Hygrothermal Analysis for Pitched Roof in Consideration of Water Penetration Through Interface Between Fastener and Roofing Underlayment

Abstract

Roofing assemblies for wood frame houses are usually made up of a combination of roofing tiles and underlayments in order to avoid water leakage. However, on the actual construction site, fasteners (such as nails and staples) that pass through the roofing underlayment have the potential to make annular spaces that cause water penetration into sheathing boards of the roofing assemblies. This study describes the impacts of water penetration on moisture behavior at the sheathing board on the pitched roof of a wood frame house. Water spray tests for a roof structure that has roofing tiles are implemented to clarify water leakage through the gaps between the tiles. Additionally, rates of leakage through the interface between the roofing underlayments and the fasteners are determined by water penetration tests. Heat, air, and moisture (HAM) analyses of the water penetration through the roofing underlayment are demonstrated using climate data in a mild climate region. Results indicate that a vented cavity under the sheathing board should be used with roof assemblies having an impermeable underlayment (such as asphalt impregnated felt) in order to avoid serious moisture accumulation. Additionally, the results show that installation of permeable underlayment (such as polymer housewrap) is effective in preventing moisture accumulation in the sheathing board.