Elevation and Layout

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Move the drawings onto the ground: position, squareness and elevation, set from a benchmark that does not move.

The big idea

Layout moves the drawings onto the ground. If the layout or elevation is wrong, every trade after you pays for it: footings in the wrong spot, a building that does not fit its setbacks, floors that do not meet the door sills. Layout does five jobs:

• Transfers the drawings to the ground
• Sets the finished floor elevation (FF)
• Sets footing, wall, slab and column elevations
• Squares and aligns the footprint
• Leaves permanent control points for the trades that follow

Survey, pins and setbacks

• Property pins are survey markers at the legal corners of the lot. If they are missing, disturbed or in doubt, have a licensed land surveyor re-establish them before you lay out near a line. In BC that is a BC Land Surveyor.
• Setbacks come from the zoning bylaw and are measured from the lot lines. A permit can be refused, or a stop-work order issued, for a structure that is too close.
• Some local governments require a survey certificate after the foundation is formed or poured, to confirm setbacks before framing starts. Ask whether yours does.
• Sites in a floodplain can have a required minimum elevation for the lowest floor, called a flood construction level, set by local bylaw.

Benchmarks and control points

• Primary benchmark (BM): a fixed elevation reference that will not move, such as a curb, a set pin, a building corner or a rock.
• Secondary benchmarks: backups placed around the site.
• Control points: gridlines, centrelines, offsets and corner points.

Find out which datum the drawings use. Some are tied to real geodetic elevations. Others use an arbitrary site number like 100.00 m (or 100.00 ft). Never mix them. Protect your benchmark, because if it moves the whole building moves.

Finished floor elevation (FF)

• Confirm FF from the approved drawings.
• Transfer it from the benchmark with a laser level.
• Mark it on stakes, batter boards or a fixed object.
• Use it to find footing elevation, top of wall, slab base, door thresholds, driveway slope and deck or landing heights.

Lasers and grade rods

Used to set footing depth, top-of-wall elevation, slab base, pad elevation, step-downs and trench depth. On commercial and civil work a total station is used for high-accuracy layout.

• Check the laser with a two-peg test before you rely on it (below).
• Re-check the height of instrument (HI) on the benchmark every time the tripod is moved.
• A grade rod can read in millimetres or in feet, tenths and hundredths of a foot. Know which one yours is.

Two-peg test

Set two pegs about 30 m (100 ft) apart. The test finds whether the laser is truly level.

1. Set up in the middle. Put the laser halfway between the pegs, 15 m (50 ft) from each. Read the rod on peg A and peg B. Any laser error is the same at both pegs, so it cancels. The difference is the true difference in height.
2. Move to one end. Set the laser about 1 m (3 ft) from peg A, on the line. Read both pegs again.
3. Compare. Compare the difference between the two readings from the new setup with the true difference from step 1. The gap is the laser error over the extra distance.
4. Judge it. Compare the error with the accuracy the maker states for your laser. If it is worse, do not rely on it. Get it checked or calibrated.

Example. Setup 1: rod on A reads 1.512 m, rod on B reads 1.438 m. B is higher by 1.512 − 1.438 = 0.074 m. Setup 2, near A: A reads 1.420 m and B reads 1.341 m, so B looks higher by 0.079 m. The error is 0.079 − 0.074 = 0.005 m, or 5 mm over about 29 m. In imperial that is about 3/16 in over 95 ft.

Batter boards

Batter boards hold your string lines outside the excavation, so the layout survives the digging.

1. Drive stakes at the corners. Place them accurately at the project corners.
2. Set the batter board stakes outside. About 1.5 m (5 ft) out from each corner stake, drive 25 x 100 mm (1x4 in) stakes. Leave room for the trench and the machine.
3. Build the boards. Each board is 1.2 to 1.5 m (4 to 5 ft) long, so the string stays clear of the trench. Level the tops to a known elevation.
4. Run the strings. Stretch line between boards for wall edges, centrelines and footing lines. Use a line level or the laser to confirm it is level.
5. Mark the string positions. Cut small kerfs in the boards so the strings can be restrung in exactly the same place.
6. Transfer with a plumb bob. Hang the plumb bob from the string crossing to the pin to mark the corner. It takes two people.
7. Re-check. Check the boards after rain, frost and machine traffic.

Squaring the structure

• 3-4-5: measure 3 units along one line and 4 along the other from the same corner. The diagonal between the marks must be 5. In metric use 900, 1200 and 1500 mm, or 3 m, 4 m and 5 m. In imperial use 3 ft, 4 ft and 5 ft. Bigger multiples such as 6-8-10 or 9-12-15 give more accuracy over distance.
• Diagonals: on a rectangle, the two diagonals must be equal.
• Laser alignment for long runs, and a total station for commercial and civil layout.

Working out the diagonal

Diagonal = √(length² + width²). For a 12 m × 8 m building: √(144 + 64) = √208 = 14.42 m. For a 40 ft × 26 ft building: √(1600 + 676) = √2276 = 47.71 ft, which is 47 ft 8 1/2 in (0.71 × 12 = 8.5 in). Measure both diagonals. If they match each other and match the number, the building is square and the size is right.

Elevation changes and step-downs

Mark these clearly before excavation or forming: garage step-downs, basement transitions, frost wall elevation changes, slab depressions (showers, mechanical rooms, elevator pits), loading docks and machine bases, and drainage channels.

• Residential: footings, walls, slabs, columns, beam pockets.
• Commercial: gridlines, column centres, shear walls, slab depressions.
• Industrial: machine bases, anchor bolt templates, heavy pads.
• Civil: centrelines, drainage slopes, culverts, utility depths.

The layout workflow

1. Check the current drawings and the pins. Confirm the revision. Find the legal pins. Call a surveyor if any are missing or in doubt.
2. Set the benchmark. Choose something that will not move, write its elevation, and photograph it.
3. Set the corner stakes. Use the drawing dimensions and the setbacks.
4. Square it. Use 3-4-5 at each corner and check both diagonals.
5. Set the batter boards and strings. Outside the excavation, levelled to a known elevation.
6. Transfer elevations. Set the laser on the benchmark, find the height of instrument, and mark FF and footing depths.
7. Protect and re-check. Re-check after the dig, after rain and frost, and after machines have been working around the layout.

Common mistakes

• A wrong or disturbed benchmark
• A laser out of level, or reading from the wrong datum
• Wrong offsets
• Sagging string lines
• Layout moved by the excavator
• Reading the drawings wrong, or working from an old revision
• Laying out from a neighbour's fence or a guess instead of the legal pins
• Not re-checking after weather or machine movement

What the inspector or surveyor checks

• Footprint dimensions match the drawings
• Footing elevation is correct
• Wall thickness and alignment
• Elevations are consistent across the site
• Setbacks to the property lines