Designing a PVC plumbing waste system for a single-family home or townhome involves understanding drain, waste, and vent (DWV) principles, applying local building codes, and clearly communicating the system through a plumbing riser diagram. Below is a step-by-step guide, including rules of thumb and code references to help with both design and documentation.
This guide aims to help start designing and documenting a code-compliant PVC waste system for a home, and drawing a clear plumbing riser diagram for plan submission and construction.
System Design Basics
A residential DWV system includes:
Branch waste lines from fixtures (toilets, sinks, showers, etc.)
Stack(s) for vertical transport
Building drain (horizontal main line)
Building sewer (from house to public or private sewer/septic)
Use only PVC Schedule 40 for residential DWV systems unless the local code specifies another material.
Pipe Sizing Rules of Thumb
Common recommended minimum pipe sizes:
Toilets: 3" diameter
Main building drain: 3–4" diameter
Bathtub/shower: 2" diameter
Washer: 2" diameter
Lavatory sink: 1-1/4" diameter
Kitchen sink: 1-1/2" diameter
Horizontal branch serving two or more fixtures: Use the minimum size for the largest fixture
Slope for horizontal pipes:
Minimum: 1/4" per foot for 2-1/2" diameter or less (UPC sec. 708.1; IRC P3005.3)
For larger pipes, 1/8" per foot can be permitted per code
Separation from water line: IRC P2602.2 (building sewer)
Section numbers may vary based on the year the version was published, so always check your local amendments and plumbing code for any variations.
Drawing a Plumbing Riser Diagram
A plumbing riser (or isometric) diagram shows a simplified 3D view to illustrate how waste and vent pipes connect vertically and horizontally within the building.
Use clear symbols for each fixture
Vertical lines for stacks/risers, horizontal lines for branch drains
Show each fixture group, cleanouts, and all vent connections
Annotate with pipe sizes, slopes, and code references
Indicate floor levels and pipe transitions between floors
Here's a basic approach:
Start with the main stack location.
Branch out the horizontal waste lines to each fixture group.
Draw vents connecting up from each fixture or group.
Indicate cleanout positions along horizontal runs and at the base of the stack.
Use consistent symbols as per national standards.
Example One-Story Schematic:
One Story Plumbing Example.pdf
28.1 KB
View full-sizeDownload Example Two-Story Schematic (With Plan):
Importance of Slope for Sewer and Septic Connections
Determining the overall slope of the plumbing waste system is critical—especially when connecting to a sewer system or, even more importantly, a septic tank. Adequate slope ensures that waste and water flow efficiently by gravity, preventing blockages, minimizing odor, and limiting the risk of solids settling in the pipe, which is particularly vital for the performance of septic systems.
For any gravity-fed waste system, the minimum slope of horizontal drainage pipes must typically be:
1/4 inch per foot for pipes 2-1/2 inches diameter or less (IRC P3005.3; UPC 708.1)
1/8 inch per foot for larger pipes, where permitted by code
This slope promotes “scouring velocity,” carrying solids with wastewater toward the main sewer or septic tank. An inadequate slope can cause buildup and blockages, while an excessive slope can cause water to outrun solids, also causing deposits.
When connecting to a septic tank, a consistent and adequate slope from all fixture drains to the tank inlet is essential because:
Septic systems rely entirely on gravity to move both solids and liquids
Any dips or “bellies” in the line can accumulate solids, leading to pipe clogs or system failure
Proper slope ensures waste enters the tank at the right velocity, supporting the correct settling and decomposition processes inside the tank
Practical Steps for Determining and Maintaining Slope
Measure the vertical drop available from the lowest fixture drain outlet to the point of connection at the septic tank or sewer lateral before planning your home’s foundation and drain layout
Calculate the total allowable horizontal run and divide by the minimum slope requirement to ensure there is sufficient fall to meet the code over the distance of the pipe run
Use a builder’s level or laser level to mark and verify pipe slope during rough-in and again before inspection
Local or state code amendments may provide additional septic tank connection guidelines
By carefully verifying the overall slope early and throughout the rough-in stage, designers and installers protect the long-term performance of both sewer and septic systems, ensuring code compliance, homeowner satisfaction, and system longevity.
When the distance to the sewer or septic tank is so great that the PVC waste line ends up lower than the intended system connection, gravity drainage is impossible—wastewater cannot flow uphill. In this scenario, specialized solutions are required to ensure proper waste disposal and code compliance.
When gravity drainage cannot be achieved due to site elevation, installing a sewage ejector or grinder pump is the universally accepted and code-compliant solution, ensuring waste reaches the sewer or septic tank regardless of the height of the final connection.
Common Solutions
Sewage Ejector/Grinder Pump System: Wastewater from the home flows by gravity into a holding basin or sump located at the lowest point on the property. When the basin fills to a certain level, a float-activated pump automatically lifts sewage up to the elevation of the sewer or septic tank connection. These systems are reliable, with grinder pumps able to process solid waste and pump it uphill over significant distances. Maintenance typically only involves periodic pump replacement, with modern systems featuring easy-access service chains or connections.
Above-Floor Collection Tank Units: For limited-space renovations, manufactured units combining a small holding tank and pump can be installed above the floor (e.g., QuickJohn or similar), allowing fixtures to drain directly into the system, which then ejects wastewater into the higher sewer line.
Alternative Site Engineering: In rare cases, civil engineering solutions such as site regrading, forming a new trench, or lowering the septic tank itself may be feasible; however, these options are often costly and disruptive.
Design and Code Considerations
Check Local Plumbing Code: Codes require a check valve on the pump discharge to prevent backflow, and often mandate alarms or dual pumps for redundancy in multi-family or critical-use buildings.
Sizing the Pump: Ensure the pump system is adequately sized for the total fixture unit flow, pumping distance, and elevation gain.
Extra Maintenance Needs: Grinder/ejector pumps need periodic service and accessible locations for repairs.
Plumbing Fixture Placement Requirements and Clearances
The 2024 International Residential Code (IRC) plumbing fixture placement requirements and clearances are primarily found in Chapter 3: Toilet, Bath, and Shower Spaces, as well as referenced in select sections and tables. Key regulations include minimum distances, clearances, and accessibility standards.
Relevant IRC Sections for Fixture Placement and Clearances
Section R326 – SANITATION
Section P2705.1 – “Clearances for Plumbing Fixtures”
This section requires that plumbing fixtures be installed to allow sufficient access for usage, cleaning, and maintenance.
Examples: At least 15 inches from the centerline of a toilet to any wall or obstruction; at least 21 inches of clearance in front of a toilet, bidet, or sink; and at least 24 inches in front of a shower entry.
Section P2706.3 – “Lavatory Placement”
Specifies requirements for the location and accessibility of lavatories (bathroom sinks).
Section P2708.1 – “Shower Compartment Size”
Mandates a shower compartment’s minimum interior area of 900 square inches, not less than 30 inches in any horizontal dimension.
Section P2710.1 – “Water Closet Spaces”
Details the minimum space requirements for water closets (toilets).
Ceiling height requirements
IRC notes a minimum ceiling height of 80 inches above fixtures within bathrooms (this is important when designing powder rooms below stairs).
Additional guidance
Table P2705.1 provides a detailed summary of required fixture clearances and minimum dimensions, useful for plan review and compliance.
