Read the main features of an architectural floor plan and a civil site plan.
Why it matters
Buildings and civil works are built from sets of drawings, sometimes hundreds of sheets. Electricians lay out boxes and runs from floor plans, civil engineers design grading and utilities on site plans, and mechanics and engineers use them to find equipment rooms and access routes. Reading the set quickly, and knowing which sheet to open, is a core site skill.
How a drawing set is organised
Sheets are grouped by discipline, usually with a letter prefix (the US National CAD Standard uses this convention):
| Prefix | Discipline |
|---|---|
| G | general (cover, index, code summary) |
| C | civil (site, grading, utilities) |
| L | landscape |
| S | structural |
| A | architectural |
| M | mechanical (HVAC) |
| P | plumbing |
| E | electrical |
| FP | fire protection |
A sheet like E-201 is electrical, floor plans series, sheet 1. Start with the sheet index on the cover and the general notes and legend sheets, which define the symbols used in that set.
Floor plans
A floor plan is a horizontal section: imagine cutting the building about 4 feet (1.2 m) above the floor and looking down.
- Walls: thick lines (cut), often with hatching or wall-type tags that refer to a wall schedule.
- Doors: a gap in the wall with a thin arc showing the swing, and a door tag that refers to a door schedule.
- Windows: thin lines within the wall, with window tags.
- Stairs: a series of lines with an arrow and UP or DN; a break line where the cut passes through.
- Room names and numbers.
- Dimension strings: usually the overall dimension on the outside, then openings and walls inside it. Check the general notes for whether dimensions are to the face of stud, the centre of walls or the face of finish.
- Grid lines: lettered and numbered column lines in bubbles (A, B, C... and 1, 2, 3...) for locating everything: "the panel is at C-4".
- North arrow and scale (often 1/8" = 1'-0" or 1/4" = 1'-0").
- Section and detail markers: a circle split in two, with the detail number on top and the sheet number below. "3 over A-501" means detail 3 on sheet A-501.
Electrical floor plan symbols (typical)
- Duplex receptacle: a circle with two short parallel lines; GFCI written next to it for ground-fault protected outlets.
- Switch: an S; S3 is a three-way switch.
- Ceiling light: a circle, often with a fixture letter that refers to the lighting fixture schedule.
- Panelboard: a rectangle, often filled, with its name (LP-1).
- Home runs: an arrow with a circuit number to the panel.
Always check that set's legend: symbols vary between firms.
Site plans
A site plan shows the building in its surroundings, at an engineering scale such as 1" = 20'.
- Property lines with bearings and distances (for example N 45°12' E, 150.00').
- Setbacks and easements (areas where you may not build, or where utilities have rights).
- Existing contours (dashed) and proposed contours (solid), with elevations.
- Spot elevations at corners, curbs and drains, and the finished floor elevation (FFE) of the building.
- Utilities: water, sanitary sewer, storm drainage, gas, electric and telecom lines, with structures such as manholes and catch basins.
- Paving, parking, sidewalks, and the benchmark that elevations refer to.
- North arrow, scale, and legend.
Worked example: slope from contours
Two proposed contours, 101.0 and 100.0 (1 ft interval), are 25 ft apart measured on the plan.
Slope = rise / run = 1.0 / 25 = 0.04 = 4% falling away from the building. Steeper ground shows contours closer together.
Worked example: finding a detail
The site plan shows a catch basin with a marker "5 / C-501". Open sheet C-501 and find detail 5, which shows the basin's construction.
Reading order for a new set
- Cover sheet and index.
- Legends, abbreviations and general notes for your discipline.
- Site plan to understand the building's position, access and utilities.
- Floor plans for your discipline, using grid lines to locate work.
- Sections and details referenced from the plans.
- Schedules (doors, fixtures, panels, equipment).
- Revision clouds and addenda: check you have the latest issue.
Common mistakes
- Using symbols from memory instead of the set's legend.
- Missing a revision cloud (a scalloped outline marking a change).
- Confusing existing and proposed contours.
- Scaling a dimension that is written elsewhere (or one that was resized when printed).
Recap
- Sets are organised by discipline prefix; start with the index and legends.
- Floor plan = horizontal cut at about 4 ft: walls, doors, windows, stairs, grids, dimensions, section and detail markers.
- Site plan: property lines, setbacks, contours, elevations, utilities; slope = rise / run.