T. KVANDE and A.M. WALDUM Norwegian Building Research Institute

This paper presents results of a test program emphasising the possibilities of making “watertight” rendering systems for façades exposed to very severe driving rain conditions. Mortars of general purpose-type with inorganic binders were used as renders and panels of lightweight expanded clay block masonry were used as substrates. One main conclusion from the tests was that the type of binder in the mortar was found to be of great importance for water penetration. Only rendering systems with a spatterdash with a high content of Portland cement or masonry cement in the binder resisted heavy attacks of wind-driven rain without discernible water penetration. Further, the results showed that a two-coat rendering system would not meet the basic requirements of driving rain resistance. Hence, a three-coat rendering system has to be used. The final coat can, however, be a thin layer inorganic product like silicate paint.