Revision C looks like revision B
A specification is re-issued with a new revision letter and a one-line description in the revision block. The description is written by the person who made the change and reflects what they remember changing. On site, the changed tolerance in section 4.3 that nobody mentioned is what causes the rework.
This is not carelessness so much as the way documents get edited. Three people touch a specification between revisions. Each makes a change, one of them writes up the revision block, and the other two changes ride along undescribed.
A worked example
Say revision C of a concrete specification is issued with the note "Section 6 updated for curing requirements". Section 6 has indeed changed. Running C against B also shows that the minimum cover to reinforcement in section 4 went from 40 mm to 50 mm, and that a clause on cold-weather placing was deleted from section 7.
Neither of those is in the revision note. The cover change affects formwork and bar schedules already being prepared from revision B. The deleted clause matters to whoever is pouring in January. A reader checking section 6 against the revision note would have found nothing wrong.
That is the whole argument for comparing every revision rather than reading the revision block: the revision block describes intent, and the comparison describes the document.
Documents worth comparing
- Specifications and method statements between revisions.
- Bills of quantities between the tendered version and the contract version, for changed quantities and rates.
- Drawing sheets with text: notes, dimensions and schedules. Purely graphical line-work changes are outside the scope of a text comparison.
- Contract documents and variation orders against the base contract.
- Tender documents against their addenda, before pricing.
Tables and quantities
A BOQ is mostly tables. A naive text diff reports every cell of every table because the extraction order differs slightly between the two PDFs. The engine on this site fingerprints tables by content, holds identical tables out of the diff, and for tables that did change reports only the cells that changed, with their page number.
In practice that means a re-issued BOQ where three rates and one quantity moved shows four entries, not four hundred. It also means a quantity that changed from 1,250 to 1,520 is reported even though the two numbers look alike at a glance, which is the kind of transposition a person reading two printouts side by side will miss.
Drawings: what is and is not compared
Drawings exported to PDF from CAD usually carry real text: general notes, dimension values, schedules and title-block fields. All of that is compared like any other text, so a note that changed from "M16 bolts" to "M20 bolts" or a dimension that moved from 2400 to 2450 shows up.
What the comparison does not do is detect a wall that moved with no change to any text or dimension on the sheet. That is a graphical change, and it needs a visual overlay or a drawing register check. Use both where drawings matter; a text comparison is not a replacement for reviewing the drawing itself.
Making it a document-control step
When a revision is received, compare it against the previous revision and attach the change list to the transmittal. When a revision is issued, compare it against the previous one and confirm the revision block describes everything in the change list. If it does not, fix the revision block before issue rather than leave the site team to discover it.
Both steps take a minute with the web tool. Document control teams handling a steady flow of revisions can trigger the comparison from their document management system through the API and attach the result to the record automatically. The construction industry page has more on team use and private deployment.
Things that trip people up
Comparing against the wrong baseline is the most common one. Compare revision D against revision C, not against revision A, or you will see every change since the first issue and lose the ones that matter now. The second is file size. Large drawing sets can run past the 20 MB per file limit of the web tool; split the set by discipline or compare the specification separately from the drawings.
The last is scanned documents. A scanned specification with no text layer cannot be compared and is refused. If the only copy is a scan, run it through OCR first and expect some noise in the result.
Finally, remember that the change list reports text that changed, not whether the change is right. A tolerance that tightened may be a correction the designer intended. Route each substantive entry to the person responsible for that section, record the decision against the change, and keep the redlined PDF with the transmittal.
Try it on your own documents. Upload two versions on the homepage. Documents up to 30 pages need no account, and nothing is stored.
Industry page: PDF comparison for construction and engineering