Seafront bench slats: material choice on a fixed budget
A seafront café in Marsaxlokk is replacing the slats on 12 outdoor benches. Each slat is 2.4 m × 95 mm × 22 mm. The existing slats were treated pine, fixed with A2 stainless screws, and failed after 14 months: surface checking, black staining around the screw heads, and salt crystallisation in the grain. The café owner has a materials budget of €1,700 and wants at least five years’ service. Quotes are: treated pine €9 per slat, marine plywood €14, recycled HDPE €22, powder-coated aluminium €28. Each bench needs six slats. The benches face the sea, receive full summer UV and salt spray, and are wiped down daily. The workshop has a circular saw with a fine-tooth blade, a pillar drill and stainless steel fixings. The owner asks the class to recommend a material and a fixing detail, not just a price.
1. Explain why the original pine slats failed, referring to at least two mechanisms and the role of the stainless screws.
Read the worked answer
Pine failed through a combination of moisture movement and UV. The slats are 22 mm thick, so when rain soaks the top face and the sun dries it, the surface shrinks faster than the core; repeated cycles cause checking and raised grain. The black staining around the screw heads is not simply dirt: it suggests water is sitting in the fixing holes and that the stainless screw is not the only factor. A2 stainless is adequate for salt spray, but if the screw is countersunk too tightly it crushes the wood fibres and creates a small pond around the head. The salt crystallisation in the grain shows that spray is reaching the surface and drying there. The screws also restrain the slat, so seasonal movement cannot occur; the slat splits or the hole ovalises. A five-year life needs either a material that does not absorb water or a detail that lets water drain and allows movement.
2. Calculate the material cost for 12 benches with each option and recommend one, justifying the decision against the five-year requirement.
Read the worked answer
72 slats are needed (12 benches × 6). Costs: pine 72 × €9 = €648; marine ply 72 × €14 = €1,008; HDPE 72 × €22 = €1,584; aluminium 72 × €28 = €2,016. The budget is €1,700, so pine, marine ply and HDPE are within it; aluminium is not. Price alone would favour pine, but the existing pine failed in 14 months, so it cannot meet five years without a maintenance regime that is not in the budget. Marine ply is better, but its edges and screw holes are vulnerable unless sealed, and the daily wiping will wear the coating. HDPE is the only within-budget option that is inherently water-resistant and UV-stabilised; it will not check or support mould, and it needs no finish. I would choose HDPE at €1,584, leaving €116 for stainless screws and spacers. The recommendation is based on whole-life cost: replacing pine every 14 months would cost about €2,600 over five years, more than HDPE.
3. Specify a fixing detail for your chosen material that reduces corrosion and accommodates movement.
Read the worked answer
Fix HDPE with A2 stainless screws through oversized clearance holes, e.g. 9 mm holes for 6 mm screws, with a stainless washer and a 2 mm neoprene washer under the head. HDPE expands about 0.2 mm per metre per °C; a 2.4 m slat can move roughly 5 mm over a 20°C range, so the clearance and slotted holes allow movement without buckling. Do not countersink HDPE: it creeps under load and the head sinks. Use a pan head or a dome head with a washer. Space fixings at 600 mm centres and leave 10 mm at each end. Isolate the screw from any aluminium frame with a nylon washer to avoid galvanic corrosion, although stainless-to-HDPE is not a galvanic pair. The detail matters because the original failure was at the fixings, not only in the timber.