Grading

On this page:

• Design Criteria
• Design Considerations

Complete and proper street grading can be a challenge in Seattle because of steep slopes and variable topography. Construction of street improvements requires grading the right-of-way to the standard design cross section and below the maximum street grades. SDOT must approve street grades for permanent improvements of each street and alley to minimize adverse impacts on adjacent private property.

Private developments must be designed to accommodate a planned permanent street grade that when fully improved with pavement, curbs, and sidewalks, the street grade will not:

• Result in driveways that are too steep;
• Require retaining walls to protect foundations and landscaping; and
• Create the possibility of an inaccessible or unsafe condition

Design Criteria

Grading for Standard Design Cross Section: The standard design cross section consists of a crowned roadway centered in the right-of-way, sloping down at 2% from the crown to the gutter line, with a 6-inch high curb and a 2% slope up from the top of the curb to the right-of-way line. See the standard design street details in Figure H: Design Cross Section. New development shall be designed to accommodate the standard design cross section.

Centerline Profile: The centerline profile shall have a constant slope from cross-street to cross-street, with vertical curves as needed at street intersections. Additional slope changes within the block are permitted only when a constant slope cannot be obtained or when needed to accommodate street drainage.

Maximum Slope: The project shall be designed so that any new grading on site will not exceed the maximum slope permitted if it becomes necessary in the future to adjust site grades to accommodate grading for street improvements. The maximum slope permitted without a retaining wall is two horizontal to one vertical (2H:1V). Grades steeper than this require installation of an approved retaining wall or structure.

Survey: Because of the relationship between street grades and site grades, it is essential that survey information for both on-site and off-site improvements be based on NAVD-88 datum, using City of Seattle bench marks and monuments as reference points. When no profile has been established for the streets abutting and leading to the development site, the developer shall provide a survey of the street area by a licensed surveyor for the purpose of establishing the proposed centerline profile. SDOT must approve the centerline profile of the street and accompanying centerline elevations proposed by the developer’s professional civil engineer.

The survey shall extend the full length of the block plus 50 feet on either side, show adequate cross section, and be based on NAVD-88 Datum. Include NAVD-88 marks with id numbers, descriptions, locations and elevation. Get more complete information on survey requirements.

Maximum/Minimum Roadway Grade: The following tables show crown elevations along the centerline of a traveled way.

Street Classification	Maximum roadway profile grade permitted
Principal Arterials	9%
Commercial Access Streets	9%
Collector Arterials	10%
Minor Arterials	10%
Residential Access Streets	17%
Alleys	17%
Sidewalk	Less than or equal to roadway slope or 5% if not adjacent to roadway

 

Surface Material	Minimum roadway centerline profile grade permitted
Asphalt roadway	1%
Concrete roadway	½%

If the grade of the street or alley exceeds 10%, asphalt concrete or Portland cement concrete is required, crushed rock will not be permitted. If the proposed roadway cannot be kept below the maximum slope the project may meet the street improvement exception criteria defined in the Land Use Code, SMC 23.53.

Vertical Alignment: The design and placement of vertical curves must take into account ADA compliant crosswalk slopes and curb ramps. The placement of the point of vertical curvature (PVC) at intersections must be carefully considered.

Vertical curves must be based on appropriate design standards (City and County Design Standards, AASHTO or WSDOT Design Manual) and should not be less than the following minimum vertical curves:

Roadway Posted Speed Limit	Minimum Vertical Curve
35 mph or less	3 times the design speed (V d) where V d is no less than the posted speed limit
Greater than 35 mph	3 times the design speed (V d) where V d is 5 mph greater than the posted speed limit

For downtown streets with steep grades, shorter vertical curves may be required to avoid excessive regrading. Deviation requests for proposed vertical curves not meeting the standards must be submitted with analysis showing that vehicles will not bottom out on the curve.

Horizontal Alignment: Design speeds are established by the City Traffic Engineer based on current engineering standards and practices. A minimum horizontal radius with a maximum 4% super elevation for urban conditions are as follows:

Design Speed	Design Radius
20 mph	125 feet
25 mph	205 feet
30 mph	300 feet
35 mph	420 feet
40 mph	565 feet

Regrading for Construction: The design of on-site improvements such as foundations, footings, floor elevations, building entries, driveways, and utility service connections shall be compatible with all grading that will be required to install future street improvements. This is especially critical to project design when the building and doorways are at or near the property line, where driveway slopes are at or near the maximum allowed, and where building height is at or near the maximum permitted by the Land Use Code.

Design Considerations

• Consult the standard profiles in the early stages of project development. Profiles for many Seattle streets have already been determined by SDOT, including some streets not yet open to traffic. These profiles may be a helpful starting point in designing profiles for unopened rights of way. The established street profiles are available at the SPU Record Vault. Consideration of private and public infrastructure that has been built since the profiles were established must also be taken into account.
• In general, the point of vertical curvature (PVC) shall not encroach in to a cross street.
• Foundations and footings shall be designed and constructed so they will not be uncovered or undermined by future grading required for street improvements.
• Grading at intersection approaches should consider appropriate transitions to avoid vehicles bottoming out.