The Role of Grading in New Construction: Getting It Right

Why Grading Makes or Breaks Your Construction Project

Grading is one of those construction tasks that gets little attention when done right and causes enormous problems when done wrong. Before the foundation is poured, before framing begins, the land itself must be shaped to support everything that follows.

Poor grading leads to water intrusion, foundation settlement, flooded basements, and landscape erosion. These are not cosmetic issues. They are structural problems that cost thousands to fix after the fact, and they are almost entirely preventable with proper grading at the outset.

What Grading Accomplishes

Drainage Control

The primary purpose of grading is directing water away from structures. Oregon receives 35-45 inches of rainfall annually in the Willamette Valley, with most falling between October and May. Every drop of that water needs somewhere to go, and proper grading ensures it goes away from your building, not toward it.

The Oregon building code requires a minimum of 6 inches of fall within the first 10 feet from any foundation. This 5% minimum slope moves surface water away from the structure and toward designated drainage points, whether that is a French drain, a swale, or the street.

Building Pad Preparation

The building pad is the level area where your structure will sit. Grading establishes this pad at the correct elevation relative to:

• The street and public utilities
• Adjacent properties
• Drainage easements
• Flood elevation requirements (if in a floodplain)

Getting the building pad elevation wrong by even a few inches can create drainage problems that persist for the life of the building.

Erosion Prevention

Exposed soil on a construction site is vulnerable to erosion, especially during Oregon's rainy season. Proper grading establishes slopes that minimize erosion velocity and directs runoff to sediment control features like silt fences and detention basins.

Oregon's DEQ 1200-C permit requires erosion control plans for any project disturbing more than one acre. Even smaller projects must meet local erosion control requirements.

The Grading Process for New Construction

Phase 1: Site Assessment

Before any equipment moves dirt, the site needs assessment:

• Topographic survey: Maps existing elevations across the property
• Geotechnical report: Identifies soil types and conditions that affect grading
• Drainage analysis: Determines where water currently flows and where it needs to go
• Utility locates: Identifies underground utilities that must be protected or relocated

Phase 2: Rough Grading

Rough grading is the heavy lifting. This phase involves bulk earthmoving to establish approximate final grades:

• Stripping topsoil: The organic layer (typically 6-12 inches) is removed and stockpiled for later use
• Cut and fill: Soil is moved from high areas to low areas to achieve design elevations
• Compaction: Fill areas are compacted in lifts (typically 8-12 inch layers) to achieve specified density
• Subgrade preparation: The building pad area is compacted to support the foundation

Rough grading tolerances are typically plus or minus 0.1 feet (about 1.2 inches) of design elevation.

Timing matters in Oregon. Rough grading during the dry season (June through September) is significantly easier and less expensive than wet-season work. Willamette Valley clay soils become extremely difficult to work with when saturated, and compaction testing often fails on wet clay.

Phase 3: Utility Installation

After rough grading, underground utilities are installed:

• Water service lines
• Sewer laterals
• Storm drainage pipes and French drains
• Electrical conduit
• Gas lines

These trenches are then backfilled and compacted to grade, maintaining the drainage patterns established during rough grading.

Phase 4: Finish Grading

Finish grading happens after the building is largely constructed. This final phase establishes precise grades for:

• Driveway and walkway subgrade: Providing a stable base for asphalt or concrete surfaces
• Landscape areas: Creating planting beds and lawn areas at proper elevations
• Final drainage swales: Directing water to its ultimate discharge point
• Retaining wall backfill: Grading behind walls with proper drainage material

Finish grading tolerances are tighter, typically plus or minus 0.05 feet (about 0.6 inches).

Grading Standards and Code Requirements

Oregon Building Code Requirements

Oregon's building code sets minimum standards for grading around structures:

| Requirement | Standard | |---|---| | Minimum slope away from foundation | 6 inches in 10 feet (5%) | | Impervious surface slope | Minimum 2% toward drainage | | Swale side slopes | Maximum 3:1 (horizontal:vertical) | | Fill compaction | 90-95% of maximum dry density | | Topsoil replacement | Minimum 4 inches for vegetated areas |

Compaction Standards

Compaction is the most critical quality control measure in grading. Under-compacted fill settles over time, causing:

• Foundation cracks and settlement
• Pavement failure
• Utility line breaks
• Landscaping depressions that collect water

Oregon requires compaction testing by a certified testing laboratory. The standard specification is 95% of the material's maximum dry density (per ASTM D698 or D1557) for structural areas and 90% for landscape areas.

Common Grading Mistakes

Grading Toward the Building

The single most common and costly grading mistake is creating slopes that direct water toward the foundation rather than away from it. This happens when:

• Fill settles after construction, reversing the designed slope
• Landscaping changes alter drainage patterns
• Downspouts discharge too close to the foundation

Always verify final grades slope away from the building on all sides before accepting the work.

Ignoring Adjacent Properties

Oregon law prohibits directing surface water onto neighboring properties. Your grading plan must account for where water goes after it leaves your site. Drainage easements, swales, and storm connections must be designed to handle your site's runoff without impacting neighbors.

Skipping Compaction Testing

Compaction testing costs a few hundred dollars. Foundation repair costs tens of thousands. Every lift of structural fill should be tested. There is no shortcut here.

Working in Wet Conditions

Attempting to grade and compact clay soils in wet conditions leads to poor compaction results and unstable subgrades. If work must proceed during the rainy season, plan for additional soil management costs and potential delays for drying periods.

Neglecting Erosion Control

Graded sites without erosion control measures can lose inches of soil in a single rain event. Silt fences, erosion blankets, and temporary seeding protect your investment and keep you in compliance with Oregon DEQ requirements.

Cut and Fill: Balancing the Site

Effective grading minimizes the amount of soil that needs to be imported or exported from the site. This "cut and fill balance" reduces trucking costs and project duration.

A skilled excavation contractor evaluates the design grades and calculates the net earthwork volume. Small adjustments to building pad elevation or lot grades can sometimes achieve near-perfect balance, saving thousands in hauling costs.

When balance is not achievable:

• Excess cut material: Can often be used for landscape berms, raised planting beds, or offered to nearby projects
• Fill needed: Must be clean, engineered fill meeting compaction specifications. Native topsoil and organic material cannot be used as structural fill

When to Hire a Professional

Grading for new construction is not a DIY project. It requires:

• Equipment: Dozers, excavators, laser-guided grading equipment
• Engineering: Civil engineering plans showing design grades and drainage
• Testing: Compaction testing by certified laboratories
• Permitting: Grading permits and erosion control compliance

Cojo provides professional grading and excavation services across Oregon. We work from your engineer's plans and coordinate with building inspectors to keep your project on schedule.

Request a free grading estimate or learn more about excavation costs in Oregon.