A roof terrace retrofit can appear straightforward until the existing build-up is opened up. In this case study of an A2 terrace system retrofit, the central challenge was not selecting a replacement surface. It was coordinating a compliant, maintainable build-up around an existing waterproofed roof while retaining the architectural intent for planters, seating and a clean aluminium deck finish.
The project team needed a solution that could be detailed with confidence around drainage outlets, threshold levels, service penetrations and substantial perimeter planters. The terrace also had to remain practical to install on a live building programme, without introducing avoidable trade interfaces or combustible materials into the proposed surface build-up.
The retrofit brief
The existing terrace had reached the point where local repairs and isolated product substitutions no longer addressed the underlying coordination issues. Decking, support components and landscape features had been considered as separate packages. That created uncertainty over loading, drainage access, finished floor levels and responsibility at interfaces.
The revised brief called for a fully coordinated terrace platform comprising adjustable pedestals, baseboard rails, rubber shock pads, A2-rated aluminium decking and bespoke metal planters. Rather than treating these as independent products, the design team needed them to operate as one system over the waterproofing layer.
Fire performance was a defining requirement. On regulated buildings, the language used in schedules matters: a product described as metal is not automatically evidence of compliant system performance. Each relevant component requires appropriate classification evidence, with fire performance assessed against EN 13501-1 where applicable. The proposed build-up therefore had to be reviewed component by component, alongside the project fire strategy and the requirements of the design team, building control body and insurer.
Why an A2 terrace system retrofit required coordination
A terrace surface is only the visible layer. Beneath it, every decision affects the performance and serviceability of the roof. Pedestal locations influence load distribution. Rail spans affect deck stability. Deck board direction affects cut lines at planters and thresholds. Planter positions can restrict access to outlets if they are fixed before the drainage strategy is resolved.
For this retrofit, the key risk was coordination failure at the interfaces. A deck installer could set out boards accurately, while a planter fabricator could manufacture to approved drawings, yet the completed terrace could still fail to provide inspection access or achieve clean junctions if the two packages were not designed together.
The system approach removed that gap. The substructure was set out first as a level-adjustable support grid, designed to accommodate the existing roof falls rather than working against them. Rubber shock pads were specified below the pedestals to protect the waterproofing layer and help spread local point loads. Baseboard rails then created a stable framework for the aluminium decking, allowing the surface geometry to align with the planter layout.
This is not a universal answer for every roof. Structural capacity, roof warranty conditions, waterproofing type, falls, upstand heights and wind exposure must all be checked before a raised system is selected. Where a roof has restricted load allowance or very limited threshold depth, the detailing may need to change. The value lies in resolving those constraints early, not forcing a standard terrace detail onto an unsuitable roof.
Surveying the existing roof before design freeze
The design process began with the existing condition, not a preferred board profile. The team established finished floor levels at doors, the position and height of upstands, drainage outlet locations, service routes and the usable extent of the waterproofed area. Existing falls were reviewed so water could continue to reach outlets beneath the raised finish.
Load information was equally important. Planters filled with growing medium, irrigation water and mature planting impose a very different demand from an empty fabricated steel box. Seating, people loading, maintenance activity and local loads at pedestal positions all needed to be considered against the structural engineer’s information.
This survey stage also determined where access panels or removable deck sections were required. Drainage outlets should remain inspectable. A well-finished terrace that prevents maintenance of the roof drainage system creates a predictable long-term problem.
Building the coordinated terrace platform
Once levels and constraints were confirmed, the design was developed from the roof upwards. Adjustable pedestals allowed the finished deck level to be controlled while maintaining clearance over the waterproofing. The rail arrangement was then coordinated with board spans, perimeter zones and planter bearing points.
A2-rated aluminium decking provided the visible walking surface. It gave the scheme a crisp, contemporary finish while supporting the project’s non-combustible material strategy. Aluminium is also useful in retrofit work because boards can be cut and detailed precisely around existing obstructions. However, board geometry still needs careful planning: narrow slivers at irregular walls, poorly considered end conditions and unplanned penetrations can undermine both appearance and installation efficiency.
Bespoke metal planters were designed as part of the same layout rather than added after the deck had been set out. This allowed planter faces, deck joint lines and integrated seating edges to align. Depending on the visual brief, corten steel, mild steel or aluminium can be selected, with powder-coated finishes matched to RAL or BS references where required. Material choice should reflect more than appearance. Weight, drainage, corrosion environment, coating specification and interface detailing all influence the final recommendation.
For integrated lighting or irrigation, routes were reserved within the build-up before fabrication. This avoided the common late-stage request to drill, notch or lift completed elements in order to run services. Access, isolation points and future maintenance were considered with the same discipline as the visible finish.
Installation sequencing protected the programme
A retrofit terrace is particularly vulnerable to sequencing pressure. Waterproofing works, façade interfaces, MEP activity, landscaping and final finishes often converge in a narrow programme window. Without a single coordinated package, installers can arrive with correct components but no agreed sequence for setting out, protection or handover.
The installation strategy separated the work into controllable stages. After confirmation that the roof was ready to receive the system, protection and shock pads were positioned, followed by pedestals and rails. Levels were checked across the grid before decking was installed. Planters were placed in accordance with the approved setting-out drawings, then deck cuts and removable access sections were completed around them.
This sequence reduced rework because the concealed layers were checked before the finish concealed them. It also created clear hold points for the contractor and project team: waterproofing condition, support grid levels, drainage access and planter positions could each be reviewed before the next stage proceeded.
For projects requiring fabricated components, programme certainty depends on releasing manufacture against stable information. Metal Planters Ltd typically works to a two to six week lead time once drawings, finishes and quantities are agreed, but that window should be protected by timely decisions on levels, services and final dimensions. Bespoke fabrication is valuable precisely because it responds to the building. It cannot compensate for unresolved information released after manufacture has started.
What the project team gained
The completed proposal gave the client more than a replacement deck. It established a terrace build-up with clear relationships between the walking surface, support system, drainage zones and landscape elements. The result was a more legible package for tendering, installation and future maintenance.
There was also a compliance benefit. Specifiers could review the relevant A1/A2 classifications and material information as part of one coordinated submission rather than collecting fragmented evidence from several unrelated suppliers. This does not remove the need for project-specific fire, structural and waterproofing approvals, but it makes the evidence trail and technical conversation materially easier to manage.
Architecturally, the integrated approach preserved the intended rhythm of the scheme. Decking lines terminated cleanly at planters and seating, rather than being interrupted by improvised trims. The raised build-up concealed the necessary support and service routes while retaining access where it mattered.
Specification lessons from the retrofit
The principal lesson is that A2 performance should be considered at build-up level, not used as a late-stage surface finish decision. Ask early which elements sit above the roof membrane, what their fire classifications are, and how those classifications are evidenced. Confirm the scope of each classification rather than assuming it applies to adjacent components, fixings or assemblies.
The second lesson concerns interfaces. A planter is not simply a landscape item when it sits within a terrace build-up. It affects loading, deck set-out, drainage access, irrigation and potentially lighting. Coordinating it before fabrication reduces the risk of site-led modifications that compromise finish quality or programme.
Finally, treat maintenance as a design input. Outlets, inspection points and service connections need a deliberate access strategy. Removable sections may be visually discreet, but they should be identified on drawings and understood by the facilities team at handover.
For any proposed A2 terrace system retrofit, the strongest starting point is a coordinated survey and a single set of build-up drawings. That is where compliance confidence, clean detailing and a buildable programme begin.