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In the top photo in Figure 1, I'm conducting a sophisticated engineering test on the rain holdout characteristics of a cladding material. I'm spraying the brick veneer wall of a garage with a garden hose. I have a wager going with people inside the garage -- I'm asking them to guess how long it will take, under the conditions of this test, for water to pass through the brick veneer. It's a multiple choice test: A) seconds, B) minutes, or C) hours.

The correct answer is A, between 15 and 30 seconds. Brick veneers are essentially transparent to water. We like to think, "Hey, brick is strong. It can handle water." And it can -- in the sense that the water doesn't damage the brick. But brick is just like a sponge: It sucks water in by capillary action, holds some of it, and lets the rest drool out everywhere. Brick can protect our walls from many things, but water is not one of them.

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Figure 1.In this hose test, water penetrated brick veneer in less than a minute (bottom). The wall behind brick veneer must be designed to drain.

Wood clapboards, fiber cement siding, and vinyl siding also leak, each in its own special way. People continue to put their faith in every kind of cladding material, but in the real world all claddings leak sooner or later. They always have, and they always will.

The common fallback strategy is to use caulks and sealants. But that's not the answer, either. Sealants do not span cracks, they cannot withstand movement, and they will degrade from sunlight, temperature, and oxidation. Caulks dry up, they shrink, they freeze and crack, they decompose -- they fail. When your cladding leaks, neither caulking nor sealants will keep water out of your building.

So if all claddings leak, and all sealants and caulks fail, how can we keep buildings dry? By creating a water management system beneath the cladding: a continuous drainage plane with integrated flashings and weep holes, with an air space between the cladding and the drainage plane where water can flow (Figure 2). We overlap everything to direct water down and out, and we let gravity do the work.

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Figure 2.The key elements in rain control are a drainage plane, a drainage space under the cladding, flashings that integrate with the drainage plane, and weep holes to allow water to escape. These principles can be applied with any cladding system and with many different drainage plane and flashing materials.

Unlike caulks and sealants, gravity is free. Gravity does not wear out, and it does not require maintenance. Gravity is predictable: It always acts downward. In water-managed wall systems, gravity is one thing we can trust.

You can build a water-managed wall system with any cladding and with many different kinds of drainage plane and flashing materials. It will work as long as you follow certain basic principles. Human beings discovered those principles thousands of years ago, but some people still don't follow them; that's why people like myself, who investigate building failures, have such wonderful job security.

Elements of a Rain-Managed Wall

There are four fundamental requirements for water-managed assemblies:

* Drainage plane: some water-repellent material, overlapped to drain downward, and continuous over the whole building exterior. "Continuous" is the key word here. All it means is that you should connect your windows and doors to the drainage plane, as well as your deck, your roof-wall intersections, any service penetrations -- everything. Every single flashing must tie into the drainage plane and dump on top of it, not behind it. There are no exceptions: One reverse lap or unflashed penetration can ruin your whole wall.

* Drainage space between the cladding and the drainage plane (the space can be very narrow, but it must be there). The water needs space to move.

* Flashings at every opening, penetration, or intersection, designed to kick water out and down.

* Weep holes: openings to allow water to escape to the outside.

If you have those four elements, you have a water-managed system. If you're missing any one, or you do any of them wrong, you can expect trouble.