Adjustable pedestals solve this problem by separating the finished terrace surface from any slope or uneven substrate beneath it. As a manufacturer of adjustable pedestals working directly with terrace contractors, general contractors and commercial installers, we see first-hand how the right support system turns an irregular patio slab or multi-pitch roof into a flat, self-draining terrace that remains stable under foot traffic.
Achieving this result requires treating the pedestals, substructure and composite boards as an integrated system. Here is how to plan, lay out and build a composite terrace on adjustable pedestals that will not sag, squeak or retain water.
1. Plan the substructure and pedestal grid
While terrace pavers often rest directly on pedestal heads with spacer tabs, composite decking boards require continuous linear support to prevent creep, springing, or sagging under load. This means you almost always build a pedestal-based grid using aluminum or steel joists or high-quality, rot-resistant frames to which the composite clips are fixed.
Spacing and spans
Composite boards are flexible, especially when the sun beats down on them strongly. While standard wooden boards may have joist spacing every 16 inches, most composite manufacturers limit spans in residential construction to 12–16 inches, reducing them to 12 inches for foot traffic in commercial facilities or diagonal laying.
Pedestal spacing directly determines the size of the joist needed:
- From pedestal to pedestal (along the joist): Spacing typically ranges from 16 to 36 inches, depending on the bending stiffness of the selected joist profile.
- From joist to joist: Determined entirely by the thickness and profile of the composite board.
Calculate the total dead load (supporting structure plus boards) and the live load (intensive foot traffic, planters, furniture). Even distribution of weight on the pedestal bases protects roof membranes and prevents indentation from point loads. If you are calculating spans for unusual site geometry, our technical support team often helps project managers review structural layouts and dead load distribution before shipping materials to the construction site.
2. Matching the height range and slope correction
Every construction project has a defined height budget. On a rooftop terrace, the height of the finished terrace is determined by the door thresholds and the parapet height.
Measure high and low points
Map the substrate before ordering components:
- Establish the reference height (usually the door threshold minus the board thickness, joist height, and pedestal head profile).
- Measure the distance down to the substrate at the highest and lowest points across the entire run.
- Determine the pedestal height range.
If the highest point requires a 1.5-inch pedestal and the lowest point drops to 6 inches, check whether the selected pedestal line covers this range seamlessly. In our production line, we design telescopic pedestals with overlapping adjustment bands between height levels. This overlap gives the crew a safety margin on site: if the concrete pad sinks an additional three-quarters of an inch below the level indicated on the drawings, the flange can be turned up without having to stop work and wait for taller extenders.
Built-in slope compensation
Substrates have a slope for water drainage. If the pedestal lies flat on a sloped membrane, its head tilts, creating an angled contact point with the joist. Look for pedestals with self-leveling heads or adjustable slope-compensating bases that correct slopes from 0% to 5%. Keeping the pedestal shaft vertical ensures that vertical loads transfer straight to the base, preventing lateral shifting or joint fatigue over time.
3. Choose the right head construction and connection to the substructure
The way the joist rests on the pedestal determines whether the terrace will feel solid or hollow to the touch.
- Joist hangers and cradles: Use pedestal heads designed with integrated joist cradles or top brackets. They hold aluminium or composite frame profiles tightly without clamping, preventing the joist from twisting.
- Fixing: Fix the joists directly to the pedestal head using screws approved by the manufacturer, or use snap-on cradle clips that allow slight thermal movement while blocking vertical lifting.
- Acoustic and anti-vibration pads: High-density rubber shims placed between the pedestal head and the joist dampen footstep noise. On occupied roofs, this single detail prevents walking vibrations from being transmitted to the rooms below.
Step 1: Preparing and protecting the substrate
Make sure the concrete or membrane is clean and free from sharp contaminants. On sensitive single-ply or torch-welded roof membranes, place protective pads under each pedestal base. This prevents the edges of the base plate from rubbing against the waterproofing over years of micro-movements.
Step 2: Establish the reference level and string lines
Set the first pedestal at the critical reference point, usually directly in front of the entrance door, leaving adequate clearance for water drainage below the threshold. Stretch taut string lines across the area in both directions, marking the top surface of the joists.
Step 3: Set out and level the perimeter
Set the perimeter pedestals and place the outer joists. Turn the adjustment rings by hand to align the joists with the string line. A laser level makes this stage much easier, but an additional check with a long spirit level will ensure you don't struggle with minor bends in the joists.
Step 4: Fill the field
Place the remaining pedestals in the internal grid according to the joist layout. Rotate each pedestal head until it achieves full, solid contact with the underside of the structure.
Once the joist grid is level, walk across it. If any pedestal rotates freely or rocks, it is not transferring load. Adjust the height rings until every contact point is stable, then lock the locking rings or locking couplers to secure the threads in place.
5. Installation of composite decking boards
Once the joist frame is rigid and level, installing the boards proceeds similarly to a standard composite deck construction, with a few key rules regarding assemblies supported on pedestals.
Managing expansion and contraction
Composite material expands and contracts along its length with temperature changes.
- Gaps between boards: Always leave the gap specified by the board manufacturer depending on the ambient temperature during installation. A board laid on a cool morning at 45°F will expand significantly on a summer afternoon at 90°F.
- Perimeter gaps: Leave at least a quarter inch gap between board ends and any solid masonry wall, door threshold, or parapet. Never fasten composite boards rigidly to a fixed structure.
See also: Clever Level 80-200 mm pedestal, 3 mm for tiles & decking
Fixing methods
Most modern installations use hidden clip systems mounted in boards with grooves on the edges.
- Fix the clips directly to the joist frame using marine grade or 304/316 stainless steel screws.
- Where face fixing is required (such as starter boards, divider boards or edge trims), pre-drill and countersink the hole to prevent mushrooming around the screw head.
Airflow and drainage
The space under a terrace mounted on pedestals is its greatest functional advantage. Water drains freely through the gaps between the boards, falls onto the substrate and flows towards the drains and outlets without obstruction from continuous joists.
Keep the edges open or use perforated ventilation strips so that air circulates under the terrace. Continuous airflow prevents heat from building up under the boards and stops stagnant water from creating mould on the slab.
6. Total Cost Calculation: Quality and Lead Times
When pricing a project using terrace pedestals, looking solely at the unit price of the plastic components can ruin the project budget. The real profitability of an installation comes down to three operational factors:
- Batch consistency and thread quality: Cheap pedestals with rough, sloppy threads skew under load or seize during fine adjustment under weight. Pedestals made from virgin, UV-stabilized polypropylene with a clean, wide thread pitch allow the crew to level a 500-square-foot grid in hours, not days.
- Lead times and phased deliveries: Rooftop and multi-family projects run on strict crane schedules and narrow street-closure windows. Sourcing pedestals from a manufacturer with reliable domestic stock and clear delivery dates protects the project schedule from costly delays caused by overlapping crew work.
- On-site support: Direct access to help with layout planning, span tables, and installation advice saves hours of guesswork when site conditions change unexpectedly, for example when you discover an unmarked roof drain exactly where the main load-bearing pedestal was planned.
By combining stable pedestals with a defined load capacity with a rigid frame grid and proper gaps for composite boards, you deliver a terrace that stays flat, drains water immediately, and withstands years of intensive use without service calls.
