Sometimes, seemingly small errors result in problems that are surprisingly expensive to correct.
In the work we do, conducting thousands of facility assessments, one of the most common yet easily avoided problems we see is improper placement of ground cover landscape soil and mulch.
Pine straw and other mulch products are used around the perimeter of buildings for good reason: they are an attractive enhancement to the landscape package, and they help retain ground moisture.
While mulch helps retain ground moisture, it also creates a moisture problem in adjacent walls when it is allowed to contact a building’s exterior wall finishes (siding, brick, stucco, etc) . Topsoil and mulch are sometimes piled up against exterior walls well above the interior floor level. This condition allows a clear path for ground moisture to saturate wall systems. When moisture is allowed extended contact with building walls, decay and structural damage will result. Wood rot, decayed sheetrock and insulation, and mold growth are the inevitable result. Significant damage typically occurs well before the problem becomes visible to a casual observer.
We also often see mulch in contact with outdoor air-conditioning condensing units. Contact with the unit metal base, frame and coils results in additional moisture in and around the unit. This condition will attract dirt and insects, damage controls, and cause metal components to rust prematurely, and will reduce operating efficiency and service life, and possibly void warranties.
The question you might be asking is, “how do we prevent these problems?”
Prevent Ground Cover Nightmares with the “6-Inch Rule”
Always keep mulch, or other landscape material a minimum of 6 inches away from the base of siding, brick, stucco and other exterior finish materials, unless special design accommodations for soil or ground cover contact are implemented.
Universal building codes require exterior wall finish systems to terminate at 6” above grade to mitigate moisture issues in exterior walls. Also, finished floor elevation (the elevation of the top of the concrete slab) should be at least 6 inches above the grade elevation immediately outside the building. While most building codes require a 6” differential, we often see this requirement violated during original construction. This condition is exacerbated over time as the cumulative effect of annual mulch application can effectively raise the ground elevation above the finished floor elevation, thus creating a clear path for ground moisture to enter exterior walls.
To prevent ground moisture from entering buildings, it is imperative to remove excess mulch and soil so that water properly drains away from the building perimeter. Be cautious about creating a “moat” around the building that would retain water and divert it back toward the building. In cases where grades cannot be lowered, a French drain (consisting of a trench filled with loose gravel and a perforated drain pipe) may provide a path for proper water diversion.
We encourage building owners to conduct a perimeter check twice a year. If a contractor maintains the grounds, we suggest the owner add language into the service agreement stipulating the provider will maintain a minimum 6-inch contact clearance around the perimeter of the building and not allow mulch or soil to contact HVAC equipment.
