Every rooftop HVAC install starts with the same question: how are you going to sit the plant on the roof? On shopping centres, retail and commercial buildings across Australia and New Zealand there are three standard answers — a full mechdeck, a plinth, or timber/steel sleepers. They're not interchangeable.
When a mechdeck is the right call
A mechdeck is a raised structural steel platform. It gives you access underneath for services, distributes the unit load into the primary roof structure, and lets you screen the plant properly. On shopping centres, it's the default: multiple units, service reticulation between them, and a landlord who wants the roofline to look tidy from adjacent buildings.
When a plinth will do
A plinth — usually a concrete or steel curb — is fine for a single package unit on a substantial roof structure with straightforward drainage. Small commercial buildings and standalone retail pads often live happily on plinths. The tradeoff: no under-unit access, and no future-proofing when the tenant swaps the AHU in year five.
When sleepers still make sense
Timber or steel sleepers are the cheapest option and the shortest-lived. They're OK for a temporary install or a low-value ancillary unit. They are not appropriate for shopping-centre plant or anything the mechanical consultant needs to sign off with a design life.
Rule of thumb
Multiple units, visible from adjacent tenancies, or on a metal deck roof? Spec the mechdeck. Single package unit on concrete? A plinth is defensible. Anything else, price both and let the client see the delta.