Last Updated: Jan 5, 2023
The water around foundations is an unfortunate truth that has plagued the housing industry since we first started excavating our building sites. When trying to keep the basement and crawlspace dry, remember that the work on the outside is far more important than the inside. Keeping the concrete wall dry keeps the building dry instead of trying to manage the moisture coming in through a wet wall. It is true that different geology and soil types play a major role in how whether or not basements and crawlspaces stay dry, and local conditions should always trump my seven rules that follow.
Grading plays a critical part in keeping your basement dry and free of water. It's important to grade the slope of the land around the house so that the water will want to run away and not towards it. When the grade slopes down and away from the building, that is called a “positive slope.” A negative slope or grade is where the water runs towards the house. A negative slope adds hydrostatic pressure to the foundation walls, with the excessive water pushing on the concrete foundation. This will lead to the ultimate failure of the wall system and allow water to enter the building walls. The water movement wears down coatings on the exterior of the foundation, slowly working away at elements, especially older liquid-based sealers.
Grading does not have to be dramatic, but it must have a positive slope away from the building on all sides. This can be done with swales, trenches, or simple slopes in the areas of concern. Most builders know this, and thankfully, most grading is finished with a positive slope—setting up the home for success. So, where do problems arise?
In my experience, the most common mistake that creates the potential for moisture in the basement is when the homeowner builds gardens up against the house. Bringing in more soil, compost, and mulch to go over the top of the new soil changes the grading initially in place from positive to negative. It's also vital to ensure you don't bury the bottom layer of brickwork or siding with mulch. So, Rule #1: Do not plant a garden up against the house.
Underground sprinklers are another great feature to help grow mold in basements. People will water all around their foundation for hours on end, trying to test the waterproofing membranes to the limits. The problem is that plants will be exposed to water for a very short period. Gravity then takes over, and the water is below the root line, and often the easiest path leads to the foundation wall. Excessive watering along the foundation puts an undue strain on the foundation walls. Rule #2: do not run underground sprinklers near the foundation of the home. Instead, consider placing properly graded mulch over the gardens to reduce the amount of water needed. Another option is to plant local plant species that require less water; this is called xeriscaping. Finally, ensure that you drain the underground sprinklers before every winter and check for leaks every spring when turning it back on.
Gutters and eaves troughs (depending on where you live, you may know them by different names) exist to move water off the roof and away from the house. Some regions of North America do not use gutters, and in those climates, you should ensure that the water that does run off the roof has an easy path away from the house. This can be done with a stone splash ditch to help reduce damage to the soil and gardens around the house. For those with eaves troughs, ensuring that they slope properly towards the downspouts is critical. They should be regularly cleaned off leaves and debris, typically in the fall after the leaves have fallen and before snowfall.
For designers, they must be adequately sized to capture the water coming off of the roof. Two variables affect the speed at which the water runs off the roof: the pitch and roofing materials. A steeper pitch means faster water movement, as does a roof made out of metal. If there is enough speed, water will shoot over the gutter, making it completely useless in heavy rains when it is needed.
Gutter guards, which stop leaves from coming in the gutter, are often not helpful. This is because water loves to stick to water. If the guard has more coverage than holes, the water may stick to itself. This results in the water running across the surface and over the edge instead of into the eaves trough. (It is, unfortunately, more common than one may think.) Rule #3: Regular gutter maintenance is still the best and cheapest solution.
Downspouts are essential, as they take the large volume of water that runs off the roof and place it near the foundation. Whether a basement, crawlspace, or even a slab on grade, the corners of the concrete foundation wall are the structural cornerstones (literally) of the building. By dumping gallons of water onto these locations, we create a serious potential for water intrusion and structural concerns. The downspouts must direct water away from the foundation and away from the building. Ideally, downspouts should extend four feet away from the building. Downspouts can get in the way of the lawnmower and will be either removed or run over on many occasions. Where the water spills out, soil erosion may occur due to the water pressure and roll back towards the building. You may note a negative grade at the downspout for this reason. Consider extender pads to move the water even farther. Rule #4: Make sure to check your downspouts, as they take a severe beating.
Some homes have the downspouts feed into an underground drainage system, typically a plastic corrugated drainage tile. Once the water drops below the surface, near your foundation, with an unknown destination, you should be concerned. The potential for the corrugated pipe to clog at the 90-degree bend just below the surface with leaves will ultimately be 100%. Once a clog appears, the failure point will happen below grade up against the foundation wall. Rule #5: If you have underground drainage, consider annual inspections and flushing out of the drainage system. This can be accomplished by removing the downspout, hosing the drainpipe, and ensuring it drains away from the house.
Window wells are also crucial because they are designed to keep snow off the basement window and its ledge. Windows are not designed to have continuous, direct contact with solid water (snow) and will fail earlier than designed due to this exposure. Remember that window weeping holes covered in snow will move water back into the window, not away. Window wells also help remove high water potential from the window, which is the weakest point in the basement walls. Window wells should be at least 12-18 inches deeper than the lowest point in the windowsill to be most effective. Gravel can be placed as far down as 18-24 inches below the windowsill to help drainage away from the window.
Clear plastic window well covers can be used. They reduce the visible light to the basement, and because they are UV sensitive, they turn yellow and crack over time—making them less than ideal for outdoor usage. Rule #6: Make sure the window well is at least one foot below the window ledge.
If you have a pier-style crawlspace and soil under their home, all of the above applies with one more detail. Too often, under the crawlspace (or large deck), the grade is a mess, and water can pool in shallow areas, impacting air quality and potentially be a structural concern. You’ll want to make sure the positive slope starts from the center of your house and continues outwards. Rule #7: Ensure that the soil in the crawlspace is higher than the surrounding exterior soil.
To properly assess water concerns, I recommend that you take the time to walk around the building during heavy rain to determine where and how the water is moving around the building. This is a simple and often eye-opening revelation on how water moves, how the downspouts work, where the water moves. It can empower you to make simple changes to improve the durability of your home.
Stephen Collette is a Building Biologist, Building Science Consultant, LEED Accredited Professional, and a Heritage Professional. Stephen is the owner of Your Healthy House and lives in Lakefield, ON with his wife and 2 daughters.