Granular fill best for gap between soil, foundation

Hey there, time traveller!
This article was published 20/05/2007 (6747 days ago), so information in it may no longer be current.

QUESTION — We live in an 83-year-old, two-storey home in River Heights and before the snow flew this past winter, there was approximately a four-inch gap/space between my foundation and the soil on three sides of my home. The one side without a gap is where the sidewalk is. With the dry conditions last summer, I was hoping the soil would swell and close that gap this spring. Well, that didn’t happen. My question is: Can I backfill that gap with soil this spring, or would that cause too much pressure against my foundation wall?

— Barry Jansen, e-mail

ANSWER — You have asked a very interesting question, the answer to which will vary considerably, depending on the viewpoint of the person you ask. There may be no definitive answer, but I will try to help you by providing a couple of possible viewpoints and the explanations for my reasoning.

In the Red River Valley, we are cursed with expansive clay soil that may shrink and swell dramatically, depending on the moisture content. As you have seen, this soil can recede enough to pull away from your foundation and leave a substantial gap. You are correct that this gap may close up if we experience a large amount of wet weather this summer, but there is no guarantee. In fact, if we continue to experience the dry spell we have been seeing lately, the soil could shrink away even more. The gap itself is of little significance other than the higher probability of seepage into the foundation following heavy summer rains. If enough water runs down the foundation walls, with no soil to absorb the excess, leakage through small gaps or cracks in the foundation or footing is likely. For this reason, I think you should fill the gap, but care should be taken to prevent overfilling.

It might be simplest to suggest that regular local topsoil be used to fill the void created from the dry clay. There might be several knowledgeable people who would recommend this option. The thinking might be that the composition of this material is similar to what is already there and will allow easy blending with surrounding soil. In my opinion, this may be a mistake. I have seen many situations where newly applied topsoil washes through gaps in basement windows and small foundation cracks and ends up in the basement. This material is very fine when freshly installed and might not adequately fill the gap without significant compaction.

A better option might be to fill the majority of the gap with fine drainage stone or coarse sand. This type of material may have more flexibility than topsoil when the clay swells back toward the foundation after rehydration. The gap should be filled almost to grade with the granular fill and topped off with clay or soil, which should be built up for the first one to two metres away from the foundation to provide a gradual slope from the house. This top section of clay will help shed excess rainwater from the foundation and prevent excess swelling of the shrunken soil near the foundation.

The main reason for my suggestion of granular fill, as opposed to soil, is that this is the preferred method for remedial action when leaky foundations are repaired by local foundation contractors. It has a very good track record and may remain problem-free for many years. When this is done on that larger scale, there is much more granular fill for the bigger opening next to the foundation, but the basic principle is the same. The granular fill will allow better drainage, is not subject to expansion from excess moisture, and will be easier to pour into a small gap and fill the area more completely than soil.

No matter what method is chosen, water management is still the key to maintaining a dry and stable foundation. Eavestrough downspouts should be extended one to two metres from the foundation for proper drainage. Soil should be gradually sloped away for the first one to three metres from the house. Also, growing grass or other low vegetation will help prevent erosion as the soil dries out. Moderate watering of the soil will maintain good ground cover and prevent excessive drying of the soil. If normal summer precipitation returns, extra watering may not be necessary to maintain this area.

As previously stated, you may receive advice from reputable sources that contradict my recommendations. This subject is certainly open to different interpretations, and none may be totally correct or incorrect. It is doubtful that any minor remediation you do will cause much of a detrimental effect on an older foundation such as yours as long as the solution is not extreme or drastic. The proper maintenance of expansive clay soils next to foundations is a tricky subject, but use of a common-sense approach is the best course of action.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and president of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, 110-2025 Corydon Ave., Winnipeg, MB, R3P 2G9. Ari can be reached at (204) 291-5358.

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