Reliability Engineering of the Exterior Wall Envelope – Parallel Versus Series Drainage Systems

By Ed Wrenn

February 6, 2014


There is a need for the design of exterior walls to incorporate the concept of reliability engineering and parallel systems to improve functionality of drainage systems for the lifetime of buildings.  In membrane/drainage systems, the exterior building envelope contains the drainage wall system which consists of the cladding as the primary water barrier and a secondary drainage layer (weather-resistive barrier) within the wall. Examples of claddings utilizing membrane/drainage systems include stucco, drainable exterior insulation and finish systems, manufactured stone veneer, vinyl siding, and wood sidings.  Typically, there are multiple systems to remove water/moisture from the exterior wall system, though this paper will only address liquid flow to the exterior. Currently, the exterior building envelope is usually designed and installed in a manner where failure of one drainage system allows water intrusion and cumulative water damage to building components.  Design standards and reference materials do not address the need for redundant flashing and drainage component installation to account for any failure of a single element. If drainage elements were installed in parallel, then failure of one system would be mitigated as another element could then drain water to the exterior of the building and prevent damage.

Reliability engineering and parallel systems

Exterior wall systems should incorporate the concept of reliability engineering and utilization of parallel systems to ensure proper functionality of drainage systems.  Reliability engineering of water management systems can significantly improve the chances of preventing failure of the drainage system(s), damages due to water intrusion, and allow the structure and its associated components to meet its expected lifetime.  Water intrusion is water that penetrates beyond the weather-resistive barrier (WRB) of the wall in such a volume that it cannot be removed by the mechanical system.  This water has the potential to cause deterioration of sheathing, framing, interior finishes, and furnishings [1].  Buildings should be designed and constructed in such a manner that drainage components are configured in a redundant (or parallel) manner to increase reliability of water management systems.

Reliability is defined as the probability that an item will adequately perform its specified function for a specified period of time under certain environmental conditions [2]. The reliability of a system of items is the probability that the system will remain functional and not fail. Reliability is often mistaken with quality.  Whereas quality is a static metric, reliability incorporates performance over a period of time, which is much more applicable to building performance.

Water intrusion causes damage which reduces the lifetime of materials and can lead to costly repairs.  Significant damages within walls can also remain hidden for long periods of time augmenting damages and potentially resulting in emergency repairs.  Buildings must be designed and built to resist water intrusion for as long as the designed life of the building or cladding components, which is generally twenty (20) years or more. In order to meet this requirement, designing and installing parallel water drainage systems is a more reliable method because if one system fails or is improperly installed, another will continue to drain water from the wall system.

The reliability and probability of failure of the exterior wall envelope drainage systems is not specifically addressed by the current design guidelines and literature.  Many standards are focused on limiting the precipitation that strikes exterior walls and then penetrates to the weather-resistive barrier.  Though there is some guidance stating to use systems proven to drain effectively in the appropriate climate, very little of the design guidance mentions flashings or drainage systems in parallel to create redundancy.  The only statement that promotes parallel drainage systems is from ASTM International’s E241-04 Standard Guide for Limiting Water-Induced Damage to Buildings, which states that the use of a combination of moisture control strategies is usually the most effective [3]. There was some effort in the wake of exterior insulation and finish barrier systems failures to provide designs utilizing redundant drainage planes [4].  Some of this guidance recommends multiple layers of WRB, but these layers are typically not all installed properly integrated with the flashing and drainage system and do not provide proper redundancy. Additionally, multiple layers of WRB would not be considered true redundant systems as they are the same material in a similar location experiencing equivalent conditions, thus each layer is not a system independent of the other layers.

Parallel Systems

The key benefit to a parallel system is that it will function if one (1) or more of its components function, whereas a series system will only function if all of its components are operational.  From a reliability engineering standpoint, it is imperative to install drainage components as parallel systems so that a failure of one system would not result as a system failure causing water intrusion.  It is important to note that installing flashings at every floor line of a multi-story building does not create components installed in parallel.  Typically, flashings would be the same material from the same manufacturer, the same construction drawings would apply for each set, and they would be installed similarly.  Due to these similarities, the failure of one set would make additional failures more likely.  The components of a parallel system must behave independently.  When a building is designed and constructed,  parallel drainage systems will provide redundancy through incorporating components that behave independently, thus maximizing the probability of water being shed to the exterior thus providing long term protection to the building.


Current design and installation standards should incorporate the concepts of reliability engineering in order to provide buildings that meet or exceed their expected lifetimes.  If one system fails, there should be a parallel component(s) to drain the resultant water and prevent damages to the structure.  One example of a cladding system incorporating redundant design would be a pressure equalized rainscreen (“PER”) system.  Simplistically, the PER is three (3) systems in parallel – the cladding / WRB / flashings, the capillary break formed with a cavity, and venting of the cavity removing moisture via air flow.  The current mindset is that this type of system is a “high performance” cladding [5], whereas redundancy really should be an integral part of any structure.  Redundancy will increase costs: however, there may be lower cost ways to implement redundancy and the project team should consider cost versus performance.  Incorporating redundancy and the concept of parallel systems can help ensure buildings that perform properly and significantly decrease the chances of failure of the drainage system, even when design or installation errors have occurred.


List of References

[1] ASTM International, E2266-04 Standard Guide for Design and Construction of Low-Rise Frame Building Wall Systems to Resist Water Intrusion, 2004.

[2] Lawrence M. Leemis, Reliability – Probabilistic Models and Statistical Methods, Second Edition, 2009.

[3] ASTM International, E241-04 Standard Guide for Limiting Water-Induced Damage to Buildings, 2004.

[4] Charles W. Graham, Wall Design Redundancy for Improving the Moisture Performance of Building Cladding Systems in Hot-Humid Climates, Texas A&M University, 2000.

[5] David Altenhofen, Rainscreen and back-ventilated drained cavity wall systems: practical applications for high performance buildings.

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