Snow Load Factors: Exposure, Thermal and Importance Explained

The flat-roof equation Pf = 0.7 Ce Ct Is Pg has three factors that trip people up. Here is what each one means and how to choose it.

Exposure factor, Ce

Ce reflects how wind-exposed the roof is. A roof in open, windswept terrain loses snow to wind (Ce as low as 0.7–0.9); a roof sheltered by trees or taller buildings keeps it (Ce up to 1.2). It combines the surrounding terrain category (B, C, D) with the roof's own exposure.

Thermal factor, Ct

Ct reflects how much heat escapes through the roof to melt snow from below. A continuously heated building is 1.0; a cold ventilated or barely heated roof is 1.1; an unheated structure is 1.2; a freezer is 1.3; and a continuously heated greenhouse gets 0.85.

Importance factor, Is

Is scales the load by how critical the building is (Risk Category). Ordinary buildings are 1.0; low-hazard structures like minor sheds are 0.8; substantial-hazard buildings are 1.1; and essential facilities like hospitals are 1.2.

Get these three right and the rest of the calculation follows. The calculator lets you set each one and shows the value it used.