Watch This Video Before Building on Existing Patio or Concrete Slab - Exterior Water Leak Prevention
This video tackles a common and often underestimated challenge in construction: preventing water leaks when erecting a new structure, or an addition, on top of an existing flat concrete slab. The core issue is that the original slab wasn't designed with the new walls' water management needs in mind, leading to potential pathways for moisture intrusion.
Understanding Standard Water Management (and Why Existing Slabs are Different)
To appreciate the problem, it's helpful to understand how water is typically managed in new construction where the foundation and walls are built together:
Exterior Water Diversion: Standard practice involves multiple layers of protection. Materials like stucco and siding are the first line of defense. Below them, a weep screed (a type of flashing at the base of the wall) and building paper (or house wrap) create a drainage plane. This system is designed to catch any water that gets behind the siding and direct it outwards, away from the wooden framing and foundation. Crucially, this system usually extends below the bottom of the wall framing and often overlaps the top edge of the concrete foundation, ensuring water sheds away from the joint.
Proper Grading and Clearance: Building codes typically require the ground level to be at least 6 inches below the bottom of the stucco/siding or the top of the finished floor. Furthermore, the ground should slope away from the building. This prevents rainwater from pooling against the foundation and wicking upwards or finding entry points. Concrete elements like patios or walkways adjacent to the building should also have a clearance (the video mentions a 2-inch minimum) from the siding to prevent splash-back and moisture trapping.
The Challenge with Existing Flat Slabs
When you build new walls directly on an existing flat slab (perhaps a smaller building on a larger old foundation, or an addition on an existing patio slab.
No Integrated Drainage Plane: You lose the continuous, integrated drainage plane that would normally be created between the new wall and a new foundation. The bottom of your new wall is now sitting directly on a flat surface.
Water Pooling and Intrusion: If the existing slab isn't perfectly sloped away from the new structure (or even if it is, during heavy rain), water can pool against the base of the new wall. Without a proper transition or flashing detail, this water can easily seep inwards through the joint between the new wall's bottom plate (the lowest wooden member) and the concrete slab. It can also exploit any cracks or imperfections in the slab itself.
Hydrostatic Pressure: If water accumulates against the base of the new wall on the slab, it can build up hydrostatic pressure, literally pushing its way into the building.
Video's Solution and Additional Considerations
The video highlights the primary solution: meticulously sealing the joint at the bottom of the new structure where it meets the existing slab.
Why Sealing is Critical: This seal acts as a barrier to prevent the pooled water on the slab surface from migrating under the wall and into the interior.
Additional Information & Best Practices for Sealing.
Proper Sealants: Use high-quality, durable, and flexible sealants designed for exterior use and adhesion to both concrete and wood/sheathing. Polyurethane-based or advanced silicone sealants are often good choices.
Flashing Integration: Ideally, combine sealing with appropriate flashing. This might involve L-shaped flashing tucked under the new wall's weather barrier (building paper/house wrap) and extending down the face of the wall and then out onto the slab, sealed at all edges. Self-adhered flashing membranes can be very effective here.
Surface Preparation: The concrete and bottom of the wall must be clean, dry, and free of debris for the sealant and flashing to adhere properly.
Creating a Small "Curb" or "Upturn": In some situations, if codes and design allow, creating a very slight raised curb (e.g., with a compatible mortar or specialized product) at the base of the new wall before sealing can provide an additional small barrier.
Interior Water Management as a Backup: In very problematic areas or if exterior solutions are imperfect, interior perimeter drains and a sump pump might be considered as a last resort, though preventing entry externally is always preferred.
Footing Awareness: The video makes an important point about potential footing issues. If the original building was larger, its footings (the structural base that distributes the building's weight to the ground) would have been around its perimeter. If your new, smaller structure is built inside that original footprint on the slab, there may be no dedicated footings directly under your new walls. This is a significant structural consideration that could lead to uneven settling or structural instability and should be assessed by a qualified professional.
In essence, the video underscores the need for careful detailing at the wall-slab interface when repurposing existing slabs. Skipping these steps can lead to persistent and damaging water intrusion problems. Consulting with a building professional or a waterproofing specialist is highly recommended if you're facing this construction scenario.
