Grading, Drainage, and Stormwater
CSLB lists Construction Preparation at about 12 percent of the C-27 examination, covering site planning, erosion control, and trenching and grading, and it lists drainage as part of the 21 percent Drainage, Irrigation, Lighting, and Plant Installation section. This chapter covers the grading and water-management half of that work: how the ground is shaped, where the water goes, and the stormwater obligations that attach to a construction site. The next chapter covers the hardscape you build on that graded ground.
Reading and Setting Grades
A slope can be written two ways and the exam uses both. A ratio such as 4:1 or 3:1 means horizontal run to vertical rise, so a 3:1 slope runs three feet horizontally for each foot of fall, and a 4:1 slope is gentler than a 3:1. A percentage is rise or fall divided by run, times 100. Work an example: a plan shows a spot elevation of 101.50 at a patio corner and 100.75 at a drain 25 feet away, so the fall is 0.75 feet over 25 feet, and 0.75 divided by 25 is 0.03, or about 3.0 percent. Contour lines on a grading plan connect points of equal elevation, so lines drawn close together mean the ground is steep there, and lines far apart mean it is nearly flat. A builder's level or laser level is used to establish consistent elevations and grades across a site, and the rod reading at each stake gives the elevation of that point relative to the instrument's level plane, which is what lets one person set a whole lawn panel to a uniform fall. Two grade targets recur. Finished grade around a building slopes away from the foundation, and the standard target for the first several feet is a fall of about 5 percent, roughly six inches over ten feet. A paved surface is given a cross slope of about one to two percent, which is roughly an eighth of an inch per foot, so that water sheds rather than ponds. Two clearances matter at the building. Finish grade of a lawn is set about an inch below an adjoining concrete walk so water sheds onto the lawn rather than pooling on the walk. And where a stucco wall has a weep screed at its base, the finish grade must maintain the clearance below the screed that the wall assembly requires; a crew that finds the side-yard grade two inches above the weep screed must lower the grade to restore the clearance, because burying the screed traps water in the wall.
Surface Drainage and Piped Systems
The first choice in drainage is whether to move water on the surface or in a pipe. A shallow vegetated swale graded toward an outlet moves surface runoff across a yard without a piped system, and it is cheaper, easier to maintain, and self-cleaning. A piped system is used where the surface cannot be graded to fall, and it starts with catch basins. A catch basin collects surface water and pipes it away, and its grate is set slightly below the surrounding grade so water actually flows into it rather than past it; a grate set flush or high collects nothing. Landscape drain lines are given a fall of about one percent, roughly an eighth of an inch per foot, which is the minimum that keeps velocity high enough to move the fines that inevitably enter and prevents the line from silting up. When a yard drain system backs up during heavy rain although the pipe itself is clear, the next thing to check is the outlet: a discharge that is submerged, buried, or blocked by growth stops the whole system regardless of the condition upstream. Subsurface water is intercepted differently. A French drain is a gravel-filled trench with a perforated pipe, and the perforated pipe is laid with the holes down, in washed rock, wrapped in filter fabric, so that groundwater rises into the pipe from below and the fabric keeps fines from clogging the rock. Downspouts belong in this system rather than discharging onto a bed near the foundation; the appropriate landscape correction is to connect them to a pipe that discharges away from the house, or at minimum to a splash block, which prevents erosion and ponding at the foundation. A dry well proposed to handle roof runoff on a lot with heavy clay subsoil deserves a caution: clay infiltrates too slowly for the well to empty between storms, so it fills and stops working. One legal limit applies to all of this. A grading plan that would send additional runoff onto a neighboring downhill lot is not acceptable; the contractor's obligation is to design the drainage to an approved outlet off the property.
Compaction and Fill
Compaction is measured, not judged by eye. A specification requiring 90 percent relative compaction of the subgrade is stating a percentage of the maximum dry density determined by a laboratory compaction test on that soil, so the field density is compared against a lab reference for the same material. Moisture is the variable most often missed: every soil has an optimum moisture content at which it reaches its maximum density under a given compactive effort, which is why two crews compacting the same base material at very different moisture levels get very different results. Too dry and the particles will not slide into place; too wet and water occupies the pore space the compactor is trying to close. Method matters as much as effort. Base and fill are placed and compacted in thin lifts, commonly about four to six inches, each compacted before the next is placed, because a compactor's energy only reaches so far below the surface and a thick lift is dense on top and loose underneath. Equipment is matched to the soil: a rammer or jumping jack, which delivers impact energy, is best suited to compacting narrow trench backfill in cohesive clay, while a vibratory plate suits granular material. Fill on a hillside terrace needs one more step: benching. Cutting level steps into the existing slope before placing fill gives the fill a horizontal surface to be compacted against, so the new material keys into the old ground rather than sitting on a slope where it can slide. Where a specification calls for six inches of imported topsoil over a native clay subgrade, the interface must be scarified and the two soils blended at the boundary, because a sharp texture change between two soils creates a perched water table and a root barrier. Expansive clay is California's chronic soil problem for hardscape: it swells when wet and shrinks when dry, and that seasonal movement lifts and drops footings, which is why a garden wall on expansive clay cracks in the same places every year.
Erosion Control and Construction Stormwater
A graded site with no cover loses soil to the first rain, and California regulates that loss. On a steep planted slope during establishment, the most effective combination is an erosion control blanket with plantings and fiber rolls: the blanket holds the soil surface and shelters germinating seed, the plantings provide the permanent cover, and the fiber rolls break the slope into shorter segments. Fiber rolls are placed along the contour rather than up and down the slope for a reason worth understanding: laid across the flow they intercept sheet flow and shorten the distance water travels before it is slowed, which is what limits the velocity that causes erosion. Laid up and down, they become channels. Hydroseeding a slope uses a tackifier in the slurry for the same reason, to bind the mix to the slope until the seed germinates. Sediment control on an active construction site follows a simple order: protect the inlets and install perimeter controls first, before the disturbance, because sediment that has already left the site cannot be recovered. Where a landscape project will disturb more than one acre of soil, an additional obligation typically applies: coverage under the state construction stormwater general permit, which requires a stormwater pollution prevention plan and monitoring. Stormwater management on the completed landscape is a design opportunity as well as a requirement. A bioswale slows runoff, filters pollutants through soil and vegetation, and promotes infiltration, which is why commercial landscapes increasingly include them, and permeable pavers do the same job under a traffic surface by letting stormwater pass through the joints into an open-graded reservoir base.
Last updated: September 2026