What Are Cants in Roofing?
When installing a roof, you want to do everything possible to support the roofing material. A good foundation for the roof can prevent problems with leaks and material shifting in the future. Depending on what type of roof you are installing, you could need to install cants.
Cants, or cant strips, are components of built-up roofing. A cant strip is a small strip or block of material, usually triangularly shaped, placed at the point where a roof deck intersects with a parapet wall that rises higher than the roof. Cant strips are used to support roofing material and prevent gaps or voids. They are most commonly installed in asphalt roofing systems and modified bitumen membrane systems.
Roofing cants can be constructed of of materials ranging from insulation composites to metal. Two common materials used are pressure-treated wood and wooden fiberboard. When choosing between the two, take into account that pressure-treated wood can provide structural support to the building overall, while fiberboard is only strong enough for the roof. For leak resistance as well as ability to withstand punctures, materials like pressure-treated wood and concrete stand up best.
When not using roofing cants, an asphalt roof can have a 90-degree angle between the roof surface and the wall it intersects. The sharp bend places stress on the felts used to support the roof. They can pull away from the angle, trying to straighten out. This can crack the roofing surface and creates gaps that allow moisture to get under the roof. A cant strip creates a gentler angle, closer to 135 degrees than 90. This reduces stress on support felts and provides an extra layer of protection for the flashings that line the edges of the roof.
Not all roofs require cant strips. Some modified bitumen membrane manufacturers don’t call for them during installation. Plastomeric and elastomeric single-ply roofing systems can actually be damaged by installing cant strips. Before installing a cant strip, consult the manufacturer’s installation guidelines before using them on any built-up roofing construction project.
- Manual of Low-slope Roof Systems; Charles William Griffin and R. L. Fricklas
- Commercial Drafting and Detailing; Alan Jefferis and Kenneth Smith