Recently, several discussions on ASPE Connect’s Open Forum have explored foundation drainage: Storm Drainage Connecting to Foundation Drainage, Renovating Old Foundation Drain Pipes, and Foundation drain pipe – Plumbing responsible? Or only between Civil and Structural? So, what is foundation drainage, why is it necessary, who should be responsible for system designs, and what is its purpose?
What Is a Foundation Drain?
A foundation drain is defined as a drain that is installed at the foundation of a structure to prevent water from collecting around it. These drains are installed in the exterior of the foundation at the footing. Foundation drains are installed simultaneously with the footing during the foundation installation phase of a structure.
A subcategory of a foundation drain is subsoil drainage. This is a drainage system installed below the slab to collect subsurface or seepage water and convey it to a place of disposal. Both of these systems use perforated or slotted pipes (flexible or ridged), wrapped in a geotextile material to minimize particulate entry and covered with stone or crushed rock. These drainage systems are basically the same; they only differ by location: exterior vs. interior.
Why Is Foundation Drainage Necessary?
These systems are required in accordance with IBC (International Building Code—the code referenced for this column) Section 1805.4: Foundation Drain, which states: “A drain shall be placed around the perimeter of a foundation that consists of gravel or crushed stone containing not more than 10 percent material that passes through a No. 4 (4.75-mm) sieve. The drain shall extend not less than 12 inches (305 mm) beyond the outside edge of the footing. The thickness shall be such that the bottom of the drain is not higher than the bottom of the base under the floor, and that the top of the drain is not less than 6 inches (152 mm) above the top of the footing. The top of the drain shall be covered with an approved filter membrane material. Where a drain title or perforated pipe is used, the invert of the pipe or tile shall not be higher than the floor elevation. The top of joints or the top of perforations shall be protected with an approved filter membrane material. The pipe or tile shall be placed on not less than 2 inches (51 mm) of gravel or crushed stone complying with Section 1805.4.1 and shall be covered with not less than 6 inches (152 mm) of the same material.”
Who Should Be Responsible for the Design of the System?
There seems to be some debate over this issue, as you might see details related to the system on the architectural, structural, or plumbing documents. IBC Section 1805.4.3 offers some guidance: “The floor base (subsoil) and perimeter drain shall discharge by gravity or mechanical means into an approved drainage system that complies with the IPC (International Plumbing Code).” Based on this statement, you would expect that the plumbing engineer/designer would be the logical choice to design and specify these systems. After all, these systems involve piping and carry away water waste from the building “tub” (the tub being the space created by the building portion found below grade—e.g., basement, elevator pits, etc.).
However, despite this guidance, many plumbing designers do not consider these systems to be plumbing. While they may make provisions for removal of the effluent, they think it is someone else’s responsibility for the design of the piping. This, in my judgment, is difficult to support, when the IPC addresses the subject in Section 1102.5, which covers subsoil drain materials and specifically requires the pipe to be “horizontally split or perforated pipe.” Section 1111.1 covers how the subsoil drains can be discharged: “Subsoil drains shall be open-jointed, horizontally split, or perforated pipe conforming to one of the standards listed in Table 1102.5. Such drains shall not be less than 4 inches (102 mm) in diameter. Where the building is subject to backwater, the subsoil drain shall be protected by an accessibly located backwater valve. Subsoil drains shall discharge to a trapped area drain, sump, drywell, or approved location aboveground. The subsoil sump shall not be required to have either a gas-tight cover or a vent. The sump and pumping system shall comply with Section 1113.1.” In my judgment, this clearly takes the idea of it “not being plumbing” out of the discussion.
While I can understand the confusion given that both the architect and structural consultant tend to show parts of these systems in their details and elevations, they generally do not show a complete system design on any of the floor plans. This leaves the real work to the plumbing consultant, who must work closely with the geotech consultant to determine the elevation of the groundwater and the potential volume requiring removal.
What Is the Purpose of Foundation Drainage?
Not all buildings will require foundation/subsoil drainage, as determined by the geotech report, which shows the determined level of groundwater within the area of the building footprint. If the maximum groundwater elevation is at least a foot below the foundation elevation and likely never to rise above that level, then foundation/subsoil drainage becomes unnecessary. However, some architects may still push for their installation to address rainwater infiltration, which will leach down along foundation walls to settle around the footings. Depending on the type of soil upon which the footings sit, this water may need a means of removal or it may simply leach off into the surrounding soil.
One point that came out of the ASPE Connect Open Forum is how we use many terms for the same systems—footing drains, foundation drains, subsurface drains, subsoil drains, etc.—which is just a reminder of how terms get used interchangeably, but each may be connected to a specific definition or area of the structure. (I, for one, am going to try to be more specific with my terms in the future.) One thing all of these terms have in common is the removal of water (groundwater or rain infiltration) from around or under the structure to protect it from water seeping into that structure and potentially damaging the interior or the contents contained within the building.
You must also make a distinction between removing water from around a structure and dewatering the area around that structure. Foundation/subsoil drainage is intended to remove water from around the structure, based on changing water table levels and rainwater infiltration. It is a system that does not normally see continuous flow. However, in rare exceptions the water table is such that continuous flow can occur, and depending on the volume of that flow, you must look at additional considerations. One of those additional considerations is dewatering. Most dewatering systems are temporary: pumps, piping headers, and well point heads driven into the ground to extract groundwater that are removed once the work within the dewatered zone has been completed. In some situations, however, the dewater system must be made permanent to address the volume of water and aid the foundation/subsoil drainage system.
As an example, Ohio Stadium (known as “The Horseshoe” owing to its shape) on the campus of Ohio State University in Columbus, Ohio was remodeled and updated. As part of the update and adding of stands at the opening to the horseshoe, the playing field was lowered. Anyone who knows the location of the “Shoe” knows that the Olentangy River is not far away. Under normal river levels, there is a need to continually extract water and return it to the river, and that volume increases as the river level changes due to rain events. While other areas around and within the stadium have foundation/subsoil drainage systems, their operation is changed as a result of this dewatering system pulling the “bowl” of groundwater down around the stadium footprint.
While, in my judgment, foundation/subsoil drainage is a plumbing system, it also requires the input of the geotech consultant and coordination with the architect, structural, and civil consultants; each brings specific knowledge and experience to the table. As with any project, to whom does the contract assign responsibility for the work? Read your contract and scope of work before you commit to the fee and agree to show the work on your contract documents.
About the Author
David D. Dexter, FNSPE, FASPE, CPD, CPI, LEED BD+C, PE, is a registered Professional Engineer, Certified Plumbing Inspector, and Certified Plans Examiner with more than 40 years of experience in the installation and design of plumbing systems. He specializes in plumbing, fire protection, and HVAC design as well as forensics related to mechanical system failures. Dave serves as Chair of ASPE’s Main Design Standards Committee, Chair of the Bylaws Committee, Co-Chair of the College of Fellows Selection Committee, and Co-Chair of the Professional Engineer Working Group. He also was the 2008–2009 President of the Engineering Foundation of Ohio, 2010–2011 President of the Ohio Society of Professional Engineers, and 2012–2014 Central Region Director for the National Society of Professional Engineers.
The opinions expressed in this article are those of the author and not the American Society of Plumbing Engineers.