Setting out a curved stair soffit from a drawing
You are asked to plaster the underside of a curved staircase in a new-build house. The drawing shows a radius of 2.4 m to the outer edge of the soffit, a rise of 2.7 m over 14 treads, and a specified plaster thickness of 13 mm. The drawing notes 'soffit to follow a smooth curve; no visible joints in finish'. On site, the first-fix electrician has clipped cables to the soffit and the joiner has installed a stringer that projects 20 mm beyond the line shown on the drawing. The site manager says to 'just follow the stringer'. You have a 3 m straight edge, a profile template made to the drawing radius, and a laser level.
1. How do you resolve the conflict between the drawing and the installed stringer before setting out?
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The drawing is the controlling document, so the 20 mm projection is a non-conformance unless a variation is issued. Measure the stringer against the drawing line, record the discrepancy with photographs and dimensions, and raise it with the site manager and the designer. Do not simply follow the stringer, because doing so would change the radius and thickness and could compromise headroom and the specified finish. Ask for a written instruction: either the stringer is adjusted, or a variation is issued accepting the new line with any knock-on effects to thickness and clearance. Only set out once that instruction is received.
2. Describe how you would set out and check the curved soffit using the template, straight edge and laser.
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Set the laser to a datum and mark the springing points at each end. Fix the profile template at intervals along the curve, checking each against the 2.4 m radius and the drawing line. Use the 3 m straight edge across the curve to check for fairness and to confirm the 13 mm thickness at each fixing point. Establish screeds or guide beads along the curve, then check them with the template and straight edge before applying plaster. Record checks as you go so any deviation is caught before the finish coat.
3. What do you do about the cables clipped to the soffit?
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Cables must not be buried without protection or agreement. Check the drawing for the intended cable route and raise it with the electrician and site manager. If the cables are to remain, they need suitable capping or conduit and enough cover, and the finish thickness must accommodate them without reducing the specified cover. Get the resolution confirmed before plastering, because chasing or re-routing after the finish is applied would damage the work.