Effects of outer-layer porous cover sheet parameters on wind loads of double-layer flat roof systems

The double-layer roof system with porous cover sheets (DRSPCS) is widely used in large-scale projects, but existing literature and standards for determining wind loads on the DRSPCS are insufficient. This study conducts wind pressure measurements of the DRSPCS in flat roof buildings with three typical porosities (ϕ = 6.8%, 21.8%, and 36.8%) of the outer-layer porous cover sheets and cavity depths (H = 5, 15, and 25 mm) between the cover sheets and the inner-layer roof by wind tunnel tests. The effects of volume distortion in the scale model, ϕ and H on the wind loads of both outer-layer cover sheets and inner-layer roof are investigated. The results indicate that volume distortion minimally impacts the mean and standard deviation wind pressure coefficients, allowing it to be disregarded in wind tunnel scale model tests for the DRSPCS. Internal pressures within the cavity reduce the mean and peak net wind loads on the outer-layer cover sheets, and increasing H or decreasing ϕ reduces peak wind loads on the inner-layer roof but increases peak net wind loads on the outer-layer cover sheets. In various parameter scenarios of the DRSPCS, the most critical upward wind loads on the outer-layer cover sheets and inner-layer roof are approximately 0.15–0.50 and 0.70–0.95 of those on a single-layer roof with the same dimensions, respectively. Reduction and conversion factors are proposed for estimating wind loads on the DRSPCS based on similar single-layer roofs.