Renovation & Design

Filling a void only a temporary solution for seepage

Question: We have lived in our 1986 two-storey home for six years. The concrete driveway extends the length of the south side of the house. There is a stuccoed chimney, as well as an air conditioning unit on that side of the house. The driveway concrete goes up to the chimney, and over the years, it seems to have pulled away from the bottom of the chimney, causing a gap. We can see directly underneath the chimney and there is a large void. Over the past five years, we have had some seepage in the corner of the house that was repaired via waterproofing, but not underneath the chimney. We also had a mouse issue, and the pest company that came out advised us to put three-quarter-down into the void and mound it up around the chimney since they believed this was an entrance point.

Recently with the very dry weather and then heavy rains, we have had water coming into our basement laundry room south wall. This is not seepage, and upon removing the insulation there is a small river of water coming in via the area under the chimney. Also, some water enters via the hose for the air conditioner that goes through the foundation wall. The three-quarter-down we have been putting in the gap simply falls into the void after a few weeks, causing the gap to return.

What is the correct way to address these two issues? We assume the air conditioner hose issue will be an easy fix, but we are at a loss about the void under the chimney and whether this is a foundation issue or a concrete issue, or both.

Thank you,

— Laura R.


Answer: The only way to properly address an issue of a void under a concrete slab poured improperly up against a foundation is partial removal of the concrete. That will allow access to build up the eroded soil and for future repairs and regrading as required.

Too often I have seen homes where a sidewalk, patio, or driveway has been poured directly adjacent to the foundation. Because of the nature of our clay soil, expansion and contraction with changing moisture levels, that is always a bad idea. In the best cases, the concrete may pull away from the foundation, slightly, leaving a small gap. This can be normally filled quite easily with flexible caulking or other filler like a foam rod. If you try to plug it with loose fill, as you have seen, it will just open again within a few seasons. In the worst-case scenario, the slab not only pulls away from the foundation, it sinks and slopes toward the foundation. In that situation, moisture intrusion is much more likely.

Your situation is somewhere in between these two extremes, but may be more concerning due to the location of the chimney.

The chimney foundation may not be as deep as the house foundation, and may not be as well damp-proofed as the house foundation, when constructed. Also, if there is any movement between the house and chimney, there may be small cracks in the concrete between the two, which can be one source of your leakage. The solution to this will not be possible without opening up the area for inspection.

Unfortunately, to gain access for inspection of the house and chimney foundations, a portion of the concrete covering the area will have to be removed.

This will require professional concrete-cutting saws and other equipment to break up and remove the offending sections. If the driveway is in otherwise good condition, only a small portion may require removal, but if it is deteriorated or damaged, complete removal may be the better choice. Once the concrete is removed, a minimum of 300 to 400 centimetres, then the area can be inspected. If no visible cracks or damage are seen, the damp-proofing intact, and the void visible, regrading may only be required to stop the leaks. Building up the soil to provide a slope away from the foundation and chimney may be all that is required.

If there is visible damage to the concrete below grade, further excavation may be required.

In many homes this age, the bitumen-based coating applied to the outside of the foundation, known as damp-proofing, wears out. In that situation, it should be cleaned and recoated, or upgraded using a waterproofing membrane. This will require further digging, and potentially more driveway removal, to access the foundation. Once complete, the removed soil could be replaced with better-draining stone fill, capped off with a sloped section of topsoil.

This repair may be warranted, but attempting the initial regrading may be enough to prevent the periodic water you have experienced. If it does reoccur, then the more thorough foundation repair should be the next step.

Plugging a gap between the concrete driveway and the house foundation or chimney may be a temporary solution to preventing seasonal foundation leakage, but partial removal of the concrete will be required to replace the eroded soil. This will allow for simply filling in the void and regrading, or may be the first step toward a larger excavation and foundation waterproofing solution, if needed.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at



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