David Dexter: Who designs the perimeter and foundation drainage system?

Recently, the question was asked on ASPE’s Open Forum “Foundation drain pipe — plumbing responsible? Or only between civil and structural?” As originally posted, the designer did not have a Geotech report. But the structural consultant did show the piping around the top of the elevator footing and the perimeter footing for the foundation in their details. However, there did not appear to be any coordination between the civil/site consultant and the plumbing team.

So, who is responsible for designing, showing and specifying the perimeter/foundation drainage system? As with many things in the vertical build market, the answer depends. Whose contract calls it out within their scope of work? It is always important to read the scope to which your firm has agreed to provide which services.

These systems can be designed and specified by many different members of the design team, including the architect, the structural consultant, the civil consultant, or the plumbing consultant. All have the knowledge, expertise and experience to provide these services. Regardless of who is going to design and specify the foundation drainage system, they will need to coordinate with all of the other members of the design team; architect, structural, Geotech, civil and plumbing, as it interacts with each discipline. It all goes back to the contract and the scope that each consultant agreed to provide.

One of the participants in the discussion felt that even though it is not uncommon to see foundation/underfloor drainage appear in the plumbing scope of work or even on the plumbing documents, these systems were not plumbing. Another was of the opinion that the normal installation of the perimeter piping was typically 6-inches above the top of the footing, so there would be a bit of room for slope. This piping is typically perforated (rigid or flexible), wrapped in a Geotech filter fabric, and covered with 5/8-inch washed rock or pea gravel, while the underslab ground water drainage system (subsoil drainage) would typically be a “tight” line (no holes) with the material being Schedule 40 PVC. This piping would be placed in a sloped trench and laid on a firm base. My question was, “If it is a tight line, how does it drain below slab groundwater?”

While slope might be desirable, these systems generally use polymer piping (rigid or flexible) that will not easily hold a slope. These systems, perimeter/foundation and/or subsoil drainage, are intended to move groundwater away from the foundation and from under the slab to aid in keeping the “tub” dry. They drain by hydraulic pressure from the groundwater to a point of disposal (wasting over the hill or lifted by a sump pump to a gravity drain).

And yet another contributor stated the “foundation drainage is not plumbing,” although it is not uncommon to see it appear in the plumbing scope of work or even detailed on the contract documents. I thought this to be a strange comment since it involves piping and the movement of water away from the building. And if it is in the scope of the work, one would expect the designer/engineer to provide the necessary documentation. This person went on to state that the piping used is not code-compliant, and laying any drainage piping without slope is contrary to the code — with the exception of siphonic roof drainage. In response, our resident Plumbing Inspection Supervisor pointed out the requirements of the International Plumbing Code (IPC) on the subject.

The requirement for a foundation drainage system comes from the building codes, of which the IPC is a part and is referenced for this column. Section 1102.5 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 above ground. 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.”

This clearly takes the idea of it “not being plumbing” out of the discussion.

The discussion continued with some voicing concern about how to appropriately select a pumping system. A valid question, as one must have an understanding of the potential volumetric flow of groundwater to select the pumping system. This is where the Geotech consultant report becomes important. As part of such a report, the elevation of the groundwater should be provided. And, if groundwater is near or above the foundation elevation, the report should provide a rate of flow needed to draw the water level down to an acceptable level. With this information, plus adding a factor for rainwater infiltration, the design professional can select the appropriate pumping system. Generally, these systems are simple. However, depending on the groundwater conditions and the potential for damage to the building and its contents, a duplex pumping system may be the prudent selection. One must also understand that the groundwater elevation may change on a seasonal basis, so work closely with the Geotech consultant to assure the pumping capacity is sufficient to handle the maximum volume of ground and rainwater.

Returning to our original question: Who is responsible for the design of the foundation/subsoil drainage systems? These systems are all within the magical “30-inches” of the building footprint (30 inches is established by the Plumbing code. It is at this point that one makes the transition from building drain to building sewer, although, per a longtime “gentlemen’s agreement,” 5 feet is the point between interior and exterior transition). The responsibility for this design has to be accepted by one of the disciplines developing the design: Architect, structural or plumbing.

As it is clearly called out in the IPC as well as the IBC (International Building Code), I would argue that it belongs to the Plumbing design professional. As it involves piping and the collection of water flow that needs to be routed out of the building footprint. While the architect and/or the structural consultant may show foundation drainage within various elevations or details, the complete system is not normally shown on their documents. While the plumbing design professional may make use of these elevations/details, it is the plumbing documents that need to show the complete design and appropriate routing for disposal. It is also up to the plumbing design professional to appropriately coordinate with the civil consultant to handle the site disposal of this groundwater system discharge.

Just a word of caution, remember if it shows on your documents, you become the EOR (Engineer-Of-Record) for that work. Therefore, the discipline responsible for the work should show all of the details and elevations necessary to accomplish the work, if the architect or structural consultant chooses to show this system within their elevations or details; they should include a note indicating that the information depicted on their drawings is for reference purposes only and the contractor shall refer to the plumbing contract documents for the system requirements as they apply to the project.

At the end of the day, foundation drainage belongs to the consultant/firm that has that work included within their scope of work. It is a contractual obligation to complete the scope. One should only show the work for which they are contractually obligated. They must have the expertise over that work and when they lack that expertise, bring in those that have it.