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Grading Plan to Finish Grade: Layout, Cut and Fill & Stormwater on the Lot

Grading Plan to Finish Grade: Layout, Cut and Fill & Stormwater on the Lot

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Alright, Grading Plan to Finish Grade: Layout, Cut and Fill & Stormwater on the Lot. Don't let the plain title fool you. Here's the part that actually matters on the job: Every elevation on the lot comes from the benchmark and the grading plan — shoot it, stake it, cut and fill to it, and give every drop of water a pipe or a swale with an outlet lower than its inlet. Learn it well and your patios drain, your walls land at the right height, and nobody calls you in April about water in the basement.

The intro taught you what the grading plan shows. This lesson turns those numbers into dirt: setting the instrument, converting plan elevations to rod readings, moving the right amount of soil, and building the drainage the plan assumes is there.

Working from the benchmark

Everything starts at the benchmark (BM) named on the plan, usually a manhole rim, a hydrant bolt, or a nail in a utility pole with a known elevation. Set the rotary laser or transit where it sees the whole work area and take a backsight rod reading on the benchmark. Benchmark elevation plus backsight equals the height of instrument (HI). From then on, any point's elevation is HI minus its rod reading, and the rod reading you want for a plan elevation is HI minus that elevation. Write the HI on a stake by the instrument.

Plans read in decimal feet: a hundredth is about one-eighth of an inch. Shoot the benchmark again after lunch and any time the tripod is bumped; a moved instrument quietly rebuilds the whole patio an inch off.

Stake what the plan calls out: finish floor elevation at the doors, top and bottom of wall, spot grades at the corners and high and low points of every hardscape, and the rim and invert of every drain structure. Mark hubs with cut or fill in decimal feet, and set offset stakes outside the work so they survive excavation.

Cut, fill, swell and shrink

Strip and stockpile topsoil first. Compute cut and fill by the grid method: divide the area into squares, average the cut or fill at each square's corners, multiply by its area, and total. Loose soil swells about 20 to 30 percent when excavated, so a 10-cubic-yard cut is 12 or 13 yards in the truck. Fill shrinks about 10 to 15 percent when compacted, so you need more than the neat volume. Clay moves more than sand.

Fill under hardscape is structural fill: clean mineral soil or aggregate in 6- to 8-inch loose lifts, compacted, never topsoil, organics, or debris. Fill under planting is the opposite, loosened and amended. Keep the two separate on the plan and on the ground.

Finish grade rules around the house

The intro covered the 6 inches of fall in the first 10 feet under IRC R401.3. The second rule is clearance: keep soil and mulch at least 6 inches below untreated wood siding and framing, below the weep screed on stucco, and below brick veneer weeps. Burying weeps traps water in the wall. Patios against the house sit a step below the threshold, sloped 1 to 2 percent away (one-eighth to one-quarter inch per foot). Beds against the house slope away too; a bed that holds water against a foundation is the most common cause of a wet basement after a landscape job.

Swales are built, not scraped: at least 1 percent of longitudinal slope, preferably 2, side slopes of 4:1 or flatter so they mow, and 6 inches or more of depth. A swale that flattens at the property line is a pond.

Pipe, structures and outlets

Roof water, drains, and sump discharges travel in solid pipe; only French drains and footing drains are perforated. Common choices are gasketed SDR 35 PVC, Schedule 40 PVC, and dual-wall smooth-interior HDPE. Single-wall corrugated pipe crushes under a wheelbarrow and cannot be jetted; use it only where the spec allows and nothing will drive over it. Slope pipe at least 1 percent (one-eighth inch per foot), 2 where you can, with cleanouts at changes of direction and on long runs.

Set catch basin and channel drain rims about one-half to 1 inch below the finished surface so water actually enters. Bed pipe on stone and compact trench backfill in lifts, or the trench line shows in the patio in a year. Every system ends at an outlet lower than its inlet: daylight on a slope with a rodent grate, a pop-up emitter with real fall, a dry well sized from an infiltration test, or a permitted tie-in to the storm sewer. Discharging onto the neighbor, the sidewalk, or the street is a violation almost everywhere. Protect daylight outlets with a stone splash pad.

Permeable pavement and stormwater devices

Where the civil plan shows permeable interlocking concrete pavement, bioretention, or underground detention, you are building an engineered stormwater device. Keep sediment and dense-graded base out of open-graded stone, do not compact a bioretention subgrade, use the specified media, and expect the locality to inspect it and require an as-built.

Going Deeper (Intermediate)

Size a dry well from the storm, not a guess. One inch of rain on 1,000 square feet of roof is about 83 cubic feet of water. Washed stone holds roughly 35 to 40 percent of its volume in voids, so that storm needs about 210 to 240 cubic feet of stone, or a chamber with a known capacity, and a soil that drains it before the next storm. Run a perk test first: saturate a test hole and time the drop. Clay that drops less than about one-half inch per hour will not empty a dry well, and a surface outlet is the honest answer.

Advanced / Pro-Level

Pros keep an as-built grading sketch with the elevations they hit and every pipe route, depth, and outlet, because the next crew will trench through anything undocumented. On larger sites they run laser receivers on the skid steer or GPS machine control from the engineer's surface model, then still check grade with a rod. When the architectural and civil finish floors disagree, they write an RFI rather than splitting the difference, and they photograph every outlet before it is buried.

Practice Challenge

The benchmark is 100.00, your backsight reads 4.25, and the plan shows the patio at 98.60 with the door threshold at 99.10. What is your HI, what rod reading sets the patio, and does the patio clear the door? (Answer: HI is 104.25; the patio rod reading is 104.25 minus 98.60, or 5.65 feet; the patio sits 0.50 feet, about 6 inches, below the threshold, which gives you one step down and keeps water off the door, and the patio still needs 1 to 2 percent of fall away from the house from that elevation.)

In Practice

A foreman who sets the instrument, writes the HI on a stake, and shoots the benchmark after every move builds patios that match the plan and drain the first time it rains. A foreman who grades by eye from the neighbor's fence builds a flat patio that ponds against the house and a wall that ends 4 inches short of the step.

Common Mistakes to Avoid

Takeaway: Every elevation on the lot comes from the benchmark and the grading plan — shoot it, stake it, cut and fill to it, and give every drop of water a pipe or a swale with an outlet lower than its inlet.

Educational content only — not design, engineering, or hands-on instruction, and not a substitute for training under a qualified landscape or hardscape professional or the judgment of a licensed engineer, landscape architect, or plumber. Follow the IRC/IBC, plumbing, and stormwater codes as adopted in your jurisdiction, the project drawings and specifications, CMHA (ICPI/NCMA) installation guidelines, the manufacturer's instructions, OSHA 29 CFR 1926, and your employer's procedures.

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