4.7.5 Infeasibility Criteria

Designers should be prepared to evaluate vegetated roofs for both technical and economic feasibility. For new construction, technical feasibility will involve ensuring that the roof is designed to handle the wet soil and snow loads customary to the microclimate of the site. A vegetated roof is considered infeasible if it cannot accommodate the following wet loads:
  • 10 to 35 pounds per square-foot for extensive designs.
  • 50 to 300 pounds per square-foot for intensive designs.
For retrofit applications, the wet-load structural requirements may make vegetated roofs economically infeasible.

The pitch of an existing roof may also make the use of a vegetated roof infeasible. Vegetated roofs should be avoided on roofs with a pitch greater than 20 degrees unless stabilization measures are in place (WSU-PSP, 2012).

The economic feasibility of a vegetated roof should be reviewed in the larger context of the cost to operate and maintain the building. The vegetated roof will contribute stormwater values that should be considered when feasibility analyses are performed. Other inputs to the economic feasibility analysis for a vegetated roof should include its longer service life over most conventional roofing materials and the savings in heating and cooling costs.