Plan Reading and Interpretation
Reviewed by a working tradesman with 17+ years in the field.
Some links on this site are affiliate links. If you buy through them we may earn a commission at no extra cost to you. Affiliate disclosure
How to read foundation, site and grading drawings, so you can see the building before it exists.
Tap your level and the boxes below open to the advice for you. Remembered on this device.
By the end you can
- Find the scale, sheet number and revision in the title block, and know you have the current set.
- Read a footing, wall, step-down and elevation marker on a foundation plan.
- Decode a rebar callout and work out how many bars you need.
- Follow a section cut and a detail callout to the drawing that shows how it is built.
- Read cut and fill, drainage arrows, property lines and setbacks on the site plan.
Never guess a symbol, and never build from an old revision. If two sheets disagree or a symbol is not on the legend, stop and get a written answer from the designer or engineer before you build. A stamped structural drawing is never changed in the field.
Plans are a language
Before any excavation, forming or concrete, you need to understand the drawings. If you can read them, you can see where the concrete goes, how deep to dig, which way water moves and what the inspector will check. The symbols are the words, and the legend is the dictionary. Symbols differ between designers, so the legend on your set always wins.
Metric basics on Canadian plans
- Dimensions are in millimetres, usually without the unit. 3050 means 3050 mm.
- Elevations are in metres to two decimals, such as 100.00. They can be site elevations or geodetic, tied to a real survey datum. Know which before you compare numbers.
- Sheet prefixes are commonly A architectural, S structural, C civil, L landscape, M mechanical, E electrical.
- Rebar sizes are metric: 10M, 15M, 20M, 25M and so on. The number is roughly the bar area in square millimetres divided by 10, and the table below gives the real figures.
| Bar | Diameter | Area | Mass |
|---|---|---|---|
| 10M | 11.3 mm | 100 mm² | 0.785 kg/m |
| 15M | 16.0 mm | 200 mm² | 1.570 kg/m |
| 20M | 19.5 mm | 300 mm² | 2.355 kg/m |
| 25M | 25.2 mm | 500 mm² | 3.925 kg/m |
| 30M | 29.9 mm | 700 mm² | 5.495 kg/m |
| 35M | 35.7 mm | 1000 mm² | 7.850 kg/m |
Bars are made to CSA G30.18. Values from a published engineering table. Check them against your supplier's data sheet before you quote.
An old US callout like #4 @ 12 in. O.C. does not match a metric size. A #4 bar is 12.7 mm. Do not swap one for the other without the engineer.
Feet and inches on a metric job
Canadian plans are metric, but you will still work with feet and inches: older drawings, renovations, lumber and sheet goods, and a lot of crews. Learn to move between them without a calculator.
| Metric | Imperial | Handy for |
|---|---|---|
| 25.4 mm | 1 in | The exact conversion |
| 300 mm | 1 ft (304.8 mm exactly) | Footing and rebar spacing |
| 400 mm | 16 in (406 mm) | Stud and joist spacing |
| 600 mm | 2 ft (609.6 mm exactly) | Stud spacing, tie spacing, step footing rise and run |
| 1.2 m | 4 ft (1219 mm) | The trench rule, sheet width |
| 2.44 m | 8 ft (2438 mm) | Sheet length, form boards |
| 3 m | 9 ft 10 in | The fall protection height (10 ft) |
| 1 m | 3 ft 3 3/8 in | Quick check only |
Plans in feet and inches write 10 ft 6 in as 10'-6". A scale such as 1/4" = 1'-0" means each quarter inch on the sheet is one foot on the building. Never mix the two on one calculation. Convert everything to one unit first.
To go from millimetres to inches, divide by 25.4. To go from inches to millimetres, multiply by 25.4. 16 in × 25.4 = 406 mm, and 3050 mm ÷ 25.4 = 120 in, which is 10 ft.
The title block and legend
Look here before you look at anything else.
Reading a foundation plan
Here is a generic sample. Find each symbol in the picture before you read the table.
Foundation symbols, explained like a foreman
Looks like: A heavier or dashed line around the footprint. The legend decides which.
Means: The footing under the wall or column. It carries the whole building.
How to use it: Trace the full perimeter. Find every change in width or depth and match sizes to the structural notes and details.
Common mistakes: Mixing it up with the wall line, missing width changes, missing step-downs.
Looks like: Two parallel lines, often hatched or filled.
Means: A concrete or block wall.
How to use it: Check thickness and height, and whether the wall is load-bearing or retaining.
Common mistakes: Confusing walls with footings, missing thickness or height changes.
Looks like: A change in footing elevation, noted on the plan and shown in detail on a section.
Means: The footing drops to a lower level because the ground slopes or the bearing soil is deeper.
How to use it: Find the step locations, step heights and the overlap or slope rules in the structural details.
Common mistakes: Digging everything at one depth, or steps steeper than the detail allows.
Looks like: A text note.
Means: 15M bar spaced 300 mm on centre. The M means metric bar size.
How to use it: Match each callout to its footing, wall or slab. Check size, spacing, cover and lap lengths.
Common mistakes: Ignoring spacing, swapping bar sizes, forgetting cover, not tying the bars.
Looks like: A circle or triangle with a number such as 100.00, maybe labelled FF, TOF, TOW or FG.
Means: That point is at that elevation.
How to use it: Find the benchmark. Compare all the elevations. Work out the highest and lowest point.
Common mistakes: Reading it as just a number, mixing datums, pouring at the wrong height.
Looks like: An arrow or flag with a reference such as 3/A5.2.
Means: Look at detail 3 on sheet A5.2 for a cross-section through that line.
How to use it: Follow the reference and study the detail before you build.
Common mistakes: Skipping the cut, or opening the wrong sheet.
Looks like: A circle with a reference such as 5/S2.1.
Means: Look at detail 5 on sheet S2.1.
How to use it: Details show reinforcement, thickness, connections and materials. Read each one that touches your work.
Common mistakes: Using the wrong detail, or skipping details.
A dashed line does not always mean a step-down. It means different things on different drawings, so read the legend. Maximum step height and overlap come from the structural detail and your provincial code. The model code (NBC 9.15.3.8) limits a step to 600 mm of rise with a run of at least 600 mm, and BC's Building Code Appeal Board describes the 600 mm rise as the building official's position. Check your province's edition and your detail before you rely on it.
Groundwork and civil symbols
These conventions vary more than foundation symbols. Check the legend on the civil or site-grading sheet.
- Cut and fill. Hatching, shading or a table. Cut is where soil is removed, fill is where it is added. Fill that is not compacted settles.
- Drainage arrow. An arrow with a slope, such as 2%. Water flows that way. Follow the arrows around the whole building and look for low spots.
- Existing and finished grade. Two lines for where the ground is now and where it ends up. Some drawings dash the existing contours and others dash the proposed ones. Read the legend.
- Swale, ditch and culvert. A swale or ditch is a shaped channel. A culvert is a pipe under a driveway. Note the pipe size, slope and invert elevations on the civil sheet.
Site plan symbols
- Property line. The legal boundary, drawn with long and short dashes, often with survey pin marks. On a tight lot have the pins located by a licensed land surveyor before layout.
- Setback line. The minimum distance from the property line, set by the zoning bylaw and not the building code.
- Utility lines. Labelled by letter, such as W, S, SD, G and E. Colours and styles vary, so do not assume. Lines on a drawing are only a guide. Always locate before you dig.
How to read a set, step by step
- Title block. Find the scale, sheet number, revision and designer. Make sure you have the current revision.
- Legend and general notes. They explain the symbols and override your assumptions.
- Trace the footprint. Identify every footing and foundation wall.
- Check elevations. Compare footing, floor and grade heights, and confirm the datum.
- Follow the drainage arrows. Make sure water moves away from the building.
- Study cut and fill. Understand how the ground has to be shaped.
- Read the sections and details. These show exact construction methods.
- Check property lines and setbacks. Confirm legal placement before layout.
- Read the structural notes and geotechnical report. They set bearing, frost cover and fill requirements.
Read the structural general notes before the drawings. They hold the concrete strength, rebar cover, bearing capacity and compaction rules. Highlight what applies to your pour and keep a marked-up set on site.
Worked example: how much 15M bar for a pad
The drawing shows a 4.8 m by 3.6 m slab pad with 15M @ 300 O.C. each way and 50 mm end cover. How much bar is that?
- Bars across the 4.8 m side. 4800 ÷ 300 = 16 spaces, so 16 + 1 = 17 bars. Each runs the 3.6 m direction, less 50 mm cover at each end: 3600 − 100 = 3500 mm.
- Bars across the 3.6 m side. 3600 ÷ 300 = 12 spaces, so 12 + 1 = 13 bars. Each runs the 4.8 m direction: 4800 − 100 = 4700 mm.
- Total length. 17 × 3.5 = 59.5 m and 13 × 4.7 = 61.1 m. Together 120.6 m.
- Weight. 15M is 1.570 kg/m, so 120.6 × 1.570 ≈ 189 kg.
- Order it. Add waste and lap lengths from the structural notes, and ask your supplier for stock lengths. This is a take-off for understanding, not a bill of materials.
Red flags beginners miss
- Footings drawn at different widths but built the same
- Elevations that do not agree between footing, floor and grade
- Drainage arrows pointing toward the building
- Missing or ignored rebar spacing, or old imperial bar sizes mixed in
- Detail callouts that point to the wrong sheet
- An older revision of the drawings on site
- Grade lines that do not match the cut and fill notes
The plan shows 3050 beside a wall. How long is it?
What does 20M @ 400 O.C. each way mean?
A drawing calls for #4 @ 12 in. O.C. Can you use 10M @ 300 O.C.?
The structural plan shows a footing at one elevation and the architectural plan shows it 150 mm lower. What do you do?
Why not measure a dimension straight off the print with your tape?
A footing on an old drawing is marked 16 x 8. What does that probably mean?
Drainage arrows on the grading plan point toward the house. What does that mean for you?
- Must-haveThe drawings:
- Must-haveA metric architect's or engineer's scale rule, an 8 m (25 ft) tape and a pocket calculator
- Must-haveHighlighters, coloured pens, sticky flags and a clipboard or plan tube
- Must-havePhone for dated photos of the revision stamp and title block
- Must-haveHard hat (CSA Z94.1), safety glasses (CSA Z94.3) and green-triangle footwear (CSA Z195) when you check the plans against the ground
- Rent itLaser or builder's level with a grade rod, to check plan elevations against the ground
- Nice to haveA weatherproof plan tube or a tablet with a drawing app and a stylus
Home DIY
Laborer or helper
1st-year apprentice
Apprentice, 2nd to 4th year
Journeyperson
Lead hand or foreman
Contractor or business owner
Starting a new business
Plan conventions follow CSA and NBC practice and each designer's legend, and metric bar sizes follow CSA G30.18. This topic is the same across Canada. What changes is the code edition the structural notes cite.
- BC: BC Building Code 2024 Part 9 for houses. Larger or complex work needs an engineer.
- AB: National Building Code 2023 Alberta Edition, in force 1 May 2024.
- SK, MB: SK uses NBC 2020 with few amendments from 1 Jan 2024. MB uses the Manitoba Building Code, M.R. 78/2023, from 1 Jan 2024.
- ON: 2024 Ontario Building Code (O. Reg. 163/24). Designers need a BCIN, so ask when drawings must be stamped.
- QC: Quebec Construction Code, Chapter I Building, with NBC 2020 (amended). The previous edition may apply to works begun before 17 Oct 2027.
- Atlantic: NB, NS and PE use NBC 2020. NL exempts Part 9 for one and two family dwellings, so the rules are municipal.
- Territories: YT uses NBC 2020 with changes. NWT uses municipal bylaws. NU uses NBC 2015 as modified.
Full comparison in Canada by Province.
- National Building Code of Canada 2020, Part 9 including 9.15.3 (National Research Council). Paid or licensed text, so check your province's edition.
- Engineering ToolBox, metric reinforcing bar (bar diameters, areas and masses; bars made to CSA G30.18)
- BC Building Code Appeal Board decision 1474 (step footings, 600 mm rise as the building official's position, 1992 Code)
- Provincial code adoption notices as listed in Canada by Province.
- Symbol conventions are general drafting practice and vary by designer. The legend on your drawings wins.
Last reviewed October 2026. General information, not engineering or legal advice.