External Terrace Over a Suspended Slab with Rain Exposure
You are laying 600 x 600 mm, 20 mm thick granite pavers on a suspended concrete terrace, 8 m x 5 m, exposed to rain and occasional light foot traffic from a cafe. The slab has a 1:100 fall to two scuppers, but ponding is visible after rain in two low spots. The slab is 12 years old with hairline cracks. The client wants a bonded installation with 3 mm joints and a dark grout. The building is 400 m from the coast. The cafe opens in 5 days and the client wants no raised threshold at the door.
1. Assess the slab and exposure conditions and decide whether a bonded installation is appropriate, justifying your answer.
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Bonded installation is risky here for several reasons:
- Ponding. Two low spots mean the fall is not continuous to the scuppers. Standing water on a bonded tiled terrace will find its way into joints and the bond line, and in a coastal environment with rain exposure the assembly must drain freely. The slab should be re-screeded to correct falls before any tiling, or the ponding accepted only with a drainage layer and unbonded system.
- Cracks. Hairline cracks in a 12-year-old slab may be dormant or active. A bonded rigid finish will telegraph and crack along them unless a crack-isolation membrane is used.
- Coastal exposure. Salt spray and rain mean the grout, adhesive and any sealant must be rated for external coastal use. Dark grout in sun is fine thermally but must be a suitable external grout, not a standard interior grout.
- Movement. External slabs move with temperature; bonded tiles need movement joints at perimeters and at intervals, and at the scuppers.
A safer option is an unbonded or semi-unbonded system on a drainage mat or mortar bed, which accommodates movement and drains water. If the client insists on bonded, the slab must be flattened, crack-isolation applied, and an external-rated flexible adhesive and grout used with movement joints. Document the recommendation and get the client's decision in writing.
2. The client wants no raised threshold at the door and 3 mm joints. Explain the conflicts and how you would resolve them, including the 5-day programme.
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Threshold. A bonded system with a 20 mm paver plus adhesive plus membrane will sit 25-30 mm above the slab. To keep the threshold flush, the door sill height and the slab level must be reconciled - either the door is adjusted, a ramp/transition is formed, or the slab is recessed. This is a design decision, not a tiling one; raise it with the builder and client immediately.
3 mm joints. Tight joints on an external slab with thermal movement leave little room for grout and increase the risk of edge chipping and water ingress. External work generally needs wider joints (often 5-10 mm) unless a specific system is designed for 3 mm. Propose a wider joint or confirm the designer's specification in writing.
5-day programme. With ponding correction, crack isolation, membrane cure, adhesive cure and grout cure, 5 days is very tight and weather-dependent. Sequence: day 1 correct falls/flatten; day 2 crack isolation and cure; day 3-4 lay tiles in sections; day 5 grout and movement joints. Do not grout before adhesive is fully cured, and protect from rain. If rain is forecast, the programme slips - tell the client now rather than promising the opening date.