Ventilation of Timber Flat Roofs

Abstract

m^eviewed here is a major co-operative investigation m J in Denmark into the effect of ventilation of • • timber flat (cold) roofs to combat condensation and moisture accumulation. Field measurements of moisture content in a number of test roofs over long periods and under different conditions are evaluated and conclusions drawn. They include the advice that, where moisture accumulation is a problem, it can be aggravated if roof vents are installed. C et article porte sur une étude très importante menée en collaboration au Danemark sur les effets de la ventilation des toitures-terrasses (froides) en bois pour lutter contre l'accumulation de condensation et d'humidité. Les auteurs donnent les résultats des mesures in situ de la teneur en eau pour un certain nombre de toitures de ce type sur de longues périodes et dans différentes conditions et en tirent des conclusions, dont celle-ci: lorsqu'il y a un problème de condensation, il risque d'être aggravé par la pose de bouches d'évacuation dans la toiture.O v e r recent y e a r s it h a s b e c o m e a p p a r e n t that ventilated t i m b e r flat roofs of t h e c o l d roof t y p e w i t h a c o v e r i n g of roofing felt are, d u e to excessive m o i s t u r e c o n t e n t in t h e roof d e c k i n g , subject to d e c a y a n d fungal attack. T h e c a u s e is primarily the permeation of w a r m , moist air f r o m underlying r o o m s into t h e roof cavity, w h e r e it c a n c o n d e n s e o n timber roof structures a n d roof d e c k i n g . N o investigation h a d , h o w e v e r , previously b e e n c a r r i e d o u t in D e n m a r k t o d e t e r m i n e t h e e x t e n t t o w h i c h ventilation, possibly s u p p l e m e n t e d b y roof vents, will i m p r o v e or a g g r a v a t e the prevailing moisture conditions in s u c h roofs. A major investigation of actual c o n d i t i o n s h a s t h e r e f o r e b e e n carried o u t (ref. 1) w h e r e ventilation w a s d e t e r m i n e d in a d v a n c e , in o r d e r to ascertain t h e influence of various factors. T h e p r o g r a m m e has b e e n f i n a n c e d b y the Danish Technological Advisory Council, t h e Danish Ministryof E n e r g y a n d t h e D a n i s h Building Research Institute (SBI).