Lightweight Composite Decking on Adjustable Pedestals: Weight, Build-Up and Fixing Without Drilling
On a roof, a balcony or a podium deck, the question is never which board looks best. It is what the structure is allowed to carry, how much height you have under the door, and whether anything has to be drilled through the waterproofing to hold the deck down.
This page gives the three figures those questions need — what the boards weigh per square metre, what one pedestal carries, and how the build-up stacks up — and is equally clear about the two things we cannot evidence today.
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22.4 kg/m²
Boards only, 140 × 23 mm
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8 kN per pedestal
Working load, before safety factor
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24–175 mm
Pedestal height range, five steps
🛡️
15-year warranty
Stepped, second generation
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CE / CPR
M.2024.206.C97619 · UDEM
The Numbers You Need Before You Start
Everything on this page comes back to these figures. Take them to your structural engineer before you take them to a supplier.
| Figure | Value | Why it matters |
|---|---|---|
| Board weight, per linear metre | 3.25 kg/m | The figure on the spec sheet, and the one most often quoted |
| Effective coverage width | 145 mm | 140 mm board plus a 5 mm gap. Estimate from this, not from the nominal width |
| Board weight, per square metre | 22.4 kg/m² ≈ 0.22 kN/m² | The figure the structural engineer actually needs. Boards only |
| Board section | 140 × 23 mm, four-sided co-extrusion | Capped on the back and edges as well as the face |
| Pedestal working load | 8,000 N = 8 kN ≈ 816 kgf | Per pedestal, before any safety factor your engineer applies |
| Pedestal height range | 24–175 mm in five steps | 24–40 / 39–65 / 60–87 / 80–130 / 130–175 mm, overlapping |
| Maximum joist spacing | 350 mm | For flat decking. Stair treads are 300 mm — a different case |
| Board lengths | 2.9 / 3.6 / 4.0 / 5.8 m | Longer boards cut the number of butt joints across a terrace |
| Production lead time | 20 days | Standard catalogue sections |
Why a Roof and a Balcony Are the Same Problem
Two different places, two different clients, one shared pair of constraints. Everything else follows from these.
The image at the top of this page is a rendering of the build-up, not a photograph of a completed project. It shows the three layers described below: pedestals, keel and boards.
You are adding load to a structure someone else designed
A ground-level deck is carried by the ground. An elevated one is carried by a slab, a beam or a cantilever with a load allowance that was set years ago and is often unknown. Until you have that allowance in writing, the weight of the surface is the only half of the equation you control — which is why the weight per square metre matters more here than anywhere else.
Nothing may be drilled through the waterproofing
On a roof it is the waterproof membrane; on a balcony it is the tanking or the applied finish. In both cases, a fixing through it is a leak path and, in most cases, an immediate end to the roofing manufacturer's warranty. This is the reason the deck sits on pedestals rather than being fixed down: the build-up is held by its own weight and by the frame, not by penetrations.
The falls and the drainage outlets are already there
An elevated slab is laid to a fall for a reason, and the outlets are where they are. A pedestal system is adjustable precisely so the deck can be brought to level over a sloping substrate without touching either. Do not level by packing under a membrane, and do not cover an outlet.
Where the two differ
The balcony's binding constraint is usually height — the door threshold sets a ceiling on the build-up, and that is why the 24–40 mm pedestal exists. The roof's binding constraint is usually load and wind exposure. The comparison table further down sets the two side by side.
Turning One Pedestal Into a Figure Your Engineer Can Use
A working load of 8 kN describes one pedestal, not a deck. What a structural engineer needs is a load per square metre, and that depends entirely on how densely the pedestals are set out. The arithmetic is simple; the safety factor and the verdict are not ours to give.
| Pedestal grid | Pedestals per m² | 8 kN each works out at |
|---|---|---|
| 0.5 × 0.5 m | 4.00 | 32.0 kN/m² |
| 0.6 × 0.6 m | 2.78 | 22.2 kN/m² |
| 0.8 × 0.8 m | 1.56 | 12.5 kN/m² |
| 1.0 × 1.0 m | 1.00 | 8.0 kN/m² |
Read that table carefully. It states what the pedestals can carry, at a given spacing, before any safety factor. It says nothing at all about what your roof or balcony is permitted to carry — that is a property of the building, not of the deck, and only the structural engineer for the project can establish it. In practice the building's allowance is almost always the governing figure, and it is usually far below the numbers above.
Used the right way round, the sequence is: get the permitted load for the structure, subtract the build-up you intend to place on it — boards at 22.4 kg/m², plus joists, plus pedestals, plus anything the deck will carry — and confirm the margin. The figures on this page are inputs to that calculation, not a substitute for it.
Work Out the Margin You Are Left With
Enter the permitted load for the structure and this subtracts the build-up dead load, so you can see what remains for live load and everything the deck will carry. It is an input to the engineer's calculation, not a substitute for it.
What the Build-Up Is Made Of
Three layers, bottom to top. Colour, variants and the full specification are on each product page.
Adjustable Decking Support
24–175 mm, 8 kN
Five overlapping height ranges. Brings the deck to level over an existing fall without fixing through the waterproofing.
View product →
Zinc Plated Steel Keel
Substructure
Sits on the pedestals and carries the boards. Set out at 350 mm centres or tighter.
View product →
Aluminium Keel
Substructure
Alternative substructure where the lighter section or the corrosion behaviour is preferred.
View product →
Co-extrusion Decking Board
140 × 23 mm · 22.4 kg/m²
Four-sided co-extrusion. The weight figure this whole page is built around.
View product →
Six Constraints That Only Apply Above Ground
None of these appear on a ground-level deck. All six are decided before the first board is ordered.
Elevated build-up conditions
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A load allowance you did not set
The structure was designed by someone else, often long ago. Until the permitted load is confirmed in writing, nothing else can be fixed.
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A membrane that must stay intact
Any penetration is a leak path and usually voids the roofing warranty. The deck is held by weight and frame, not by fixings through the waterproofing.
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A height budget set by the door
Pedestal plus joist plus board has to fit under the threshold. On balconies this is normally the binding constraint, which is what the 24 mm pedestal is for.
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Wind exposure rises with height
An elevated deck sees uplift a garden deck never does, and it increases with building height and edge proximity. See the section below on what we can and cannot evidence.
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Existing falls and outlets
The slab already drains somewhere. The build-up has to keep that route clear and let water pass through to it.
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It has to come back up
The membrane below will need inspection one day. A build-up that cannot be lifted in sections turns a small repair into a demolition.
Sequence on an Elevated Slab
The order is different from a ground-level deck, and the first two steps are paperwork rather than site work.
Get the permitted load in writing
From the structural engineer or the building's records. Everything downstream is sized against this figure, and no supplier can provide it for you.
Survey the existing falls, outlets and threshold
Record the fall across the area, the position of every outlet, and the height available under the door. These three set the pedestal range you need.
Protect the membrane, then set the pedestals
Use a protection layer between the pedestal bases and the waterproofing where the membrane specification calls for it. Nothing is fixed through the membrane.
Adjust to level over the fall
Bring the pedestal heads to a level plane above the sloping substrate. This is what the adjustment range is for; do not pack under the membrane.
Lay the keel at 350 mm centres or tighter
Set the substructure out across the pedestals, keeping the route to every drainage outlet clear.
Fit boards with the clip system and expansion gaps
Leave the required side, end and perimeter allowances. Keep the perimeter detail removable so the build-up can be lifted for inspection later.
Have the permitted load already?
Send the permitted load per square metre, the area, the fall across it and the height available under the door. We will come back with a build-up weight, the pedestal range that fits and the margin you are left with.
Which Pedestal Step Fits Under Your Threshold
Enter the height you have under the door and the joist section you intend to use. This shows what is left for the pedestal and which of the five height steps start low enough to be usable.
Two Things We Cannot Evidence Today
Both matter on elevated decks, and both are places where a supplier can easily imply more than they hold. Here is the exact position.
Wind uplift — no applicable data
We hold one wind test, Intertek report 250710027GZU-001, but it does not apply here. It was carried out on a 1.8 × 1.8 m fencing system — WPC boards with aluminium posts, rails and caps — under a horizontal distributed load of 372.1 N/m² for three minutes. It used an in-house method, not a recognised standard, and the result is tied to that specific configuration. It is not a decking figure, not an uplift figure, and not transferable to a roof build-up. For an exposed or high-level deck, uplift should be assessed for the actual configuration, and restraint detailed accordingly.
Roof fire classification — not held
Roof build-ups in the EU are commonly required to achieve BROOF(t4) under EN 13501-5, and the accepted evidence is normally a test on the complete build-up rather than on any single product. We do not hold BROOF(t4), and we do not hold EN 13501-1 either. If your project requires either, we cannot meet it today. Establish what the authority will accept before the build-up is specified, not after.
What we would rather you do than find out later
Ask us early. Where a project needs a classification we do not currently hold, testing can be arranged: the testing cost is paid up front and credited in full against your order. That is a slower answer than a confident one, but it is the one that survives an inspection.
What we do hold
CE marking under the Construction Products Regulation, EU 305/2011 Annex V — certificate M.2024.206.C97619, issued by UDEM International Certification, a notified body, valid to 11 March 2029. Quality and environmental systems certified to ISO 9001 and ISO 14001, valid to 29 April 2027.
Roof Terrace and Balcony, Side by Side
The build-up is the same. What governs the design is not.
| Roof terrace or podium | Balcony | |
|---|---|---|
| Usually governed by | Permitted load and wind exposure | Height available under the threshold |
| Typical pedestal range | Mid to tall, 60–175 mm | Low, often the 24–40 mm step |
| Fall across the area | Often significant; adjustment does the work | Usually slight, but drainage still has to pass through |
| Wind exposure | Higher, and increases near edges and with building height | Lower, but uplift at an open edge still needs thought |
| Access for later inspection | Plan removable sections from the start | Usually easier, but still keep the perimeter liftable |
| The question to settle first | What is this roof permitted to carry? | How much height do I have under the door? |
Lightweight Decking and Pedestals: Common Questions
Weight and load
What does composite decking weigh per square metre?
22.4 kg/m² for the 140 × 23 mm board, which is about 0.22 kN/m². That is derived from 3.25 kg per linear metre at an effective coverage width of 145 mm, and it covers the boards only. Joists, pedestals and anything the deck carries are additional.
Is composite decking lighter than timber?
Not necessarily, and it is worth being careful here. Timber weight varies widely by species and by moisture content, and wet timber is considerably heavier than dry. Rather than compare categories, take the actual figure for each product you are considering — ours is 22.4 kg/m² for the boards — and use those in the calculation.
How much load can the pedestals take?
8,000 N per pedestal, which is 8 kN or about 816 kgf, as a working load before any safety factor. At a 0.6 × 0.6 m grid that works out at 22.2 kN/m², at 1.0 × 1.0 m it is 8.0 kN/m². Those figures describe the pedestals, not your building: the permitted load for the structure is a separate and usually much lower number that only your structural engineer can establish.
Can you tell me if my roof will take the deck?
No. That is a structural assessment of a specific building and it needs the engineer for that project. What we can do is give you an accurate weight for the build-up you are considering, so the assessment has the right input.
Build-up and fixing
Does anything get drilled through the waterproofing?
No. The pedestals sit on the substrate and the build-up is held by its own weight and by the frame. A fixing through the membrane is a leak path and in most cases ends the roofing manufacturer's warranty.
What is the minimum build-up height?
The lowest pedestal step starts at 24 mm, and the total is that plus the joist section plus the 23 mm board. Measure the height under the door before choosing the pedestal range, because the threshold cannot be moved.
How do pedestals deal with the fall on a roof?
That is what the adjustment is for. Each pedestal is set so the heads form a level plane above a sloping substrate. Do not pack anything under the membrane to achieve level, and keep the route to every drainage outlet clear.
What joist spacing should be used?
350 mm maximum for the 140 × 23 mm decking board. Note that this is the flat-decking figure; stair treads are a different case at 300 mm.
Fire and wind
Do you have BROOF(t4)?
No. Roof build-ups in the EU commonly require BROOF(t4) under EN 13501-5, usually evidenced by a test on the complete build-up rather than on a single product. We do not hold it, and we do not hold EN 13501-1 either. If your project requires either, we cannot meet it today.
Do you have wind uplift data for roof decks?
No applicable data. The one wind test we hold, Intertek 250710027GZU-001, was on a 1.8 × 1.8 m fencing system under horizontal load, using an in-house method rather than a recognised standard, and it is tied to that configuration. It is not an uplift figure and not transferable to a roof deck.
What if the project needs a classification you do not hold?
Tell us at design stage. Testing can be arranged, with the cost paid up front and credited in full against your order. Find out what the authority will accept before the build-up is specified.
Supply
What warranty applies?
The co-extruded boards are second generation, covered by the 15-year limited warranty. The cover is stepped rather than flat: 100% of the value of the affected goods in years 1 to 3, 80% in year 4, 70% in year 5, 50% in year 6, 40% in year 7, 30% in years 8 and 9, 20% in years 10 to 12 and 10% in years 13 to 15, with no cover from year 16. It runs from the date the goods arrive and are signed for at the destination port.
Can we see the board before committing?
Samples are free — you cover the courier, and we credit it against your first order.
What is the lead time?
20 days for standard catalogue sections.
Send the load allowance and the height you have
Permitted load per square metre, area, fall across it, height under the door, and destination port. We come back with a build-up weight, the pedestal range that fits and the margin left — not a rate per square metre.
Start from the load, not from the board
Send the permitted load and the height you have to work with. We will confirm the build-up weight, the pedestal range and a fixed quantity take-off — and tell you plainly where our evidence stops.