The importance of creating and maintaining a positive downhill slope condition away from your foundation is paramount, and happily, is one of the most relatively inexpensive and easily achieved preventative measures available. Our focus in this article will be on the practical how's and why's of good positive drainage.
This is usually defined in most soils reports as a condition where at least 6"-10" of drop occurs in the first 10' of run out from the foundation. Another way of expressing the same thing is to express the angle of the slope by means of a percentage; 6%-10% positive slope for mulched or rock bed areas, and 2%-3% positive slope for hard paved areas, such as sidewalks, driveways, or parking lots. (A lesser slope is usually acceptable for paved areas because hard surfaces like concrete or asphalt naturally conduct water away from the foundation much more efficiently than a porous surface like a mulch bed).
With a few basic tools and a bit of patience, these slope percentages may be determined by the average property owner wishing to prevent needless structural and/or water damage to the foundation of his/her property. The process of doing so is as follows:
First, choose a point along the foundation wall and place one end of a 10' long straightedge (such as a 2 x 4) at the same point while extending it out perpendicularly from the foundation.
Next, place a carpenter's level on top of the 2 x 4 while you gradually raise the far end of the 2 x 4 until the bubble indicates a level condition has been achieved.
Finally, simply measure down from the elevated end of the 2 x 4 to the ground directly beneath is and make note of that dimension. For this example, let's suppose that our vertical measurement is 10".
Now, you have all the raw data you need to calculate your slope percentage.
The math is simple: divide the vertical dimension, known as the "rise" by the horizontal dimension, which is known as the "run". Don't forget to convert your units of measure as needed so that they are consistent. In our example, we would have 10" (our rise) divided by 120" (our run), equaling .083, which we could round to 8%. As a rule of thumb, an 8% positive slope downward would be considered to be a safe and effective amount of slope to quickly move water away from the foundation before it can penetrate to deeper layers of soil and cause problems. Sometimes, calculating slope or determining what a safe and effective grade may be difficult, so it is possible to have a geotechnical engineering firm or other foundation related company come out and do the calculations.
Now that you have a slope calculation, it’s easy to find problem spots on your property and change the grade to protect your home. You should check the slope of your property, especially when your home is a new construction, after spring melt. Settling and changes in the grade are often caused by changes in moisture and run off. Preventative maintenance like this can save from a flooded basement, a muddy crawlspace, or even cracked and settling foundations.
Check out a reprint of Jeff Kortan, P.E.'s article on Identifying Conditions for Structural Concerns, which is currently in circulation on the NAHI website and in their quarterly publication.
Identifying Conditions for Structural Concerns: Part Two of a Four Part Series
By Jeff Kortan, P.E.
Foundation Supportworks, Inc., Omaha, NE 800-281-8545
Cracked, bowed and leaning foundation walls, along with associated exterior and interior damage, may be signs of a serious structural problem. These conditions are not only worrisome to look at, but without appropriate action could lead to a much bigger problem, such as a total collapse of the walls. The good news is that restoring the integrity of failing foundation walls can be quick and cost-effective, while also giving both the home seller and buyer confidence that a home’s value is not compromised.
WHAT CAUSES FOUNDATION WALLS TO FAIL?
While there are a number of causes for cracked, bowed and leaning foundation walls, the most common is excessive pressure from the soil outside the wall. When expansive clay soil absorbs moisture, it swells and applies pressure that often exceeds the wall’s design capacity. In addition to expansive clay soil, excessive pressure is often the result of a failed or compromised foundation drain system and a buildup of moisture behind the wall. Again, this additional hydrostatic (water) pressure often exceeds the wall’s design capacity, leading to wall deflection. Poor surface grading and drainage is another factor that leads to the buildup of hydrostatic pressure.
Excessive wall deflection, evidenced by wide cracks, bowing and leaning, can reduce the structural integrity of the wall. When the wet cycles are repeated, as in the above two scenarios, this causes further weakening and inward wall movement. Other common causes of wall failure include soil expansion due to frost, large tree root systems and surcharge loads from the foundations of adjacent structures or additions. Heavy equipment and heavy storage items placed immediately adjacent to basement walls can also cause wall deflection.
HOW WILL I KNOW IF A HOME HAS FAILING FOUNDATION WALLS?
Symptoms of failing foundation walls in a home look different depending whether they are constructed of concrete block or poured concrete.
Concrete block walls typically display horizontal cracking across the center length as it begins to bow inward near mid-height. As the problem worsens, stair-step cracking at the corners can be observed. Continued inward pressure exerted on the concrete block wall can also result in horizontal shearing at a mortar joint, where the bottom row of block is held in place by the concrete floor slab as the next course of block and wall above slides in.
Vertical shearing may also be observed when the end of a wall is supported or held in place by an adjoining perpendicular wall. The end of the wall remains stable as the rest of the wall cracks and moves inward. Poured concrete walls will typically display single, diagonal cracks extending upward from the bottom corners of the wall toward the top center. Further, unlike a block wall that bows in near mid-height, the top of a poured wall tends to lean in. In finished basements, where foundation walls may not be exposed, other signs may alert you to a structural problem. As failing basement walls move inward, ceiling panels and ceiling drywall may begin to buckle.
Drywall on finished, abutting walls can also buckle between the wall studs. In extreme cases, horizontal cracking may even be seen in the home’s exterior brick veneer.
HOW DOES A HOME OWNER FIX FAILING FOUNDATION WALLS?
There are several options available for repairing failing foundation walls, some more desirable than others. One option is the total removal and replacement of the foundation walls. With this solution, landscaping, decks, and exterior concrete slabs are removed and all the soil surrounding the foundation is excavated. The home is then jacked up and alternatively supported while the foundation walls are removed and then replaced. Not only is this solution disruptive and expensive, but the problem is likely to occur again if the original cause(s) are not addressed.
A second option for repair is the installation of steel columns (although acting as beams in this application). The steel beams are typically bolted to the concrete floor at the bottom and bolted, screwed or nailed to the wood floor joist system at the top. Forces pushing on the wall are therefore transferred through the steel beam and to the floor joists. Due to the high degree of variability with home design and construction, designers of these systems should have a thorough knowledge and understanding of each and every site-specific application, most importantly the connection details with the wood framing and the additional, required joist blocking and reinforcing. Without a proper design, the steel beam may bend, the floor joists and first floor may buckle, and the walls may continue to deflect. This option also cannot straighten the foundation wall back to its original position.
A third option is carbon fiber strips or sheets. The application/installation includes an epoxy glue to adhere the carbon fiber to the wall. The carbon fiber then acts as tensile reinforcement to the interior surface of the wall to minimize any additional bowing. While these products can be used successfully to minimize additional bowing conditions, they cannot prevent a wall from tipping in at the top or to straighten a wall back to its original position.
The final option is a wall anchor system. With a wall anchor system, heavy-duty, galvanized steel earth anchors are embedded securely in the stable soil out away from the foundation wall. The earth anchors are connected to galvanized steel wall plates positioned on the foundation wall with long, threaded, galvanized steel rods. This system relies on the passive resistance of the soil at the earth anchors to resist the excessive pressures being applied to the wall. The wall anchor system does not rely on the home’s wood framing for support. Wall anchor systems are a proven method to stabilize foundation walls and offer the best opportunity to straighten the walls back toward their original position over time. Anchor systems have long been cited as an ideal solution for bowed and leaning foundation wall problems, even as the recommended option by the U.S. Department of Urban Housing and Development’s chief appraiser back in 1992.
Advantages of the wall anchor system include year-round installation, minimal disturbance to lawn and landscaping, no damage to interior floor joists, the ability to straightens walls, the prevention of further bowing/leaning, and most importantly to your clients, it restores property value. Although failing foundation walls can be scary and intimidating to home sellers and buyers, the solution of a wall anchor system can be installed quickly and often for less money than what would be deducted from the seller’s asking price to offset for the structural concern or perceived repair.
