How to Read Structural Drawings: 8 Symbols Every Homeowner and Builder Must Know
How to read structural drawings is a skill that saves time, prevents costly mistakes on site and helps homeowners understand exactly what their structural engineer has designed. Whether you are a builder picking up a set of calculations for the first time, a homeowner trying to make sense of the plans attached to your Building Control submission, or a trainee engineer learning the basics, this guide explains how to read structural drawings in plain language with real examples.
How to Read Structural Drawings: General Arrangement Symbols
General Arrangement (GA) drawings are the core output from a structural engineer. They show the size, position and level of every structural element. When learning how to read structural drawings, the first step is to recognise the standard symbols that appear on every GA drawing.
| Symbol | What it means |
|---|---|
| Elevation mark (circle with number, arrow pointing at element) | Shows which element is being viewed from the side. The lower half of the circle contains the drawing number where the elevation can be found. |
| Section mark (circle with number, line with arrows at both ends) | Shows where a vertical slice has been taken through the structure. The arrows indicate the direction you are looking. The lower half of the circle gives the drawing number. |
| Detail mark (circle with line pointing to a specific area) | Flags a location where a larger-scale detail drawing has been produced to show a complex junction or connection. |
| Level marker (e.g. 57.710 SSL) | Shows the height of an element. Common suffixes: AOD (Above Ordnance Datum), TOS (Top of Steel), SSL (Structural Slab Level), FFL (Finished Floor Level), TOF (Top of Foundation). |
| Slab thickness (number in a circle, e.g. 200) | Indicates the thickness of a floor slab or wall when shown in elevation. |
| Void / hole (crossed box) | A hole or void in a structural element, typically for services or stairs. |
| Recess (hatched box) | A recess or step-down in a structural element, usually to accommodate different floor finishes. |
| Span arrow (arrow between supports) | Shows the direction a floor slab is spanning between its supports. |
How to Read Structural Drawings: Steel Section Labels
Steel sections on structural drawings follow a specific labelling protocol. Understanding how to read structural drawings for steelwork means knowing that the numbers in a steel label are not always the actual dimensions of the section.
| Label on drawing | What it actually means |
|---|---|
| 457 x 191 x 89 UB | Universal Beam with 457mm serial depth, 191mm width, 89 kg/m weight. The actual depth is 463mm, not 457mm. The serial size is a nominal grouping, not the true dimension. |
| 203 x 203 x 86 UC | Universal Column. Actual depth is 222mm, not 203mm. A 203 x 203 x 52 UC is only 206mm deep. The serial size stays the same but the depth changes with weight. |
| 120 x 120 x 12 RSA | Rolled Steel Angle. Unlike UB/UC sections, the dimensions here are actual: 120mm leg x 120mm leg x 12mm thick. |
| 150 x 100 x 5 RHS | Rectangular Hollow Section. Actual dimensions: 150mm deep x 100mm wide x 5mm wall thickness. |
| V = 850kN, M = 0 | Ultimate end reactions shown on the design intent drawing. V = shear force, M = bending moment. M = 0 means a simple (pinned) connection is assumed. |
When a beam is drawn as a single thick line with a gap at the column, it means the beam is a separate element from the column. It is simply supported, not continuous. This is one of the most commonly misunderstood conventions when learning how to read structural drawings.
How to Read Structural Drawings: Reinforced Concrete Notation
Reinforced concrete drawings communicate the layout of steel reinforcement bars inside concrete elements. They use a notation system that looks confusing at first, but follows a simple pattern once you know how to read structural drawings for RC work.
16 = number of bars, H = high-yield steel, 10 = bar diameter in mm, 07 = bar mark (unique ID), 200 = centres in mm.
How to Read Structural Drawings: Line Weights and Drawing Scales
Understanding how to read structural drawings also means recognising what different line thicknesses represent. A thin dashed line (0.18mm) is a gridline or hidden detail. A medium line (0.3mm) shows an element in elevation. A thick line (0.5mm) shows an element that has been cut through in section. This hierarchy helps you instantly distinguish between what you are looking at and what is behind it.
| Drawing type | Common scales |
|---|---|
| Plans | 1:250, 1:200, 1:100, 1:50 |
| Elevations | 1:100, 1:50 |
| Sections | 1:100, 1:50 |
| Part sections | 1:25, 1:20, 1:10 |
| Details | 1:20, 1:10, 1:5 |
Units on structural drawings are in millimetres for setting out dimensions and element sizes. Levels are normally stated in metres. This is a UK convention and applies to all how to read structural drawings for British projects.
How to Read Structural Drawings: Revisions, Clouds and Notes
Structural drawings are living documents that change as the design develops. Understanding revisions is an essential part of knowing how to read structural drawings. When a drawing is revised, the changed area is highlighted with a revision cloud and marked with a triangle containing the revision number. The details of what changed are described in the revision column on the drawing sheet.
An abeyance cloud is the opposite: it marks an area that is on hold and should not be relied upon. If you see an abeyance cloud on a drawing, the information inside it has not been confirmed and construction should not proceed in that area.
Drawing notes are located in the top right corner of the sheet. They contain critical information such as concrete grades, cover to reinforcement, steel grades and any assumptions the engineer has made. Always read the notes before starting any work.
How to Read Structural Drawings: Common Mistakes to Avoid
A 203 x 203 x 86 UC is actually 222mm deep, not 203mm. If you cut a hole 203mm deep for this column, it will not fit. Always check the Blue Book or section tables for the true dimensions of open steel sections.
Reinforcement drawings cannot be read without the accompanying bar bending schedule. The schedule gives the shape, length and bending dimensions for every bar. Without it, the fixer has no way to fabricate the correct bar shapes.
Always check the revision number in the title block before starting work. Using a superseded drawing is one of the most common causes of rework on construction sites. If in doubt, ask the engineer to confirm which revision is current.
Never measure dimensions by placing a ruler on a printed drawing. Drawings may not print at exact scale, and photocopied drawings are frequently distorted. All dimensions must be read from the figured dimensions on the drawing, not scaled off.
How to Read Structural Drawings: Frequently Asked Questions
What does TOS mean on structural drawings?
TOS stands for Top of Steel. It is the level at which the top surface of a steel beam or column sits. SSL means Structural Slab Level and FFL means Finished Floor Level. These level markers help everyone on site understand exactly where each element should be positioned vertically.
Who produces structural drawings?
The structural engineer produces the General Arrangement (GA) drawings showing element sizes, positions and levels. For steel-framed buildings, the steel fabricator then produces fabrication drawings and shop drawings showing bolt holes, fin plates and welds. The engineer checks the fabricator's drawings before construction begins.
Do I need structural drawings for a house extension?
Yes, if the project involves structural elements such as steel beams, columns, foundations or alterations to load-bearing walls. Building Control will require structural calculations and drawings as part of the approval process. Your structural engineer provides these as part of the calculation package.
How much do structural drawings cost?
At SECalcs, structural calculation packages that include GA drawings start from £245 for simple projects such as wall removals. Larger projects such as loft conversions or extensions start from £395. Call 07359 267907 for a quote.
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