To review a revised drawing set, work in six steps: confirm you have the right pair of sheets, read what the author says changed (transmittal, revision block, clouds), compare the two revisions on the sheet, look for changes that were made but not clouded, record what you found with clouds and notes, and file the old revision as superseded. The comparison itself is only step three. Many review mistakes happen before it (the wrong pair) or after it (findings nobody can trace).
What arrives with a revision?
A revised sheet normally carries three signals that something changed, and a transmittal that tells you why it was sent:
| Signal | Where | What it tells you |
|---|---|---|
| Revision block entry | Title block | Revision number or letter, date, a one-line description, who drew and checked it |
| Revision cloud | On the drawing | The area the author changed |
| Delta (triangle with the revision number) | Next to the cloud | Which revision the change belongs to |
| Transmittal | Cover sheet or email | Which sheets were issued, at which revision, and for what purpose (information, coordination, tender, construction) |
On projects that follow ISO 19650, the sheet also carries a status code and a revision code. Preliminary revisions are usually numbered P01, P02 and contractual ones C01, C02, with status codes such as S1 (suitable for coordination) or A1, A2 and so on (authorised and accepted, numbered by project stage). In the UK these come from the National Annex to ISO 19650-2; each project confirms them in its information standard, and conventions differ elsewhere, so check rather than assume.
Step 1: Are you comparing the right pair?
Comparing the wrong two sheets is a common review error, and no software catches it for you. Before anything else, check on both sheets:
- Sheet number and title match.
- Revision: the new one is the revision the transmittal names, and the old one is the revision you last reviewed, not simply the previous number. If you skipped a revision, compare against the last one you actually checked.
- Status: a sheet issued for information should not be reviewed as if it were for construction.
- Page size and scale: both sheets on the same paper size and at the same drawing scale. If one was exported on A3 and the other on A1, many tools will show every line as changed.
Step 2: What does the author say changed?
Read the revision block description and find every cloud on the sheet before you compare anything. Write the list down. This is the author's claim, and the comparison in the next step either confirms it or shows more than was claimed. Without the list you have nothing to check the comparison against.
Step 3: How do you compare the two revisions?
Pick the method by the question you are asking:
| Method | How it works | Best for |
|---|---|---|
| Overlay | Both revisions drawn on top of each other, each in its own colour | Plans, sections, details: "what moved, what is new, what is gone?" in one look |
| Side by side | The two sheets next to each other, often with linked zoom | Reading context around a change, or sheets that do not line up |
| Report compare | A generated document that lists or highlights differences | Specifications and text-heavy sheets, where you want a list of edited words |
For drawings, an overlay is usually the quickest first pass to read, then side by side to read what a change means. Compare the whole set, not only the sheets that are clouded: a change to a grid line or a wall on the plan often shows up on sections, elevations and details that nobody clouded. For the step-by-step mechanics, see how to compare two PDF files for differences.
Step 4: Were all changes clouded?
Hold the comparison against the list from step 2. Three things can turn up:
- Clouded and changed: as claimed. Read each one for what it means for your discipline.
- Clouded but nothing changed: an old cloud carried forward, or a change that was reverted. Ask, because the revision block may now be wrong.
- Changed but not clouded: the important one. Conventions exist (the US National CAD Standard, for example) but are voluntary, and how much to cloud varies by office, client and contract. Some project specifications require every change to be clouded and tagged; others leave it to judgement. An unclouded change is still a change, so record it and raise it with the author.
Step 5: How do you record what you found?
A review nobody can trace later did not happen. For each finding:
- Put a cloud or a redline on the reviewed PDF at the place it occurs, in a colour your team reserves for reviews (see how to redline a PDF drawing).
- Add a short note: what changed, what it affects, what you need from the author.
- Reference the sheet number and both revisions, so the note still makes sense when the next revision arrives.
- Keep the findings in one list per issue (a spreadsheet or the markups list of your PDF tool), not scattered across emails.
Step 6: What happens to the old revision?
Mark the previous revision as superseded and move it out of the working folder, but keep it. You will need it the next time a question comes up about what was issued when. Keep a clean, unmarked copy of each issued set (tender, construction) next to your marked-up review copies.
A one-page checklist
| # | Check |
|---|---|
| 1 | Transmittal read: sheets, revisions and purpose of issue noted |
| 2 | Right pair: same sheet, new revision vs the last revision you reviewed |
| 3 | Same page size and scale on both sheets |
| 4 | Author's list made from the revision block and clouds |
| 5 | Whole set compared, not only clouded sheets |
| 6 | Unclouded changes recorded and raised |
| 7 | Findings clouded, noted and listed with sheet and revision |
| 8 | Old revision marked superseded and archived; clean issued set kept |
How we compare revisions in our studio
We built Ncored in our architecture studio, and comparing revisions is part of it. Open the newer revision, drag the older PDF onto the sheet (or use the Compare button at the top right), and the two appear as one overlay: blue for what the newer revision added, red for what it removed, grey for what stayed the same. A slider fades the unchanged linework so only the differences remain, and on a whole set the pages are paired by how they look, so an inserted sheet does not throw the rest off. The comparison is a view: nothing is written into either file, and it runs on your own computer on Windows and Mac.
Frequently asked questions
How do you check a revised drawing?
Confirm you have the right pair of sheets, read the revision block and clouds, compare the two revisions (an overlay is the fastest for drawings), check for changes that were not clouded, and record your findings with clouds and notes.
Do all drawing changes have to be clouded?
There is no universal rule. Many project specifications require every change to be clouded and tagged with a delta, but practice varies, so treat an unclouded change as a finding and raise it with the author.
What is a delta on a drawing?
A small triangle with the revision number placed next to a revision cloud, so you can tell which revision a change belongs to.
Should I compare only the clouded sheets?
No. A change on one sheet often affects others (sections, elevations, details) that were not clouded. Compare the whole set when you can.
What should I do with superseded drawings?
Mark them superseded and archive them rather than deleting them, and keep a clean copy of every issued set next to your review copies.
Related: drawing revision compare tools, compared, why a drawing comparison shows everything as changed, what is a transmittal in construction.