Ask the Architect

Our basement slab is leaking


Q. We’re confused about what to do with our basement slab. Water keeps coming up through it, and we have to do something. There are now cracks. We want to do the right job putting in a new one. Can you help?

A. As with most problems, there’s usually one best solution and other ways that are satisfactory, but not as responsive. Like most of the solutions in my column, someone has a better way, even if that way is flawed by negatives like cost, potential failure probability or not fully understanding the requirements.

For example, you could remove your entire basement floor slab and start over with the waterproofing membrane and internal slab reinforcing that would have prevented the problem in the first place, only to realize that the adjoining walls require additional attention to be sealed as well. You could apply a sealant that looks like paint but penetrates the concrete. If you did that, however, the less costly brands don’t have the solids and ability to bond to the cementitious surface and end up scaling off. The much more costly brands of sealant provide a molecular bond to the surface, and some are made to penetrate and set as much as an inch into the material. But then, if your intention is to cover the sealed surface with a finish, the finish has to be checked with the manufacturer for compatibility, and methods of attachment must be explored also. Some types of sealants won’t bond, or can cause the applied sealant to break down.

The outside elements must also be understood. If the slab is being applied over soft soil, if the groundwater is ever-changing due to a high water table or an adjacent body of water, or if the interior space is subjected to high humidity, the slab must be designed to be reinforced, and should probably be capable of being self-supporting, like the floor of a parking garage. This means calculating the most severe potential load, such as heavy equipment, file cabinets, etc.

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