Base flashing masonry covering

In the basement of a residential building, water infiltration can occur at the top of the foundation walls, specifically at the junction between the sill plate (made of wood) and the foundation wall. In most cases, the cause of this type of water infiltration is improper installation of the base flashing for the exterior masonry or other cladding. We have observed this problem in several latent defect cases.

The base flashing installed on the foundation walls must begin on the exterior face of the foundation wall and extend up the top of the foundation wall. The flashing must then extend at least 6 inches up the intermediate sheathing of the exterior wall, behind the air barrier. Therefore, the air barrier must be positioned in front of the base flashing (on the exterior side).

Deficiencies

There are many deficiencies encountered in the installation of this basic flashing, including:

1. The horizontal part of the flashing is too short so that we cannot see this flashing at the base of the siding;
2. The vertical part is short behind the air barrier membrane;
3. The vertical part is in front of the air barrier membrane when looking from the outside;
4. The vertical part of the base flashing is slipped behind the intermediate sheathing (OSB, Tentest or other);
5. The total absence of the base flashing and the air barrier membrane which is extended vertically on the course of the foundation wall as a base flashing;
6. The base flashing is holed or torn;
7. In older buildings, the base flashing is made of black paper that has decomposed and lost its integrity.

Waterproofing exterior coating

Various exterior cladding materials – including bricks and other masonry – are not 100% waterproof. A certain amount of water can penetrate the exterior cladding, towards the back, through mortar joints, cracks, missing or cracked sealant joints, etc. The water then runs down towards the base of the cladding and drains away from the building envelope , through weep holes if it is masonry , and through the drainage space at the base of the cladding if it is not masonry.

The base flashing directs water away from the building and prevents it from entering the wood structure installed at the top of the foundation walls. If there is a problem with the installation of the base flashing or if there are deficiencies in its watertightness, water will seep into the wood structure and flow down to the basement (from the top of the foundation walls). Water contacting the wood structure will damage it, eventually causing it to rot. This situation compromises the structural integrity of the exterior walls and attracts carpenter ants. It also leads to mold growth on the building materials in the basement.

This problem can be costly to repair, depending on the extent of the problem and the degree of rot—in the wood structure—and the degree of contamination in the basement. This requires removing the base of the exterior siding and replacing the affected structure, as well as redoing the base flashing and waterproofing (at the base of the siding). In addition, inside, rotten wood elements that can be removed from the interior, such as ceiling joists and beams, will need to be replaced. The basement will also need to be decontaminated and refinished.