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newSchoolWorks.com › VoTechWorks › Building & Construction › Masonry › Layout & Leveling › Lesson 1
BLD-MASO04-10 · Lesson 1 · 9 clock hours · theory

Layout Fundamentals: Datums, Benchmarks, and Site Plans

Objective. Students will interpret a basic site plan, identify the benchmark and datum, and explain how horizontal control lines and vertical grades are derived from those references.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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What Is a Datum?

Every construction project needs a fixed starting point for measurements. A datum is any stable, agreed-upon reference level from which all vertical measurements on a project are made. In the United States, the most common national datum is the North American Vertical Datum of 1988 (NAVD 88), which ties elevations across the country to mean sea level. On most residential and light-commercial jobs, however, the project engineer assigns a simpler local datum — often a round number such as 100.00 ft — to a convenient fixed point on site. All other elevations are then described as higher or lower than that point.

Benchmarks

A benchmark (BM) is the physical object that marks the datum elevation on or near your jobsite. It must be permanent, stable, and clearly identified on the site plan. Common benchmarks include:

The benchmark elevation is always printed on the site plan next to the symbol for that point. Before any layout work begins, verify that the benchmark has not been disturbed. Never assume a benchmark is correct — check it against a second known point whenever possible.

Reading a Site Plan

A site plan (also called a plot plan) is a scaled, bird's-eye drawing of the property showing the building footprint in relation to the lot. Key items a mason must locate before driving the first stake include:

1. Property lines — the legal boundary of the lot, usually shown as a bold or dashed perimeter line with bearing and distance labels.

2. Setback lines — minimum distances from property lines within which no structure may be built; set by local zoning ordinance.

3. Building footprint — the outline of the foundation, dimensioned from property lines or established control lines.

4. Benchmark symbol and elevation — typically a triangle or circle with the letters BM and a numeric elevation.

5. Finished floor elevation (FFE) — the design elevation of the top of the finished floor slab or wood subfloor, printed on the plan or in the general notes.

6. Existing and finished grades — contour lines or spot elevations showing existing ground (dashed) and design ground (solid) at key points.

7. Utilities and easements — locations of underground lines that affect where equipment may operate.

Always use an architect's scale to confirm that printed dimensions agree with scaled distances. If they conflict, printed dimensions govern.

Horizontal Control Lines

Horizontal control is the plan-view location of the building. Masons establish it by stretching batter boards and mason's line to define the outer edges of the foundation. These lines are derived from the property corner stakes set by the surveyor, offset by the required setbacks, and checked for square using the 3-4-5 rule or by comparing diagonal measurements.

Before touching any control line, verify that survey stakes are undisturbed. If a stake is missing or suspect, stop work and notify the supervisor — do not guess.

Vertical Grade: Cut and Fill

Vertical control means getting the ground and the masonry to the correct elevation. The site plan gives you the existing grade (what is there now) and the finished grade (what is required). The difference tells you whether the site needs cut (material removed) or fill (material added):

The mason is especially concerned with the relationship between the benchmark, the finished grade, and the finished floor elevation. The FFE is always higher than finished grade by at least the stem-wall height (or slab thickness plus base course). Calculating these differences early prevents costly errors in foundation height.

Deriving the Finished Floor Elevation from the Benchmark

The benchmark gives you a known elevation. From that point, a surveyor's level or a builder's level is used to transfer the elevation to batter boards or grade stakes. The difference between the benchmark elevation and the FFE tells you how far up (or down) to set those boards.

Formula: `Grade difference = FFE − Existing grade at a given point`

A positive result means fill is needed; a negative result means cut is needed.

Safety on the Layout Site

Key terms10

DatumA fixed reference level from which all elevations on a project are measured; may be tied to a national standard such as NAVD 88 or assigned locally by the project engineer.
Benchmark (BM)A permanent physical point on or near the jobsite whose elevation is known and printed on the site plan; the starting point for all vertical layout.
Site planA scaled, overhead drawing showing property boundaries, building footprint, setbacks, grades, utilities, and other site features needed to locate and build the structure.
Setback lineThe minimum required distance between a property line and any part of the building, established by local zoning ordinance.
Finished floor elevation (FFE)The design elevation of the top surface of the completed floor, used as a key vertical control point for all masonry and framing above.
Existing gradeThe actual ground surface elevation at a point before any excavation or fill work is performed; shown as dashed contour lines or spot elevations on the site plan.
Finished gradeThe design ground surface elevation at a point after all earthwork is complete; shown as solid contour lines or spot elevations on the site plan.
CutEarthwork that removes material because existing grade is higher than the required finished grade at that location.
FillEarthwork that adds material because existing grade is lower than the required finished grade at that location.
Batter boardsHorizontal boards nailed to stakes set back from the building corners, used to support and re-establish mason's lines that define the horizontal position of the foundation.

Worked examples4

1. The benchmark elevation on a site plan is 100.00 ft. The finished floor elevation is listed as 101.50 ft. The existing grade at the building corner is 99.80 ft. How much fill is needed at that corner to reach finished grade if finished grade at that point is 100.20 ft?
  1. Identify the finished grade at the point: 100.20 ft.
  2. Identify the existing grade at the point: 99.80 ft.
  3. Calculate the difference: Finished grade − Existing grade = 100.20 − 99.80 = 0.40 ft.
  4. Because the result is positive (finished grade is higher than existing grade), material must be added — this is a fill situation.
  5. The amount of fill required at this corner is 0.40 ft (about 4¾ inches).
Answer: 0.40 ft of fill is required at that corner.
2. The benchmark is set at elevation 100.00 ft. The finished floor elevation is 101.25 ft. What is the vertical distance from the benchmark to the FFE, and is the FFE above or below the benchmark?
  1. Write the subtraction: FFE − Benchmark = 101.25 − 100.00 = 1.25 ft.
  2. The result is positive, so the FFE is above the benchmark.
  3. When transferring this elevation with a builder's level, the rod reading at the FFE location must be 1.25 ft less than the rod reading at the benchmark (the instrument shoots a flat horizontal plane, so a lower rod reading means the ground is higher).
Answer: The FFE is 1.25 ft above the benchmark.
3. A site plan shows three spot elevations for existing grade: Point A = 101.60 ft, Point B = 100.30 ft, Point C = 99.50 ft. The required finished grade at all three points is 100.00 ft. Determine whether each point needs cut or fill and by how much.
  1. For each point, calculate: Difference = Finished grade − Existing grade.
  2. Point A: 100.00 − 101.60 = −1.60 ft. Negative result → existing grade is too high → CUT 1.60 ft.
  3. Point B: 100.00 − 100.30 = −0.30 ft. Negative result → existing grade is slightly high → CUT 0.30 ft.
  4. Point C: 100.00 − 99.50 = +0.50 ft. Positive result → existing grade is too low → FILL 0.50 ft.
Answer: Point A: cut 1.60 ft; Point B: cut 0.30 ft; Point C: fill 0.50 ft.
4. A mason's batter board must be set at the finished floor elevation of 102.40 ft. The builder's level is set up and a rod reading of 4.85 ft is obtained at the benchmark (elevation 100.00 ft). What rod reading should the instrument person call out when the rod is held at the batter board and the board is at exactly the right height?
  1. Calculate the height of instrument (HI): HI = Benchmark elevation + Rod reading at BM = 100.00 + 4.85 = 104.85 ft.
  2. Calculate the required rod reading at the batter board: Rod reading = HI − Desired elevation = 104.85 − 102.40 = 2.45 ft.
  3. When the rod reads 2.45 ft at the batter board location, the top of the board is at elevation 102.40 ft — the correct FFE.
Answer: The rod reading at the batter board should be 2.45 ft.
In class: Instructor-led walk-through of a sample residential site plan: students locate the property lines, benchmark elevation, building setback lines, and finished floor elevation, then annotate a printed copy. · Small-group discussion: compare three site plans of increasing complexity (slab-on-grade, crawl space, full basement) and list the layout information a mason needs from each before driving a stake. · Individual written exercise: given a benchmark elevation and a finished floor elevation, calculate the difference in grade and identify whether fill or cut is required at three given points on a sketch.
Materials: Sample site plans (residential and light commercial), architect's scale, highlighters, pencils; NCCER Masonry Level 2 module on Layout; tape measure; safety glasses
Check: Students correctly identify all required layout data on a new sample site plan and calculate two grade differences; 80% accuracy required on a 10-question written check.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Masonry — Layout & Leveling
1. What is the primary purpose of a datum on a construction project?
NCCER Masonry — Layout & Leveling
2. On a site plan, the benchmark elevation is shown as 100.00 ft and the finished floor elevation is 102.75 ft. What is the vertical difference between these two points?
NCCER Masonry — Layout & Leveling
3. The existing grade at a building corner is 101.20 ft and the required finished grade at that corner is 100.50 ft. What earthwork operation is needed, and how much?
NCCER Masonry — Layout & Leveling
4. Which of the following best describes a benchmark?
NCCER Masonry — Layout & Leveling
5. A builder's level is set up and gives a rod reading of 5.20 ft at a benchmark of elevation 100.00 ft. What is the height of instrument (HI)?
NCCER Masonry — Layout & Leveling
6. When dimensions printed on a site plan conflict with distances scaled from the drawing, which takes precedence?
OSHA 10 — Construction Safety & Health
7. Before driving stakes for layout on a site you have not worked on before, what safety step is required for any ground penetration?
NCCER Masonry — Layout & Leveling
8. A mason is comparing site plans for three foundation types. Which piece of information is critical to collect from the site plan before driving the first layout stake, regardless of foundation type?
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