Properly installed wall flashing is critical to your building’s health. Many builders and designers believe that inadequate vapor barriers are the main cause of moisture problems and wood decay in walls. However, the vast majority of water problems in walls are caused by water leakage from the exterior — not by poor vapor barriers.
These leaks are primarily around windows and doors, but also occur at other penetrations and joints in the siding and trim. The long-term solution is proper flashing. Caulking can help in some cases, but never lasts for long and in some cases can do more harm than good. See Window Flashing for details.
Building corners are prone to water leakage due to wind exposure and shrinkage of the corner boards. A simple approach that works well is to add a spline of asphalt felt paper at outside corners that extends 6 in. beyond the corner boards. The asphalt felt paper should go over the sheathing wrap, which should also wrap the corners at least 6 in. each way. Inside corners also benefit from a spline (see Illustration).
WATER TABLE FLASHING
Many traditional homes have a wide board called the “water table” that runs around the house along the foundation and supports the first piece of siding. The water table should overlap the foundation by about 1 in. and be capped on top with either a preformed metal drip cap or a custom flashing installed under the sheathing wrap. Cut a slit in the sheathing wrap along the entire length of the water table and slip the upper leg of the flashing under the wrap (see Illustration).
Metal termite shields are still widely used as a physical barrier to termite entry at the top of foundation walls in the southern US and in tropical climates. The metal barriers sit directly on top of foundation walls, piers, and other supports before the first piece of wood is installed (see Illustration).
Subterranean termites. Termite shields were originally developed to block the entry of subterranean termites, the most widespread and harmful wood-destroying insect in the United States. The termites nest in the soil outside the home and enter the building to feast on the wooden portions of the house. Unfortunately, the tenacious creatures will find and penetrate the tiniest gaps in termite shields to reach the wooden portions of a house.
Inspect for tunnels. Since subterranean termites cannot tolerate open air for long, they build mud tunnels from the soil to the house, along exposed foundation walls and around termite shields if necessary. Although the shields rarely stop termites, they impede their progress and force them to build tunnels that are easily seen during inspections.
Seal all joints and penetrations. To be effective, termite shields must have tightly sealed joints and be sealed around foundation bolts and other penetrations. Joints should be either soldered or mechanically interlocked. If the metal barrier is not tightly sealed at joints and penetrations, termites will easily bypass the shield.
Termite shield details. Termite shields should be at least 6 in. above grade and extend out 2 in. on either side of the foundation downward at 45 degrees. In addition to making infestations visible, termite shields also form a capillary break between the foundation and sill, protecting the sill from moisture.
Trouble spots. Areas that are hidden from inspection and where termite shields cannot be used are at high risk of infestation. For example the crevice between concrete stairs and the foundation is a common entry point. The same problem occurs where a raised porch or patio slab attaches to the main building. Another common entry point is where stucco or exterior foam insulation extends below grade, providing a perfect hidden entry route for termites.
Chemical treatment. In termite-prone regions, the only reliable way to prevent termite damage is to use treated wood in critical locations and treat the surrounding soil with termiticide. Modern baiting systems are more costly than traditional treatment, but use smaller quantities of less toxic materials, making them appealing to many homeowners.
DECKS & LEDGERS
Most backyard decks are supported along the house by a “ledger,” a length of framing bolted or nailed (not a good idea) to the house. It is critical to protect this area from water problems, since decay in the ledger or the framing it attaches to can cause a deck to collapse.
For water protection here, at a minimum install a cap flashing that tucks under the sheathing wrap and goes over the ledger. Adding a second flashing, either peel-and-stick membrane or metal flashing, between the sheathing and ledger, as shown, is a worthwhile backup should any water get over, around, or through punctures in the cap flashing. Since pressure-treated wood is corrosive to aluminum, it’s best to use membranes, plastics, or corrosion-resistant metals such as copper or lead-coated copper in direct contact with pressure treated wood. Read more on Flashing Deck Ledgers.
All flashings must integrate with adjacent materials, so water always flows to the exterior, following the shingle principle. Where a slope roof meets a sidewall, slip the upper legs of the step flashing under the sheathing wrap (see Illustration).
In areas where snow may build up, add a wide band of peel-and-stick membrane over the step flashing but under the sheathing wrap, as shown.
Where the step flashing terminates along a sidewall is a common trouble spot for moisture problems. The best solution is a preformed or custom-bent kick-out flashing to divert the water away from the siding. Otherwise, dripping water will leave its mark along the siding.
Where wood or composite siding meets the roofing or any other horizontal surface, leave a gap of 1 to 2 inches to protect the siding from moisture damage. The end grain of wood siding will wick up moisture, leading to paint failure and wood decay. The risk is greatest in cold climates where snow buildup can keep the area wet for extended periods (see photo).
In wall areas frequented wetted by splashing rain (splashback) or snow buildup, flashing membranes up to 36 inches wide can be used to protect the wall sheathing and structure. Water damage from splashback is common in wall areas located:
- under the eaves of a roof with little overhang and no gutters
- where an attached backyard deck meets the house
- above a lower roof where it abuts the wall
In all cases, make sure to detail the flashing membrane so that it sheds water. It should tuck under the sheathing wrap above and over the flashing below. If installed along the foundation, the membrane should cover the joint where the sill meets the foundation.
Do not apply more membrane than is needed to protect the wall – up to 3-feet wide in climates with substantial rain or snow. Applying too much will create a cold-side vapor barrier, which can lead to serious moisture problems in cold climates.