What Is Subterranean Catch Basin Routing? (PVC Slope Rule)
Subterranean catch basin routing carries collected surface water through buried pipe to an approved gravity outlet. The PVC slope rule is not one universal grade: choose the route and outlet first, then set pipe size and slope to local code, design flow, and site conditions. Verify continuous fall from measured pipe inverts before covering the trench.
What subterranean catch basin routing means
Subterranean catch basin routing is the planned path from a below-grade basin to a lawful discharge point. The basin collects runoff; a buried gravity pipe carries it away. Success depends on a suitable outlet, enough elevation drop, adequate pipe capacity, sound bedding, and access for inspection and cleaning.
A catch basin often receives runoff from a driveway, patio, or low spot in a yard. Its outlet pipe is sometimes called a catch basin lateral. That lateral may be governed by local stormwater or site-drainage rules, not the same rules used for a building storm drain.
I start with the destination, not the trench. A pipe can have a neat, steady slope and still fail if its outlet is too high, too small, blocked, or not approved. The first task is to confirm where water may go and whether it can get there by gravity.
Planning the basin route and approved outfall
An outfall is the point where the pipe releases water to an approved destination. Before digging, identify the basin, route, property limits, foundation, buried utilities, and outlet. Confirm that the outlet is lawful, lower than the pipe’s start, and able to accept the expected flow.
Possible destinations vary by site and local rules. They may include an approved surface outlet or a permitted stormwater system. Do not assume a nearby sewer, footing drain, or neighboring lot is acceptable. In many places, stormwater connections to sanitary sewers are prohibited because they can add flow and contribute to sewer surcharge or backups. Get written approval when the destination is not clear.
Record the proposed pipe inverts. An invert is the inside bottom of the pipe, where water flows. Measure the basin outlet invert and the outfall invert, then compare them with the route length and required grade. Also check the outlet for erosion risk, backwater, and maintenance access.
- Locate underground utilities through the required local service before excavation.
- Check property boundaries and any easements.
- Confirm the discharge point with the local authority or system owner.
- Identify likely pipe cover, soil, frost, and traffic loads.
A route is workable only when its outlet, elevation, and permitted discharge all check out.
Checking inverts, utilities, and access
An invert survey checks the pipe’s inside-bottom elevations along the route. It turns a proposed slope into a measurable plan. Before installation, use the survey to spot conflicts with utilities, foundations, pavement, and other buried lines, and to confirm that the planned outfall is low enough.
The required drop depends on both the run and the selected grade. For a constant slope, multiply the pipe length in feet by the slope in inches per foot. For example, a 40-foot run at ⅛ inch per foot needs 5 inches of fall. The outfall invert must be low enough to allow that fall, plus any changes needed to make the route and connections fit.
Do not use trench depth alone as proof of slope. Trench bottoms can vary, and the pipe may shift during bedding or backfill. Set and check the pipe by its inverts, using a suitable level or survey method. Check intermediate points too, especially where the route changes direction or crosses a utility.
Plan access for cleaning. Bends, long runs, and buried connections can make blockages harder to clear. The design or local authority may require cleanouts or other access points. Keep access clear and use fittings that suit the drainage system.
Applying the PVC slope rule
Slope is the vertical drop divided by horizontal run. For gravity drainage, the pipe needs a continuous design fall toward the outlet. The familiar ⅛-inch-per-foot figure equals about 1.04%; ¼ inch per foot equals about 2.08%. Neither value is a universal minimum for every buried catch basin lateral.
The 2021 International Plumbing Code (IPC) §1106 and Table 1106.2 address sizing of building storm drains and sewers. Table 1106.2 includes ⅛ inch per foot as a listed design slope. That is a condition in a sizing table, not a blanket rule for every landscape drainage pipe. Check the locally adopted code, stormwater requirements, and approved design before choosing a slope or pipe size.
| Grade | Percent slope | Drop over 20 ft | Drop over 40 ft |
|---|---|---|---|
| ⅛ in./ft | 1.04% | 2.5 in. | 5 in. |
| ¼ in./ft | 2.08% | 5 in. | 10 in. |
Use the table to check elevation, not to choose a grade by itself. Pipe diameter and slope must suit the design flow and the rules that apply to the project. A larger slope does not automatically solve a capacity or outlet problem. Confirm the route’s full length, elevations, and any local limits before excavation.
Selecting and installing PVC
Use solid-wall PVC sewer pipe that is accepted for the application. ASTM D3034 is one product standard used for sewer pipe, but the designation alone does not confirm that a pipe is right for a particular cover depth or traffic load. Verify the pipe diameter, stiffness class, manufacturer instructions, and local acceptance.
For gasketed joints, check that the pipe and joint system is compatible and meets applicable requirements. ASTM D3212 covers joints for drain and sewer plastic pipe using flexible elastomeric seals; ASTM F477 covers elastomeric seals. Use the manufacturer’s parts and assembly steps rather than mixing components.
Embedment supports the pipe and helps it keep its shape and grade. Where specified, ASTM D2321 provides installation practices for thermoplastic gravity sewer pipe. Bedding, haunch support, compaction, and cover must fit the soil and expected loads. There is no single burial depth that works for every site.
A careful installation sequence is:
- Excavate to the planned line and grade, while protecting utilities and the foundation.
- Prepare uniform bedding that supports the pipe barrel without point loads.
- Set the basin and pipe to the measured inverts. Keep joints supported and aligned.
- Assemble compatible joints as directed by the pipe maker.
- Place and compact embedment around the pipe, including the haunches, without shifting the line.
- Recheck grade and alignment before completing backfill.
Do not let a coupling bridge a gap or leave a joint unsupported. Small shifts during backfill can create a low spot that holds water and debris.
Testing flow, joints, and final discharge
A pre-backfill check confirms that the pipe still has continuous fall and that water reaches the intended outlet. Inspect the line before it is hidden, then flush or test it using a method suited to the installation and local requirements. Confirm free discharge, no visible leakage, and no standing-water pockets caused by a sag or backfall.
Check the basin connection, joints, bends, and outlet. If flow slows or water remains in the pipe, investigate before backfilling. Possible causes include a shifted pipe, debris, a blocked outlet, or an elevation error. Record the final invert elevations and route so later repairs do not depend on guesswork.
Backfill in the specified lifts and compact as required for the pipe and site. Avoid heavy equipment directly over a pipe until the required cover and support are in place. Keep any required inspection or cleaning access available.
Worked routing examples
These examples show the arithmetic and checks, not approved designs for every site. Actual pipe size, slope, cover, and discharge rules depend on the project and local authority.
Example 1: A 40-foot yard lateral. The proposed grade is ⅛ inch per foot. The required fall is 40 × ⅛, or 5 inches. If the basin outlet invert is 100.00 inches on a project datum, the planned outlet invert is 95.00 inches, assuming a straight, constant-grade route. Confirm that the real outfall is low enough and approved before digging.
Example 2: A tight route near a foundation. The outfall elevation may allow only 3 inches of fall over a 30-foot run. That is 0.1 inch per foot, or about 0.83%. Do not silently substitute that grade for a code or design requirement. Recheck the route, outlet, and applicable criteria with the authority or a qualified designer. A different route or approved discharge point may be needed.
Example 3: A driveway crossing. The route may be short, but vehicle loading affects pipe selection, embedment, and cover. Do not assume a product’s ASTM designation sets a safe cover depth. Confirm the pipe class and installation detail for the actual soil and load.
Quick comparison of common choices
| Situation | Check first | Practical next step |
|---|---|---|
| Landscape catch basin to surface outlet | Local discharge rules and outlet elevation | Survey inverts and verify erosion control |
| Catch basin near a storm system | System owner approval and connection detail | Obtain permission before tying in |
| Proposed connection to sanitary sewer | Explicit written authorization | Do not connect unless expressly allowed |
| Pipe below a drive or parking area | Loading, pipe class, cover, embedment | Use approved project or manufacturer details |
| Route with little available fall | Required grade and outlet elevation | Revise route or seek an engineered solution |
Conclusion and FAQ
Subterranean catch basin routing is a gravity-drainage design problem, not simply a matter of burying PVC at a familiar pitch. Confirm a lawful outlet, calculate available fall from surveyed inverts, size the pipe for the design flow, and install it with proper support. Test and document the line before covering it.
Is ⅛ inch per foot the required PVC slope for every catch basin?
No. It is a slope listed in the 2021 IPC storm-drain sizing table, not a universal rule for every landscape lateral. Check local requirements.
What percent grade is ⅛ inch per foot?
It is about 1.04%. The calculation is 0.125 inch of fall divided by 12 inches of run.
How much fall does a 30-foot pipe need at ¼ inch per foot?
It needs 7.5 inches of fall: 30 multiplied by ¼ inch per foot.
How do I know if gravity drainage will work?
Compare measured basin and outlet inverts with the required fall over the full route. Confirm the outlet is approved and can accept the water.
Can I connect a catch basin to a sanitary cleanout?
Not unless the authority expressly permits it. Many jurisdictions prohibit stormwater connections to sanitary sewers.
Does ASTM D3034 tell me how deep to bury PVC?
No. The designation does not, by itself, set allowable cover or traffic loading. Verify the pipe class and project installation requirements.
What do ASTM D3212 and ASTM F477 cover?
They relate to gasketed plastic-pipe joints and elastomeric seals. Use components compatible with the pipe maker’s system.
Why does a pipe need haunch support?
Support around the lower sides helps the pipe resist movement and maintain its shape and grade. Follow the project requirements for bedding and compaction.
Should I test the line before backfilling?
Yes. Check continuous fall, joints, free discharge, and signs of standing water before the pipe is hidden.
Is there one standard burial depth for catch basin pipe?
No. Cover depends on the pipe, soil, frost conditions, and surface loads. Use local or engineered requirements for the site.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
