Contour lines are the language of terrain on a grading plan. If you cannot read them fluently, you cannot design a site, review a grading plan, or understand where water goes during a storm. This is not an abstract skill. Every pad elevation, every drainage swale, every retaining wall on your project starts with reading the existing contours and deciding what the proposed contours should be.

This article covers the fundamentals of reading contour lines, the process for setting pad elevations on a development project, and the common grading design decisions that drive constructability and cost.

Contour Line Basics

A contour line connects all points at the same elevation. If you walked along a contour line on the actual ground, you would never go uphill or downhill. Standard conventions on civil engineering plans:

• Index contours are drawn heavier (thicker line weight) and labeled with their elevation. They typically occur at every 5-foot or 10-foot interval, depending on the terrain and map scale.
• Intermediate contours are drawn lighter and fall between index contours. At a 1-foot contour interval, there would be four intermediate contours between two 5-foot index contours.
• Contour interval is the vertical distance between adjacent contour lines. A 1-foot interval means each successive contour represents a 1-foot change in elevation. Steep terrain may use 2-foot or 5-foot intervals; flat parking lots may use 0.5-foot intervals.
• Closely spaced contours indicate steep slopes. If 1-foot contours are 5 feet apart on the plan, the slope is 1:5 or 20 percent. If they are 50 feet apart, the slope is 1:50 or 2 percent.

Calculating Slope from Contours

Slope is the rise divided by the run, expressed as a percentage or a ratio. From contour lines:

The direction of steepest slope is always perpendicular to the contour lines. Water flows perpendicular to contours, downhill. This is fundamental to drainage design. If you can read the contours, you can trace where every drop of stormwater goes on the site before it reaches a drain inlet or leaves the property.

Setting Pad Elevations

The pad elevation (also called the finish floor elevation or FFE) is the elevation at the top of the building slab. Setting it is one of the most consequential decisions in site grading because everything else keys off of it: the parking lot grades, the drainage pattern, the retaining walls, the utility depths, and the building entry accessibility.

Factors That Drive Pad Elevation

1. Flood elevation. If the site is in or near a FEMA flood zone, the pad must be set above the base flood elevation (BFE), typically by at least 1 foot. Many jurisdictions require 2 feet of freeboard above the BFE. This is non-negotiable and often the single biggest constraint on pad elevation.
2. Drainage away from the building. The finished grade must slope away from the building at a minimum of 2 percent for the first 10 feet (IBC Section 1804.4). In practice, 2 to 5 percent for the first 10 feet is standard. This means the pad needs to be high enough relative to the surrounding grades to maintain positive drainage.
3. Accessible entry. At least one building entry must be accessible, which means a maximum slope of 2 percent (1:50) at the entry landing and a maximum running slope of 5 percent (1:20) on the accessible route to it. If the pad is significantly higher or lower than the parking lot, you will need ramps, which cost money and consume space.
4. Utility connections. Sanitary sewer flows by gravity. The sewer lateral from the building needs to fall at a minimum slope (typically 2 percent) to the sewer main. If the sewer main is shallow, the pad cannot be too low or you will not have enough fall. If the main is deep, this is less of a constraint.
5. Earthwork balance. A pad elevation that is too high relative to existing grade creates excess fill, which may need to be imported. Too low creates excess cut, which may need to be exported. The ideal pad elevation minimizes earthwork while satisfying all of the above constraints.

The Iterative Process

Setting a pad elevation is not a single calculation. It is an iterative process where you balance competing constraints:

1. Start with the flood elevation constraint (if applicable). This sets the floor.
2. Check the sewer connection. Can you maintain gravity flow to the main with the proposed pad elevation?
3. Sketch the parking lot grades. Can you drain the lot to inlets while maintaining ADA slopes on the accessible route and positive drainage away from the building?
4. Calculate preliminary earthwork. Is the site reasonably balanced, or are you creating a massive import or export situation?
5. Adjust and repeat. Raise or lower the pad 6 inches and see how it affects everything else.

On complex sites with multiple buildings, this process runs simultaneously for each building, with the added constraint that the grades between buildings must also work for drainage and fire access.

Common Grading Design Principles

Positive Drainage

Every point on the site must drain to a defined collection point (inlet, swale, or sheet flow off-site). There should be no ponding areas that are not designed as detention or bioretention. A minimum slope of 1 percent is needed on paved surfaces to move water; 2 percent is preferred. On unpaved areas, 2 percent minimum with 3 to 5 percent preferred.

Maximum Slopes

Paved parking lots: 5 percent maximum (8 percent for short runs in structured parking). Paved drive aisles: 10 percent maximum for fire access roads; 15 percent for non-fire-access driveways in some jurisdictions. Landscaped slopes: 3:1 (33 percent) maximum for mowable slopes; 2:1 (50 percent) maximum for non-mowed with erosion control. Steeper than 2:1 generally requires a retaining wall or engineered slope stabilization.

Transition Grades at Property Lines

The proposed grading must match existing grades at the property line unless there is a retaining wall. You cannot raise or lower the grade on your property in a way that redirects drainage onto adjacent properties or undermines their improvements. This sounds obvious but it is one of the most common grading plan check comments.

Reading a Grading Plan

A complete grading plan shows:

• Existing contours (dashed lines) and proposed contours (solid lines)
• Spot elevations at building pads, parking lot high and low points, inlet rims, and grade breaks
• Slope arrows showing direction and percentage of flow
• Top-of-wall and bottom-of-wall elevations for retaining walls
• Flow line elevations for swales and ditches
• Earthwork quantities (cut and fill) in the grading notes

When reviewing a grading plan, the first thing to check is whether every area has a path for water to escape. Trace the contours and spot elevations from every high point to its outlet. If you find a low point with no inlet and no contour path out, there is a grading error that will become a ponding problem in the field.

The second thing to check is whether the accessible routes actually work. It is common for the grading design to achieve drainage goals but violate ADA slope maximums on the path from accessible parking to the building entry. These two objectives often conflict, and resolving them is where the skill in grading design shows up.