How to Read a Drainage Plan

If you are building a new home, adding an addition, or dealing with a drainage dispute with a neighbor, you may encounter a drainage plan (also called a grading plan, stormwater plan, or site drainage plan). These drawings look intimidating, but once you understand the basic symbols and conventions, you can read them with confidence.

This guide teaches you how to interpret the most common elements of a residential drainage plan.

What Is a Drainage Plan?

A drainage plan is a scaled drawing that shows how water moves across a property. It includes existing and proposed ground elevations, the direction water flows, and the location of drainage structures like pipes, catch basins, and swales.

Drainage plans are typically required by your local building department for:

• New home construction
• Additions that change the footprint of the building
• Significant grading changes
• Stormwater management compliance
• Subdivision development

The plan is usually prepared by a civil engineer or land surveyor and is drawn on a site plan (an aerial view of the property).

Key Elements of a Drainage Plan

Contour Lines

Contour lines are the curved lines that represent elevation. Each line connects points of equal elevation above sea level.

• Existing contours are usually shown as dashed lines
• Proposed contours are shown as solid lines
• The number on each line is the elevation in feet (for example, 102.0 means 102 feet above sea level)
• Water flows perpendicular to contour lines, from higher elevations to lower elevations
• Closely spaced contour lines indicate a steep slope; widely spaced lines indicate a gentle slope

The contour interval (the elevation difference between adjacent lines) is noted in the legend. Residential plans typically use 1-foot or 2-foot intervals.

Spot Elevations

Spot elevations are specific elevation points marked on the plan. They look like an X or a + with a number next to it.

Common spot elevation locations include:

• Building corners (finished floor elevation)
• Driveway edges
• Property corners
• Drainage structure inverts (the bottom of a pipe or basin)
• High points and low points in the yard

Reading spot elevations: A spot elevation of “FF=103.5” means the finished floor is at 103.5 feet. “TC=102.0” means the top of curb is at 102.0 feet.

Flow Arrows

Flow arrows show the intended direction of surface water flow. They are drawn as arrows pointing downhill. If you follow the arrows, you can trace the path water takes from any point on the property to the ultimate discharge point (usually the street, a storm drain, or a natural drainage way).

Pipe Networks

Underground pipes are shown as lines with specific symbols:

Example: A label reading “8” RCP @ 1.5% INV=98.5” means an 8-inch reinforced concrete pipe at a 1.5 percent slope, with the bottom of the pipe at elevation 98.5 feet.

Drainage Structures

Common drainage structures on a residential plan:

• CB: Catch basin (collects surface water through a grate)
• MH: Manhole (access point for underground pipes)
• DI: Drop inlet (a type of catch basin)
• FES: Flared end section (pipe outlet at daylight)
• RIP: Riprap (loose stone at a pipe outlet for erosion protection)
• SD: Storm drain or subdrain
• FD: French drain or footing drain

Swales and Ditches

Swales are shown as V-shaped or U-shaped contour bends that point uphill. They channel water along the surface. On a plan, swales are often labeled with their slope (for example, “1% swale”) and may include cross-section details showing the depth and width.

Impervious Area

The plan may show the total impervious area (roof, driveway, patio, sidewalk) as a percentage of the lot. Many municipalities limit the impervious area to control runoff. Typically, you will see a note like “Impervious coverage: 35% (max allowed: 40%).”

Common Abbreviations

How to Read Slopes

Slopes are expressed in different ways on drainage plans:

• Percentage: 2% means the ground drops 2 feet for every 100 feet of horizontal distance. This is the most common format.
• Ratio: 4:1 means 4 feet horizontal for every 1 foot vertical. Steeper ratios like 2:1 are used for embankments; gentler ratios like 10:1 are used for swales.
• Feet per foot: 0.02 ft/ft is the same as 2%.

For residential grading, a minimum slope of 2 percent (quarter inch per foot) away from the foundation is standard practice for the first 6 to 10 feet from the house.

Reading the Plan Step by Step

Here is how to systematically read a drainage plan:

Step 1: Find Your House

Locate the building footprint on the plan. Note the finished floor elevation. This is your primary reference point.

Step 2: Trace the Water Path

Starting from the highest point of the property, follow the contour lines and flow arrows to see where water goes. Ask yourself: does all surface water flow away from the house? Where does it ultimately end up?

Step 3: Check Foundation Grades

Look at the spot elevations around the foundation. The ground should slope away from the house in all directions. A minimum of 6 inches of fall in the first 10 feet is recommended (that is about 5 percent slope).

Step 4: Identify Low Points

Find the lowest elevation on the property. This is where water will collect. There should be a drainage structure (catch basin, swale, or pipe outlet) at every low point. If there is a low point with no drainage solution, ask the engineer about it.

Step 5: Check Pipe Sizes and Slopes

Look at the pipe network. Verify that pipes flow downhill (upstream invert elevation should be higher than downstream invert elevation). Use the Manning’s Pipe Flow Calculator to verify that the pipe size is adequate for the expected flow.

Step 6: Check for Off-Site Drainage

Determine whether any water from your property flows onto neighboring properties, or whether water from neighboring properties flows onto yours. This is one of the most common sources of drainage disputes.

Questions to Ask About Your Drainage Plan

If you receive a drainage plan for a project on your property, here are the important questions to ask:

1. Where does the water go? Trace the complete path from roof to discharge point.
2. What is the design storm? Most residential plans are designed for a 10-year or 25-year storm. Ask what happens during a larger storm.
3. Is there a maintenance plan? Catch basins need periodic cleaning. Swales need mowing. Know what maintenance is required.
4. What about the 100-year flood? If any part of the property is in a flood zone, the plan should address it.
5. Does the grading match what was built? After construction, have the surveyor verify that the final grades match the plan. This is called an as-built survey.

Using Calculators to Verify a Plan

You do not need to be an engineer to spot-check a drainage plan. These calculators can help:

• Use the Rational Method Calculator to estimate the peak flow from a drainage area and verify it against the pipe sizes shown on the plan.
• Use the Manning’s Pipe Flow Calculator to verify that a pipe shown on the plan can handle the design flow.
• Use the SCS Curve Number Calculator to estimate total runoff from the site for comparison with detention or retention requirements.

When to Call a Professional

You should consult a civil engineer or licensed surveyor if:

• You cannot identify the discharge point for water from your property
• The plan shows water flowing toward the house in any location
• You need to challenge or modify a drainage plan submitted by a developer
• You have a drainage easement on your property and do not understand its implications
• You are in a drainage dispute with a neighbor and need an independent evaluation
• You suspect the as-built grades do not match the approved plan

A professional plan review typically costs $300 to $1,000 depending on the complexity.

Related Calculators

• Rational Method Calculator — Verify flow estimates on the plan
• Manning’s Pipe Flow Calculator — Check pipe sizing
• SCS Curve Number Calculator — Estimate site runoff
• Catch Basin Calculator — Verify catch basin sizing