Most water control layers are composed of a mixture of waterproof and water-resistant materials, employed to meet the needs of a particular climate in a particular position on the building.
Exposed and hidden layers Although we tend to refer to the water control layer in the singular, its importance often warrants two (or more) layers of protection. This is common in modern construction on the roof, where the water control provided by the roofing material is backed up with a water control layer in the form of roof underlayment, and on walls, where the exterior cladding is often installed over a weather resistant barrier (WRB). These hidden layers of water control are often critical components in maintaining water protection. They are especially important for providing protection at punctures, penetrations, and transitions such as windows and skylights because they provide a surface for adhering tapes, caulking, and other sealants that protect those joints from direct weather and UV exposure.
Sealing of openings/penetrations Punctures in the water control layer (such as an electrical service entrance) must be properly flashed or sealed. Modern construction offers a range of tapes, sealants, and membrane materials that can be used to ensure water is kept out at these vulnerable locations.
Using caulk wisely Caulking is often used as an important part of the water control layer, and if a quality caulking is properly applied in the right conditions, it can play an effective role in the water control layer. Caulked joints should be sized properly not too small, and if large, a backing rod should be used. A high-flexibility and UV-resistant caulk is preferable. Always ask yourself what happens to water when it makes its way past the caulking, and be sure the answer is adequate enough to trust with the long-term health of your building.
Positive lapping The elements of a water-resistant layer are always installed in a “shingle lap,” with materials lower in the assembly covered by materials higher in the assembly. Bulk water is encouraged to continue to run off at seams, rather than behind the next layer.
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Drip edges While positive lapping encourages water to be shed, the phenomenon of surface tension can cause water to move against gravity behind lapped joints. Drip edges are flashings that cause water to bead and fall away from the building instead of creeping in behind. They can also be incorporated into parts of the assembly, such as window sills or trim.
Prevent capillary transmission Moisture barriers are employed between the soil and foundation materials and between foundation materials and walls and flooring, to prevent water movement into assemblies via capillary action.
Design for water control Roof overhangs, gutters, downspouts, site grading, and foundation drainage, while not directly part of the water control layer, are critical components of a water control system because they minimize the amount of water that needs to be controlled in more vulnerable areas.
Design flood-smart Design your building to account for known flood plains and predictions for 100- or even 200-year flood events.