Investigations on the Durability of Load-Bearing, Directly Rendered, External Sheathings of Wood-Framed Houses

Several years ago some German companies built wood-framed houses with directly rendered external sheathings. After some time, the exterior walls of these houses exhibited large deformations. As a result, the industry mostly discontinued building with such types of construction. Now the German building industry has new and improved sheathing products on the market that are available for directly rendered, exterior walls. Exterior walls must be resistant to changes in temperature, solar radiation, driving rain and dampness. Additionally, these walls need to bear a variety of loads, for example: the vertical load of the construction itself, snow, traffic, and loads resulting from wind load moment, as well as horizontal wind load. Note: Insulation materials between rendering and sheathing as used in ETICS are not part of this work. Possible damages on directly rendered, external sheathings of wood framed houses follow: • large vault / bending of the sheathing • failure of the bonding between the sheathing and the rendering • fungal attack in the construction materials • cracks in the rendering due to hygro-thermal impact, allowing penetration of rain, and • cracks in the rendering at nailed joints of the sheathing caused by horizontal loads. Therefore, it is worthwhile to consider a durable wall system because a directly rendered sheathing is an inexpensive construction alternative. Compared to other linings, it has good protection against mechanical impact as well as low maintenance costs. Last, but not least, a rendered facade is appealing to many because of its solid appearance. The criteria for long-term durability of directly rendered sheathings of wood framed walls were defined with the following necessary tests: • determine the mechanical properties of different sheathings for calculations concerning load bearing capacity • determine the hygro-thermal properties of different sheathings and plasters/stucco for hygro-thermal calculations of the wall construction • test material combinations (plaster/stucco on sheathings) under extreme hygro-thermal conditions on small samples in the climate chamber • determine the shear behavior of joints relative to possible cracks in the plaster/stucco • determine the bonding between sheathing and plaster/stucco and • test natural sized walls under combinations of extreme hygro-thermal conditions and static loads relative to cracks. Hygro-thermal calculations on moisture behavior and mold growth, as well as mechanical calculations on load bearing capacity compared to experiments of natural sized walls, were realized.