BRE IP19/01 PDF

Introduction

New materials and methods of construction used in pitched roofing have focused attention on the provision of ventilation under roof slates. Little is known about the temperature and humidity variations in the batten void (the space directly beneath the slates). Using insulation at rafter level and ‘breathable' membranes may alter these conditions from those understood to exist in previous forms of roof construction, and possibly require ventilation of the batten void. Breathable membranes allow moisture to pass out of the roof void (and into it) but resist the passage of liquid water. Water vapour that passes through the membrane is assumed to naturally disperse through gaps between the slates. If it does not disperse, perhaps because slates are manufactured to tolerances that increase the air tightness of pitched roofs, there is a risk of condensation when the external temperature is low.

This Paper covers a study of the effects of forced condensation cycles on fibre cement replica slates.