Water Leaks in Houses: When the Cause of the Leak is not one of the Usual Suspects

Water Leaks in Houses: When the Cause of the Leak is not one of the Usual Suspects

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When a water leak happens in a house, we usually look to one of the usual suspects: the roof, the windows or the doors. It’s often easy to overlook one of the lesser known culprits: the exterior walls.

The exterior walls of any building must be designed and constructed to keep rain and snow from entering the building. A common source of moisture exposure to exterior walls is wind-driven rain.

In typical Ontario wood-frame construction, the exterior walls of houses are built with “two planes of protection” from rain (and snow) exposure. The first plane of protection is the exterior wall cladding. Some of the common types of exterior wall cladding found on houses in Ontario are siding (vinyl, aluminum, wood), stucco and brick/stone veneer. These building materials block and re-direct the bulk of the precipitation that hits the wall.

The second plane of protection typically consists of a membrane that is applied to the exterior surface of the wood framing of the home, behind the cladding. This membrane is commonly referred to as the “moisture barrier”. The purpose of the moisture barrier is to protect the wood wall frames from being exposed to any moisture that makes it through the exterior cladding, and to re-direct that moisture to the exterior of the building. Common types of moisture barriers seen in Ontario homes include asphalt felt paper, commonly known as tar paper, or a synthetic sheathing wrap, commonly referred to as house wrap.

As an example, let’s look at one of the more common types of exterior wall assemblies: a wood-framed wall with brick veneer cladding. How does this wall system work to prevent water from entering the house, and what are the common types of problems that we encounter with it?

The brick veneer assembly, including bricks and mortar, is the first plane of protection. The majority of moisture that hits the brick wall will be prevented from entering the building by the brick/mortar. Brick (and mortar) can absorb and allow moisture to pass through it. This is why the second plane of protection, the moisture barrier, is so important. It protects the wood framing of the house from the water that gets past the brick veneer. The moisture barrier is installed against the wood framing of the house, and should be continuous and free of gaps or openings, so that water cannot get behind it.

In a typical brick wall assembly, the brick and moisture barrier are separated by a 25 mm (1”) air space. This air space (or “brick cavity”) provides a path of travel for any moisture that makes it past the brick to find its way to the bottom of the wall. At the bottom of the wall, openings (called weep holes) are constructed in the brick mortar joints. These openings allow the water in the brick cavity to drain to the exterior. They also allow air into the cavity, to help equalize the air pressure between the exterior and interior of the house, which also helps to prevent water migration through the brick.

A layer of flashing should also be installed at the bottom of the brick cavity, to direct the water towards the weep holes, and to prevent it from coming into contact with the wood wall framing. This diagram shows how a brick wall is constructed and functions.

Common problems with brick walls include gaps or holes in the brick assembly, or damaged and deteriorated bricks (and mortar). These kinds of problems are usually very easy to identify and repair, since they are often visible from the exterior of the home.

Problems with the moisture barrier or airspace detailing are much more difficult to identify, since they are usually hidden behind the brick veneer. Moisture barriers and flashing that are improperly installed, missing, or have been torn and damaged during construction, can have gaps and holes, which could lead to water leaks into the house. It is important that the moisture barrier is properly installed around door or window openings. An improperly installed moisture barrier around a window or door opening can cause leaks at these locations, leading homeowners and insurance professionals to mistakenly suspect the windows or doors themselves as the culprit.

Problems with the air space construction can also lead to water leaks. Air spaces can become blocked when too much mortar is used in the brick construction. The excess mortar overflows into the airspace, where it hardens. When water enters the brick cavity, it can pool on the hardened mortar, and subsequently leak into the house. Also, excess mortar at the bottom of the wall can block the weep holes, preventing water in the brick cavity from draining to the exterior.

It’s not always easy to tell if water leaks are caused by an improperly constructed wall assembly. That’s where Roar Engineering can help. We have investigated several water leak cases, and have the expertise to determine the cause of water leaks into your home.

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