![]() ![]() But, I bet you didn’t know that the main reason we care about controlling airflow is not about drafts, energy efficiency, or air quality (though those are all important). They’re used to-you guess it-control airflow. You’ve probably heard about air barriers. That same pinhole under a few inches of standing water will leak profusely. A pinhole in the roof membrane probably won’t leak much if the water flows by quickly. The reason is that slope to drain makes deficiencies in the water-control materials less likely to cause an actual problem. Generally at least 1/4 inch per foot is a good rule of thumb, but more is better. Makes sense, right? Unless you’re building a swimming pool, make everything slope to drain. Slope to drain means that the surface that water is flowing on is pitched, or sloped, towards a drain, roof edge, flashing edge, or other location where you want the water to go. When the lap finally fails water will have an entry point, as opposed to a properly shingled lap which does not require any type of adhesive or sealant to shed water. Even when materials are well-adhered or sealed, a lap that bucks water is more likely to deteriorate over time. This seems obvious when talking about shingles, yet often materials like membrane underlayments are installed in such a way as to “buck” water. When one material laps onto another, the higher one should lap over the lower one-like the shingles on a roof. Use this knowledge when designing and building. It’s not hard to figure out how water will move. And you’ll be less likely to end up on the wrong end of lawsuit. Keep water from getting where it is not supposed to be and your buildings will last longer and cost less to maintain. Mold, rot, and corrosion are big problems that cost big money to fix. But people are getting sued all day long over water leakage that causes mold, rot, and corrosion. It’s pretty rare that someone gets sued because a building is drafty or ugly. The most important job of the building envelope, however, is to prevent water from getting to where it is not supposed to be. The building envelope has a lot on its plate, from reducing energy use to aesthetics. Start with the code and manufacturer’s instructions.Match the expected service life of adjacent materials.Think of these principles as a roadmap-an algorithm-that will always guide you to the right destination. There are a few key principles that can guide you to the answer to almost any building envelope question. It’s meant to be a helpful resource so we welcome feedback on if it’s accomplishing that goal, or on ways we can make it better. Note: This page is an ever-evolving work in progress. ![]()
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