Do you need to waterproof a slab foundation?

Since slab foundations do not enclose the space below ground level, traditional waterproofing is often not required. It's a common misconception that only basements need solid waterproofing strategies. While the error is understandable (after all, slab foundations sit on the ground), the reality is that all foundations can develop problems with invasive moisture. Slab foundations are very common in commercial buildings.

The construction methods are similar, only on a larger scale, as are the potential problems. There's really no reason to have a shoe drain or moisture protection with slab above grade. If you have a groundwater problem (which you say no), a case could be made to keep water out of the EPS insulation, as it could affect the R value. You should have a good vapor retarder under the slab.

If the foundation wall is blocked, a good large layer or equivalent will protect it from prolonged freeze-thaw damage. Most waterproofing systems need protection during backfilling. Some manufacturers have their own protective plate for this purpose. A drainage or insulation mat can also function as a protective board.

An inexpensive 1-inch thick expanded polystyrene foam board works well as a basic protective layer. However, a manufacturer has a protection plate that functions as insulation, protection plate, and drain medium in one. If you're using a waterproofing subcontractor, recognize that good waterproofing materials can be in high demand during peak season. Another waterproofing agent once told me that in one of his jobs the general contractor sent a worker, after applying the waterproofing, to break the ties of form inside a foundation.

Gwenda Hunt
Gwenda Hunt

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