Ask any installer what actually goes wrong on a rooftop mechanical platform and you'll get the same three answers. The unit ended up too close to the screen so the coil can't be pulled. The deck was priced at the light load rating and the plant schedule changed. Or the platform was drawn as a rectangle and the roof it landed on was anything but.
Fair Dinkum Mechdecks is built around those three failure points — clash zones, load ratings, and custom-shaped decks — and every one of them behaves differently depending on whether you're fixing to metal deck or concrete. This guide walks through each, how the app enforces it, and where standalone screens fit in.
1. Clash zones: the 600 mm amber buffer
A clash zone is the strip of deck you are not allowed to build into. On every platform with a screen or guardrail, the app draws a 600 mm buffer inboard of the platform edge and shades it amber on the canvas and in the 3D render. HVAC objects physically cannot be dropped into it — drag a unit toward the edge and it clamps at the boundary.
Six hundred millimetres is not an arbitrary number. It is the working room a maintainer needs to walk a unit's perimeter, swing a panel, and get a body between the plant and the screen without leaning over an edge. Lose it and you've built a platform that passes on paper and fails on the first service call.
The clash zone follows the deck, not a bounding box. On an L-shaped or notched platform it hugs every internal and external corner, so a concave return gets the same protection as a straight run. Cut-outs and known obstructions — a vent penetration, an existing duct riser, a skylight — carry their own exclusion ring, so the layout is blocked around the object rather than just around the perimeter.
The practical benefit is that the layout you send to the client is the layout the installer can actually build. Nothing gets placed that later has to move, and the deck area you're quoting is usable area, not gross area.
2. Load ratings: 2.5 kPa vs 5.0 kPa, and what changes underneath
Every platform is designed against a uniformly distributed live load — either 2.5 kPa for standard maintenance access or 5.0 kPa for heavier plant and trafficked decks. Picking the number is the easy part. What matters is that the structure under the mesh changes with it, and the app enforces that automatically rather than leaving it to a note on the drawing.
At 2.5 kPa, truss spacing cannot exceed 1000 mm. At 5.0 kPa, spacing tightens and top battens are added running across the trusses at a maximum of 700 mm centres, sitting under the mesh. Those battens are drawn in the model and counted in the bill of quantities, so the steel you quote is the steel that gets delivered.
On top of the structural rules, the app runs a live capacity check. Total HVAC plant weight is compared against the deck area and the selected rating, so if someone adds a second condenser to a deck sized for one, the warning appears while it's still a drawing rather than after the crane is booked.
Design flags sit alongside it: any platform edge with a height difference over 300 mm from the roof deck picks up a mandatory guardrail automatically, edges over 1500 mm raise a high-edge warning, and a roof pitch beyond 30 degrees stops and refers the job to engineering and a costs review. These aren't advisory footnotes — they change the model and the pricing.
3. Custom-shaped decks: trace the roof you've actually got
Very few plant areas are rectangles. Tenancy boundaries, roof ridges, plant rooms and existing services all push the platform into an L, a T, or something with a bite out of it. Rather than approximating with a rectangle and adding a fudge factor, you trace the real shape.
Upload the architectural PDF, set a calibration scale once with the measure tool, and draw the polygon straight over the plan with orthogonal snapping. Cut-outs and obstacles are drawn the same way. Each edge of the polygon is numbered — E1, E2, E3 and on — and each number can independently be a screen or a guardrail, at its own height, with stairs, ladders and gates numbered against the same edges.
Roof fall is handled with fall lines. Drop a ridge or valley line across the polygon and each region takes its own pitch direction, set on a slider in 30-degree increments. The solver propagates heights across the regions so the planes meet cleanly at the ridge, and post legs are sampled off the resulting surface rather than a single assumed plane. Trusses follow the fall direction in each region and battens bridge across them, so framing turns correctly at every ridge and mitre.
The result is a 3D model, plan and side views, shop drawings and a BOQ that all describe the same odd-shaped platform — including split-pitch platforms that span a ridge with a different pitch on either side.
Metal deck vs concrete: same rules, different fixings
Substrate is set under Job Type and it flows through the whole model. On metal deck roofs the bottom truss chord is fixed with a tech screw and rivet combination into the deck profile, and the load path has to land on primary structure — the app's connection details sheet calls this out per fixing location. On concrete, the same chord is anchored with an appropriate anchor bolt and the detail sheet changes accordingly.
Everything else — clash zones, load ratings, framing rules, guardrail triggers — behaves identically on both. That's deliberate. The compliance logic shouldn't change because the roof does; only the connection detail and the fixing line item in the BOQ should.
Standalone screens: no platform required
Not every job needs a deck. Plenty of rooftop work is purely visual screening around plant that already sits on the roof, and the app treats that as its own job type. There are three configurations: screen only fixed to concrete or metal deck with no platform at all; a platform with the screen attached to the platform, following the platform perimeter; and a platform with a separate standalone screen fixed independently to the roof.
For screen-only jobs, the workspace switches to a flat plan view with a line-drawing tool. Set out each screen run as a polyline with grid snapping, drag segments to reposition them, label them, and give each run its own height. Posts are 75 x 75 mm RHS steel with configurable custom spacing, and pricing is split in settings between screen-to-platform and screen-to-concrete rates so the quote reflects what's actually being fixed.
Because it's screen only, the irrelevant metrics drop away — no deck area, no plant capacity, no HVAC total weight. What you get is a schematic plan, side views, a 3D render, a BOQ and a quote covering exactly the screening scope, with the same view-lock and report workflow as a full platform job.
Why the three work together
Individually each feature saves a bit of rework. Together they change the shape of the job. The custom polygon means the platform matches the roof, the load rating means the steel under it matches the plant, and the clash zone means the plant sitting on it can actually be serviced. Change any one and the other two update — move a unit and the capacity check recalculates; redraw an edge and the clash zone, guardrail trigger and framing follow.
That's the point of doing it in a model rather than a spreadsheet: the quote, the drawings and the bill of quantities can't drift apart, because they're all reading the same geometry.