Reading Construction Drawings

A set of construction drawings is how an architect and engineers tell the builders what to do. It can run from a few pages for a small addition to hundreds of sheets for a hospital. Nobody reads all of it. What you need is the skill to find your sheet, understand what it is showing, follow the references to the detail you need, and notice what has changed. Doing that well saves rework and arguments on any job, whatever your trade.

What You Will Be Able to Do

• Find a sheet by its number and discipline.
• Tell a site plan, floor plan, elevation, section and detail apart.
• Use architectural and engineering scales, and know when not to scale at all.
• Follow callouts, grid lines and schedules.
• Work from the current revision, and know what to do when drawings conflict.

How a Plan Set Is Organized

Sheets are grouped by discipline, and each sheet number starts with a letter. The first pages list every sheet in the set and give general notes, abbreviations and symbols. Read these before you start.

Sheet A2.1, for example, is usually an architectural floor plan. Firms number differently, so check the index.

The title block on each sheet gives the project, sheet title, sheet number, scale, date and revision history. Check the revision column first.

The Main Drawing Types

Plans show where things go. Sections and details show how they are built. Schedules show what they are.

Scale

Construction drawings are drawn smaller than real life, and the title block or the drawing label states the scale.

Architectural scale gives a fraction of an inch for each foot. A common one is 1/4" = 1'-0", which means each 1/4 inch on paper is 1 foot of building.

Worked example: A wall measures 2-1/2 inches on a sheet at 1/4" = 1'-0".

1. 2.5 inches / 0.25 inch per foot = 10
2. The wall is 10 feet long.

Engineering scale uses whole numbers, and appears on site and civil drawings. A scale of 1" = 20' means 1 inch on paper is 20 feet on the ground.

Worked example: A fence line measures 4.25 inches on a sheet at 1" = 20'.

1. 4.25 x 20 = 85
2. The line is 85 feet long.

A scale ruler has several scales on its faces, so pick the one that matches the drawing. Sheets are also printed at half size for the field, so check the graphic scale bar, if there is one, before you trust any paper measurement.

Warning: Do not scale a drawing to get a number you will build from. If a written dimension is there, it wins, even if your ruler says something else. Many drawings say "DO NOT SCALE." If the dimension you need is missing, ask rather than guess.

Reading Dimensions

Dimensions are written in feet and inches, such as 24'-6". Watch what a dimension is measured to:

• Face of stud, face of finish, or face of concrete.
• Centerline of a wall, column or opening.
• Rough opening or finished opening.

A note at the front of the set usually says how walls are dimensioned. A framer and a drywall finisher can both be correct and still land in different places if they read the reference line differently.

Plans also use elevations as heights. A spot elevation such as 102.50' is the height above a set reference point called a benchmark. Finish floor is often labeled FF.

Following the References

Drawings refer to each other using symbols. A circle divided by a line is a common one.

• Section or detail bubble: the top number is the drawing number, and the bottom is the sheet. A bubble reading 3 over A5.1 means detail 3 on sheet A5.1.
• Grid lines: letters one way, numbers the other, usually through the columns. "Column C-4" tells everyone exactly where something is.
• Door, window and room tags: a tag number that links to a schedule. Door 104 is listed on the door schedule with size, type, hardware and fire rating.
• Keynotes: numbers on the drawing that match a list on the same sheet.
• North arrow: tells you which way the plan is turned. Plans are often not drawn with north at the top.
• Match lines: show where one part of a large plan continues on another sheet.

Each discipline has its own symbols for outlets, valves and fixtures. The legend is usually near the front of that discipline's sheets.

Revisions and Changes

Drawings change after they are issued. These are the common ways you will see changes:

• Revision cloud: a wavy line around whatever changed, with a triangle showing the revision number.
• Addendum: a change issued before the contract is signed.
• Supplemental instruction or bulletin: a change issued during construction.
• Request for information (RFI): a written question to the design team, with a written answer.
• As-builts: marked-up drawings showing what was actually built.

Throw out or clearly mark old sets. Building from a superseded sheet is one of the most common reasons for tearing work out.

Specifications and Conflicts

Drawings show quantity and location. The project manual, or specifications, says what materials to use and how to install them. When two documents do not agree, do not pick the one you like. Stop and raise it through your foreman, so it gets answered in writing. Fixing it on paper costs far less than fixing it after a wall is closed in.

How to Approach a New Set

1. Read the sheet index and the general notes.
2. Find the abbreviations and symbol legends.
3. Find your discipline's sheets, plus the architectural floor plan they rest on.
4. Check that you have the latest revision.
5. Find where your work happens on the plan.
6. Follow every bubble that touches your work.
7. Check the schedules.
8. Check other disciplines where your work crosses theirs, such as ducts and beams in the same ceiling space.

Common Mistakes

Check Your Understanding

1. On a sheet at 1/8" = 1'-0", a line measures 3 inches. How long is it in real life?

3 / 0.125 = 24 feet. Use the written dimension if one is shown.

2. Your scale ruler reads a wall at 18'-3", but the dimension says 18'-0" and the sheet is stamped DO NOT SCALE. Which do you use?

18'-0". The written dimension governs. If it does not fit the field conditions, ask.

3. A bubble on your plan reads 5 over S3.2. Where do you go?

To drawing 5 on structural sheet S3.2.

4. You need to know the hardware on door 212. Which part of the set should you check?

The door schedule. The tag on the floor plan links to it.

Key Takeaways

• Sheet letters show the discipline. Start with the index, notes and legends.
• Plans show where, sections and details show how, and schedules show what.
• A scale of 1/4" = 1'-0" means every 1/4 inch on paper is 1 foot, and 1" = 20' means each inch is 20 feet.
• Written dimensions beat your ruler every time.
• Check for the latest revision, and look for clouds.
• Raise conflicts in writing before you build.