Heritage Terrazzo Floor Restoration with a Level Threshold
You are restoring a 1930s terrazzo floor in a heritage shopfront, 6 m x 4 m. The terrazzo is sound in the centre but has lifted and cracked within 500 mm of the walls, and there is a 15 mm step down to the street entrance. The heritage consultant requires the original terrazzo appearance to be retained and any new work to be visually indistinguishable. The client wants the step removed for accessibility. You have matching aggregate but no original chips. The floor is over a concrete slab on grade with no damp-proof membrane visible. Moisture readings at the wall edges are high.
1. Explain how you would investigate and treat the moisture and edge failure before laying any new terrazzo.
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The edge lifting and high moisture readings point to moisture rising through the slab on grade and possibly to thermal/moisture movement at the perimeter.
Investigate. Take calibrated moisture readings across the floor, not just at the edges, using the correct meter for concrete and allowing for temperature. Check for a DPM by inspection or by drilling a small test hole. Confirm whether the high readings are surface or deep-seated. Identify whether the edge cracking is debonding of the terrazzo from the slab.
Treat. If there is no DPM and moisture is rising, a surface damp-proof membrane compatible with the new terrazzo system must be applied after the slab is prepared, or the specification changed to a system tolerant of moisture. Lifted and cracked terrazzo within 500 mm of the walls should be removed back to sound material, the substrate prepared, and a repair mix matched to the original. Perimeter movement joints should be provided. Do not lay new terrazzo over high moisture - it will debond and stain. Get the moisture results and proposed treatment signed off by the heritage consultant and client.
2. The client wants the 15 mm step removed and the heritage consultant wants an indistinguishable finish. How do you reconcile these, and what are the risks of matching the aggregate?
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Step removal. Removing a 15 mm step means either raising the floor by 15 mm or ramping/transitioning. Raising the whole floor by 15 mm changes door heights, skirtings and the heritage fabric, and adds load. A formed transition strip or ramp is often the practical answer but must meet accessibility requirements and be approved. If the floor is raised, the new terrazzo must be bonded to a prepared substrate and the perimeter detail resolved. This is a design decision to put to the client and consultant, with cost and heritage impact noted.
Matching aggregate. Without original chips, an exact match is unlikely. Risks: colour and chip-size variation, different binder yellowing over time, and a visible patch line. Mitigate by sampling several mixes, curing and comparing under the same light, and by blending new work into existing at a natural break rather than a straight line. Where an exact match cannot be achieved, propose a deliberate feature band or threshold detail agreed with the consultant. Record the agreed approach in writing before starting, because 'visually indistinguishable' is subjective and disputes are common.