Water Always Wins
NOVember 7, 2017
When an architect is focusing on detailing the exterior envelope of a building, they must respect rule number one, “water always wins”. An improperly detailed envelope is an opportunity for water to intrude and damage finishes, underlying structure, and cause metal surfaces to rust prematurely or promote mold growth.
For buildings that experience all four seasons, detailing the exterior envelope is tough, since water or water vapor can become frozen, thus the architect must respect water in all three of its physical states. Unlike most liquids, water expands up to as much as 9% of its volume when it freezes. This little fact makes a big difference because if water gets inside the envelope and freezes it can exert pressure in different directions and make small openings larger.
Water weighs how much?!
When it comes to water and your roof, it’s important to get water off quickly. Water weighs 8.35 pounds per gallon. Imagine a small 2500 square foot flat roof during a rainy day. One inch of rain equals approximately 1,558 gallons of water, which represents approximately 13,000 pounds of water weight. If the roof drainage isn’t designed properly this water weight can cause ponding to occur. Too much weight in one area can lead to a structural collapse and a very large hole in the roof.
In winter many residential homes with a shingled roof can experience icicle growth and ice damming along the eaves. While at times this might occur simply due to cycling temperature swings, it could also occur from an improperly designed roof. Ice dams typically form when a ‘hot roof’ melts snow. This water then freezes at the ends of the roof when exposed again to freezing temperatures. As the roof melts more snow, the water rolls down and joins the growing patch of ice. This can form large dangerous icicles or work its way back up and under the roof itself, pulling things apart thanks to the expansive properties of ice.
It gets everywhere....There are plenty of opportunities for liquid water and water vapor to enter the wall assembly and cause damage. Liquid water generally finds a path into the exterior wall through poorly detailed windows, doors, and/or vent penetrations. Interior rooms with a lot of water sources such as bathrooms and kitchens are areas where water can get into a wall. Of course, these rooms have a lot of water vapor and the warm, moist environment is ideal for growing mold. Without proper ventilation mold can grow to be a health problem. A balanced ventilation system with the correct vapor retarder and weather barrier can help keep most of the water and vapor from entering the walls.
While these are just a few examples of what could go wrong, there are more examples of bad details due to a unique site, climate, assembly or combination thereof. A good architect knows this and won’t simply rely on details from previous unrelated projects to solve problems. A great architect will thoughtfully develop the right detail and constantly reevaluate those details to meet the needs of building owners and maintenance team. These details matter because water has the ability to move around a building and cause damage. These damages cost more money to fix later in life than if constructed properly from the start. Remember, if given the opportunity . . . water will take it because water always wins.
Written by: Jason Arnold AIA NCARB CDT, Associate