13.2 Waterproofing, Flashing & Sealants

Everything Below the Roof

Quick Answer: Water management on a building is a system of planes and laps: a weather-resistive barrier behind the cladding, flashings at every interruption, weeps to let water back out, and sealants at the joints. The rule that governs all of it is that every layer laps over the one below, shingle-fashion, so water always travels out and down.

Section 12.3 covered insulation, vapor retarders, and air barriers. This section covers the wet side: how water is kept out of walls and out of below-grade space, and why it gets in anyway.

Dampproofing Versus Waterproofing

These are two different specifications and the exam tests the difference.

The code's logic is straightforward: dampproofing is the default for a foundation wall enclosing habitable or usable space, and waterproofing is required where groundwater will actually stand against the wall. Specifying dampproofing on a site with a hydrostatic condition is a design error the contractor should flag rather than build.

Both depend on drainage. A perimeter foundation drain in clean gravel, wrapped in filter fabric, discharging to daylight or a sump, does more to keep a basement dry than any coating. Arizona sites add a wrinkle: caliche layers perch water, so a site that is bone dry in May can hold water against a wall for days after an August storm.

The Drainage Plane and Through-Wall Flashing

Modern wall assemblies are not assumed to be perfectly sealed. They are designed so that water that does get past the cladding is intercepted by a weather-resistive barrier (WRB), carried down, and discharged back out.

Making that work requires three things:

1. Continuity. The WRB is continuous, with laps and seams sealed as the manufacturer requires.
2. Correct lapping. Upper layers lap over lower ones. Reverse-lapped housewrap at a window head funnels water into the wall — the single most common wall-leak cause in residential construction.
3. Integration with flashings. Every penetration and interruption gets a flashing that ties into the WRB in the right order: sill pan first, then jambs, then head flashing tucked under the WRB above.

Through-wall flashing appears at the base of masonry walls, at shelf angles, above and below openings, and at roof-to-wall intersections. It requires:

• End dams turned up at each end, so water cannot run off the end of the flashing into the wall.
• Weeps at the flashing line so collected water exits — typically spaced about 24 to 33 inches on center in masonry, formed with rope wicks, open head joints, or manufactured vents.
• A clear cavity. Mortar droppings that bridge the cavity and cover the weeps defeat the whole assembly. Mortar-collection mesh at the flashing line is cheap insurance.

Sill pans at window and door openings are the residential equivalent, with the same rules: back dam, end dams, and sloped to drain outward.

Sealant Joints

Sealant is not glue and it is not a substitute for flashing. It is a movement-accommodating closure, and it works only if the joint is designed.

For Arizona exterior work, the combination of intense ultraviolet exposure and 40-degree daily temperature swings argues for high-movement silicones or high-performance urethanes at exterior joints. A cheap acrylic caulk at a stucco control joint is a one-summer product.

Warning: the two failure modes have different fixes. Adhesive failure — sealant peeling cleanly off the substrate — means preparation or primer was wrong. Cohesive failure — sealant splitting down its middle — means the joint was undersized or the sealant lacked the movement capability. Refilling the same joint with the same product repeats the failure.