Some roofing types are installed over minimal wooden strapping, while others require a solid deck of plywood or lumber that can significantly raise costs and impacts. Certain roof types will require specific underlayment membranes, and these too can add to the environmental and financial tally. Be sure to understand the requirements of the full roof sheathing system before making decisions.
Building Science Basics for Roofing
It is the job of roof sheathing to keep water out of our buildings and to resist any forces that might deter the sheathing from keeping water out. While it is only one task, it’s a difficult one.
The roof’s job is made more difficult by design choices we make to change pitches and add ridges, hips, valleys and dormers to the roof shape. The situation is further complicated by the need to add penetrations through the roof sheathing for things like chimneys, vents and plumbing stacks. The simpler the roof design and the less penetrations through the roof, the easier it will be to provide reliable, long-term protection with the roof sheathing. A simple shed roof with a minimum pitch of 3:12 with no penetrations is not difficult to keep leak-free for decades. Every seam added to the roof sheathing increases the chances of leakage.
Successful roof sheathing is all about providing positive lapping wherever there is a seam and ensuring that water and snow have as free a path as possible to leave the roof with the help of gravity. Each roof sheathing material will have specific means of lapping and flashing at intersections, and care must be taken during the installation to ensure that all seams have been properly detailed. It is wise to avoid strategies that rely on caulking to provide the primary protection against leakage. Proper lapping of sheathing materials and flashing should be the primary protection, with caulking used as a backup defense under extreme conditions. Don’t skimp on the width or depth of ridge, hip or valley flashings and be sure to specify the highest quality flashing kits for penetrations like chimneys and plumbing stacks where they absolutely must go through the roof.
Ice damming on roofs in cold climates was one of the issues that initiated the study of building science, as the issue causes many premature roof failures. Ice damming is usually caused by heat leaking from the house into the attic space under the roof, which causes snow to melt. At the edge of the roof where warm air no longer leaks from the house, this runoff refreezes and builds up along the edge of the roof and potentially sends water under the sheathing. Most building codes enforce design regulations intended to prevent ice damming, including sufficient attic insulation and air and vapor control layers at the ceiling so that heat and moisture do not leak into the attic space, as well as ventilation strategies to encourage outside air to move under the roof sheathing to maintain a similar temperature on the top and bottom sides of the sheathing. If you live in a cold climate, be sure to detail your attic space according to codes and/or best practices.
In warm weather and under direct sunlight, a roof can get very warm. This heat can be transmitted into the building and result in much higher cooling loads. The solar reflectance index (SRI) of a roofing product is defined by ASTM Standard E1980-01 and is a calculation that uses solar reflectance and thermal emittance. The US Environmental Protection Agency summarizes SRI as “the relative steady-state surface temperature with respect to the standard white (SRI=100) and standard black (SRI=0) under the standard solar and ambient conditions.” The color of a roof product has a larger effect on SRI than the material used, though certain roof types will have better SRI ratings than others. The Cool Roof Rating Council maintains a database of products in the full range of available colors to help homeowners choose cooler roofs that will help to reduce cooling loads and improve energy efficiency.