A Systemic Approach to Building Component Roles and Failures

When analyzing the failure of a component, whether part of a building, mechanical system, or other type of system, its role in the long-term performance of the system must be considered. Buildings, for example, are single objects made up of thousands of parts. Each component is a portion of a specific system and is responsible in part for the overall performance of that system. For building construction, these systems can be broken into several global categories: structural, electrical, mechanical, building envelope, and so on. Some components have more than one role. A roof deck, for example, can be part of the structural system to convey loads to the purlins, beams, and columns and also is part of the building envelope. 

A good example of a building component taking on two roles is the exterior wall sheathing behind a masonry veneer. The sheathing attached to the studs provides the shear resistance from wind and seismic loads. When wind is pushing in one direction on a roof, imagine that the walls are resisting that force. Sheathing can be made of oriented strand board (OSB) or plywood as well as other products. 

During a flood event, sheathing might get wet for a few hours or maybe for days. We know from research that plywood and OSB that have an Exposure 1 rating hold up pretty well in flood events, provided that they are allowed to dry adequately. For fiber backerboard and paper-backed gypsum products, the residual strength after a flood might not be as straightforward. 

For repairs, one option is to remove the veneer and replace the sheathing, which is costly. From a purely structural standpoint, another option would be to put the sheathing on the inside of the wall. The forces we are trying to resist do not really care which side of the wall you put it on. Sure, your living-room might be a half-inch smaller, but it is a lot less work than replacing the masonry. There are some other things a structural engineer might suggest in order to compensate for the unknown shear resistance of the exterior sheathing. These might include diagonal braces, modified fastening patterns of interior wallboard, blocking, and proprietary shear resisting panels.

So we have figured out a way to compensate for the potentially reduced strength of the sheathing behind the brick. Done, right? Not really. What else is the sheathing doing? If the lateral force resistances were achieved by other means as mentioned above, could we just have studs then masonry with nothing in between? 

That might work in the Atacama Desert, but in most places in North America, there is at least some annual rainfall. Brick and stone veneers are porous, and though they shed a lot of rainwater, some water still enters the system. These exterior wall assemblies rely on the wall cavity and the weather-resistant barrier behind the brick to keep water out of the exterior walls and direct it to thru-wall flashing and weep holes toward the exterior of the structure. What does this have to do with the sheathing? That is what the weather resistance barrier is attached to. This sheathing fulfills a dual role of structural component and building envelope component. 

This distinction is critical because it plays a role in the decision-making process with respect to recommendations and scope of repairs for flood-related losses. Some might opine that the exterior sheathing should be removed from the interior (without removing the veneer) and that rigid insulation should be filled in between the studs. This condition does not meet either of the two roles discussed above. It does not provide the structural component of providing lateral force resistance, nor does it provide a continuous drainage surface for the proper function of the weather-resistant barrier. Had the wallboard been left in place and allowed to dry, the drainage component would be intact, and the strength component could be dealt with on the interior as mentioned above. But how does the exterior face of the backerboard dry out? The nice thing about the porous nature of the masonry is that it works in both directions. The masonry gets warm during the day and dries out the wall cavity. Keep in mind that there is a little different thought process when the exterior finish is siding or an exterior insulating finish system. 

When assigning a loss to an expert, make sure the scope of the loss is discussed and the expert understands what question is to be addressed. When assessing a property after an event, it is important to be mindful of the integrity of the different building systems that are in play and also to be aware of components that have more than one purpose. Each of its roles must be addressed in order to ensure that the property is put back to a safe, pre-loss condition that is compliant with governing agencies having jurisdiction over the property.