Reading a Floor Plan When the Dimensions Do Not Add Up
Priya is setting out a timber floor frame for a single-storey extension. The architectural plan is drawn at 1:100 and shows an overall internal width of 5400 mm, with three bays marked 1800 mm, 1800 mm and 1800 mm between bearer centrelines. A separate structural sheet shows the same width as 5400 mm but with bearer centres at 1900 mm, 1600 mm and 1900 mm. The specification says bearers are 140 x 45 mm treated pine at maximum 1800 mm centres, and joists are 90 x 45 mm at 450 mm centres. The plan's revision block shows the architectural sheet is Revision C and the structural sheet is Revision B, both dated the same week. Priya has already cut two bearers to the architectural bay lengths.
1. Explain which document Priya should follow and why, and what she should do about the bearers already cut.
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The two sheets conflict: the architectural sheet gives equal 1800 mm bays, while the structural sheet gives 1900/1600/1900 mm. Both total 5400 mm, so the overall width agrees; the difference is bearer position. Priya should not guess. The specification sets a maximum bearer spacing of 1800 mm, and the structural sheet's 1900 mm spacing exceeds that maximum, so the structural sheet as drawn is inconsistent with the specification. She should stop, mark up both sheets, and raise a request for information to the builder or designer, asking which layout governs and whether the 1900 mm spacing is acceptable for the bearer size. Until answered, she should not cut more bearers. The two bearers already cut to 1800 mm may still be usable depending on the final layout, so she should keep them aside and not modify them further. Following the wrong sheet risks a floor that is out of level or under-supported, and rework.
2. Describe how Priya should check the set-out before fixing the frame, using the plans and simple site checks.
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She should first confirm the overall width on site against the 5400 mm internal dimension, allowing for the wall thicknesses shown on the plan. She should establish a square corner using the 3-4-5 method or diagonal measurements: for a rectangle, the two diagonals should be equal. She should mark bearer centrelines from a single datum edge rather than stepping bay by bay, so errors do not accumulate. She should check the joist spacing of 450 mm centres against the specification and confirm the bearer size and treatment match the schedule. She should compare her measurements with the scaled plan but trust the written dimensions over scaling. Any discrepancy greater than a few millimetres should be recorded and raised before fixing.
3. The specification and drawings are both Revision B/C. Explain how Priya should manage document control on site so this conflict does not recur.
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Priya should keep a single, current set of drawings in a known location, with superseded sheets removed or clearly stamped 'superseded'. She should check the revision block each time she receives a new sheet and record the revision and date in the site diary. She should highlight the dimensions she is working to and initial them. When a conflict appears, she should raise an RFI in writing and keep the response with the drawings. The builder should maintain a drawing register. Good document control prevents the same problem recurring and gives a clear record of what was built and why.