In his latest “Building Science Insights” article, Joe Lstiburek addresses rain control, tackling three general approaches to managing the water flow – draining it, blocking it and storing it – depending on the type of cladding used.
This article should be required reading for all members of the crew, as well as the crews of your siding and roofing trade partners. Like a lot of Lstiburek’s work, it builds a comprehensive view of things – in this case, of the all-important task of any building: controlling bulk water – beginning with the practical basics that have been part of our building tradition, and continuing point by point through the critical details that make each type of cladding assembly – drainable, barrier and mass work.
Here’s an excerpt from the first part:
Reducing the amount of rainwater deposited on building surfaces and assemblies has traditionally been a function of siting and architectural design. Old timers view all of this as common sense:
• Site buildings so that they are sheltered from prevailing winds, to reduce exposure to wind driven rain.
• Provide roof overhangs to shelter exterior walls from rain deposition.
• Provide architectural detailing that sheds rainwater from building faces.
• Slope sites away from buildings.
The next bit requires an understanding of the physics of rain entry. Again, old timers view this as obvious. Five typical driving forces are responsible for rain entry:
• Gravity (hydrostatic pressure).
• Surface tension.
• Capillarity.
• Momentum, and
• Air pressure differences.
Each of these driving forces can act independently or in combination. And again, old timers figured out pretty simple approaches to dealing with them …