BSP-020: Water Management Details for Residential Buildings
Section 1: Water Management Concept
Controlling rain and ground water is the most important factor to designing a durable building. One of the best methods for controlling rainwater is by ensuring that water is effectively drained down and out of the building and assemblies. The various levels of drainage that are considered are as follows:
- Drain the site
- Drain the house
- Drain the assembly
- Drain the opening
- Drain the component
- Drain the material.
Gravity is the driving force behind drainage, and while it can be of great use to us, if the details that we create to manage water are poorly designed, poorly installed, or deteriorate, it can create severe problems as well.
Risk Factors
The severity of the failure is based on several risk factors in the design and construction of the building.
- Water Resistance: As the water penetration resistance of the assembly increases, the risk of moisture problems decreases. Risk = Low
- Moisture Tolerance of Assembly: As the moisture tolerance of the materials that comprise the assembly increases (masonry and concrete vs. wood and steel) the risk of moisture related problems decreases. Risk = Low
- Exposure: As the exposure to rainfall increases, the risk of moisture related problems increases. Risk = High
- Rainfall: As the amount of rainfall increases, the risk of moisture related problems increases. Risk = High
- Drying Potential: As the ability of an assembly to dry increases due to the climate, design, or both, the risk of moisture related problems decreases. Risk = Low
- Workmanship: As the craftsmanship of the construction of the assemblies and their connection details increases, the risk of moisture related problems decreases. Risk = Low
Some of the factors we have limited to no control over. Factors such as rainfall and exposure are often facts of life that we need to accommodate in how we design. We can reduce some of the risk with the use of water resistant (or tolerant) materials and assemblies, or by maximizing the drying potential of the systems. When it comes to liquid water intrusion however, these strategies can only protect us to a point.
Some of the most severe failures occur due to poor design or installation of water management details. Blame cannot be put on one person or group for these failures, they are often a combination of tricky or difficult installation sequences, a lack of understanding of what is trying to be achieved through the detail, time pressure, and occasionally unforeseen reactions between materials. While in some locations in the assembly greater tolerances can be endured, there are critical points in any design where extra care must be taken and more refined tolerances observed in order for the system to function properly for the long term.
Building Science Corporation has been working for years to develop simple, effective, and low cost methods to provide effective water management for houses. These details have been incorporated into numerous homes built under the Building America Program. Through these numerous installation, guidance for proper installation and areas that need special care, has been developed based site inspections of common reoccurring problems and improper installations, as well as from field-testing of different assemblies.
