Roof Terrace Decking on Adjustable Pedestals — It Sits On the Roof, It Does Not Fix Into It
A deck over a flat roof, a garage roof or a terrace above occupied space has one requirement that outranks everything else: the waterproofing stays intact. Every other question — colour, board width, price — is downstream of it. On a pedestal system the base plate rests flat on the membrane and the weight of the assembly holds it there. Nothing is screwed, bolted or anchored through the roof.
This page is the technical side of that: six pedestal models covering 30 mm to 320 mm of build-up, how the pedestals take up the fall the roof already has, why the deck has to come up again over a drain, and how to work out the dead load for your own structural engineer. We are the plant — WPC decking extruded on our own lines in Sihui, Zhaoqing, Guangdong, on dies we own and cut ourselves.
Specifying a board rather than a system? Every section in our schedule has its metre weight published, which is the figure the load calculation on this page needs.
Specification guides: Profiles, sections and metre weights
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No penetration
Base sits on the membrane
30–320 mm
Six pedestal models
Takes up the fall
Level deck over a sloped roof
Lift and relay
Access over drains
MOQ 70 m²
Well under a container
CE / CPR
EN 15534-1 + EN 14041
The Whole Point: The Membrane Stays Intact
Ask a roofing contractor what worries them about a roof terrace and it is never the boards. It is a leak that appears two years later somewhere nobody can find. A pedestal system exists to take that risk off the table.
The base plate rests on the membrane — nothing goes through it
The drawing at the top of our own catalogue page shows it plainly: a wide circular base plate sitting flat on the roof surface, an aluminium joist in the pedestal head, and the boards above. No screws into the deck of the roof. No anchors. No brackets. No drilled holes. The assembly is held down by its own weight, which is also why the load calculation further down this page matters.
The practical consequence is the one your client cares about: the roof build-up is exactly as the roofer left it, and the warranty on that build-up is undisturbed. Whoever signs off the waterproofing should still see the drawing before work starts — that is their call, not ours — but there is nothing on our side of the detail for them to object to.
What it looks like half-finished
One of our own installations, mid-build. The aluminium joists are sitting on the roof surface and the boards clip down onto them — lift one board and you can see straight through to what is underneath. Nothing is drilled, and nothing needs to be.
To be precise about what this particular photograph shows: here the joists sit directly on the surface, which is the simplest version of the principle. On a pedestal build-up the joists sit in the pedestal heads instead, which raises the deck clear and lets it take up the fall. What does not change between the two is that nothing is fixed through the roof.
And it comes back up again
A deck fixed down is a deck that has to be destroyed to inspect what is underneath. A pedestal deck is laid, not built: boards unclip, joists lift out of the pedestal heads, pedestals pick up off the membrane. That matters more over a roof than anywhere else, because sooner or later somebody needs to get to the outlet, the upstand or the membrane itself.
The Six Pedestal Models, and What Build-Up Each One Covers
Straight from our catalogue. Pick the model by the height you need to make up between the roof surface and the finished deck level — remembering that the figure has to swallow the fall of the roof as well as the joist and board thickness.
| Model | Height adjustment | Typical use |
|---|---|---|
| MB-DPAO-A | 30 – 45 mm | The lowest build-up. Where a threshold leaves very little height to play with |
| MB-DPAO-B | 45 – 75 mm | The common residential terrace case |
| MB-DPAO-C | 80 – 130 mm | Where the fall is steeper or an insulation layer has eaten the height |
| MB-DPAO-D | 130 – 175 mm | Bringing a deck up level with a raised threshold |
| MB-DPAO-E | 160 – 260 mm | Larger level corrections; services or drainage running under the deck |
| MB-DPAO-F | 190 – 320 mm | The tallest. Substantial level changes across a single terrace |
How the Pedestal Is Put Together
It is a modular stack rather than six separate products, which is what makes the height range continuous from 30 mm to 320 mm.
Five parts, and what each one does
Tabs hold the joist and set the spacing. The mounting head is the part that threads up and down to fine-tune the level. The fix circle locks the height once it is set. A connecting coupler extends the stack for the taller models. The supporting base is the wide plate that spreads the point load across the membrane.
Look back at the model table and you can read the logic: the A is a single piece; the B adds a base; the C and above add the coupler and a taller base. Same head, different stack.
Why the base plate is wide
Everything the deck carries reaches the roof through the pedestals, so the plate is sized to spread that load rather than concentrate it. How many pedestals, and at what centres, is a function of the joist span and the load — send us the layout and the profile and we will set it out with you rather than leave it to the installer's judgement on the day.
A Flat Roof Is Not Flat — and That Is the Point of Adjustable
Every flat roof is laid to a fall so it drains. A terrace, on the other hand, wants to be level. The pedestals are what reconcile those two facts.
The fall stays where it is; the deck comes level on top of it
The roof keeps draining exactly as designed, because nothing about the fall is altered — no screed, no levelling compound, no packing. Instead each pedestal is threaded to its own height, taller at the low end of the fall and shorter at the high end, and the finished deck reads level underfoot. Water runs underneath the deck to the outlet as it always did.
Which means the height range is a design input, not a detail
Work it out at drawing stage rather than on site: the difference in height across the terrace, plus the joist depth, plus the board thickness, plus any threshold you have to meet. That total is what picks the model from the table above — and if it lands near the top of one model's range, take the next one up so the adjustment is not at its limit. Send us the levels and the threshold height and we will tell you which model the job needs.
The threshold is usually the constraint
On a terrace off a living room or a bedroom, the door threshold fixes the finished deck level and everything else has to fit underneath it. That is where the lowest models earn their place. Where the height available is genuinely tight, tell us the figure you have to work within and we will tell you what fits.
Leave a Way Back In
The most common regret on a finished roof terrace is not the colour. It is that nobody can reach the drain.
Plan the access before the boards are cut
A roof outlet needs to be reachable for cleaning and inspection, and on a pedestal deck that is easy to arrange if it is designed in and awkward to retrofit. Make the boards over and around each outlet a liftable section — cut to the joist line so a panel comes out cleanly rather than leaving somebody to unpick half a terrace. The same applies over any upstand, movement joint or service run you may need to inspect.
Mark it on the as-built drawing
Whoever maintains the building in five years will not know where the outlet is under 60 m² of identical boards. A drawing showing which panels lift is worth more than the panels themselves, and it takes ten minutes at handover. This is the sort of thing a distributor can add to a quotation and win the job on.
Keep the drainage path under the deck clear
Leaves and grit find their way under any raised deck. The board gaps that let water through also let debris through, so the space under the deck needs to be considered part of the drainage route rather than a sealed void. Fall, clear outlets and liftable access are a system — the deck is just the top of it.
Working Out the Dead Load
This is the number your client's structural engineer will ask for, and the honest answer is that it depends on the board you choose. Here is how to arrive at it properly rather than borrowing a figure from someone else's page.
The method
Board self-weight per square metre is metre weight × linear metres per square metre. The linear metres come from the covered width, not the nominal board width: a board plus its gap. So a 138 mm board laid with a 5 mm gap covers 143 mm, which is 1000 ÷ 143 ≈ 7.0 linear metres per square metre.
Take ZSGJ138H23A at 2.85 kg/m from the model schedule: 2.85 × 7.0 ≈ 20 kg/m² of boards. Do the same arithmetic with the profile you are actually buying — metre weights across the deck schedule run from 2.00 to 7.27 kg/m, so the answer moves a long way depending on the section.
What that figure does and does not include
It is the boards alone. The joists, the pedestals, the clips and the trim are on top of it, and so is everything that will stand on the deck — furniture, planters, people, and in some climates snow or standing water. The total imposed load on the roof is a structural question and it belongs to your engineer, who will also want to know the pedestal centres because that is where the load reaches the structure.
What we do is give you the inputs in writing: the metre weight for your profile, the covered width, and the pedestal layout, on the quotation, so the person doing the calculation is working from our numbers rather than an estimate. Ask and we send it. The model schedule has the metre weight for every section.
Hollow or solid
Hollow sections weigh less per metre than solid ones of a similar size, which is the usual reason they get specified over a roof. Solid sections get specified where the traffic justifies them. That trade-off is worth having with us before the quotation, because on a large terrace it changes both the dead load and the container quantity.
Wind, and Why a Loose-Laid Deck Needs Thinking About
A deck that is not fixed to the building is held down by weight and by the way the assembly is put together. On an exposed roof that deserves a sentence rather than a shrug.
Uplift acts on the assembly, not on the board
Wind over a roof produces suction, and it is strongest at the edges and corners. What resists it is the mass of the whole assembly and the clips tying boards to joists — not the weight of an individual board. So the clip specification and the edge detail are the parts that matter, and they are the parts most often left to whoever is on site.
The exposure is a local question and it has a local answer
Wind loading is set by your own national code against the building's height, location and terrain, and the person signing off the roof will already be working to it. Give us the exposure and we will give you the fixing specification and the pedestal layout to suit it, in writing with the quotation. Edge and corner zones usually want closer pedestal centres and more clips than the field of the deck — that is normal, and it should be on the drawing rather than improvised.
Near the coast, the fixings are the weak link
Salt air attacks fasteners long before it affects the composite. Specify A4 / 316 stainless for clips and screws rather than A2 on any coastal or estuary site, and put the grade in the quotation. Replacing corroded fixings under a roof deck means lifting it — which, to be fair, this system at least lets you do.
Boards That Suit a Roof Deck
Six of the sections most often specified over pedestals. Metre weight is the column that matters here, because it is the one that feeds the load calculation above.
Co-extrusion 23 × 138
Capped, hollow · 2.85 kg/m
The main capped section and the lightest of the common choices. This is the profile our slip test was run on.
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Co-extrusion 23 × 148
Capped, hollow, premium
Wider capped board, four-sided cap. Fewer boards and fewer clips across a large terrace.
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Co-extrusion solid 20 × 140
Capped, solid core
Solid throughout. Heavier per metre, and specified where a commercial terrace justifies it.
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3D embossed, round chamber
Deep-grain surface
Heavier embossing than a standard woodgrain, in the capped construction.
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3D embossed, square chamber
Deep-grain surface
Same surface, square-cell core. Lower cost per square metre than the round-chamber section.
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Classic 140 × 25
Uncapped, square chamber
First-generation grade, shown in Walnut. Cost-led work with routine cleaning.
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Fixing the Boards, and the Gaps That Have to Be Left
From our own installation guide. Over a roof these figures do double duty — they are what lets the deck move, and they are what lets water through to the roof below.
The four-step method on pedestals
One: the aluminium joist clips into the tabs on the pedestal head. Two: joist connectors go on at both ends and are tightened. Three: the boards are fixed down to the joists with clips. Four: the L-shaped trim closes the edge. Note what is absent from all four steps — at no point does anything fix into the roof.
The gaps
5–7 mm between the sides of adjacent boards. About 5 mm between a capped board and a wall. 3–5 mm at butt joints where two board ends meet. Butt joints sit on double joists — each board end gets its own joist, never a shared one, and this is the detail most often missed. On a roof terrace an upstand or a parapet counts as a wall for the purposes of that 5 mm.
Accessories, with sizes
Stainless steel start clip, plastic clip, expansion screw, L-shaped corner trim 50 × 50 × 3000 mm, WPC joist 40 × 30 × 3000 mm, aluminium joist 40 × 25 × 2900 mm. Over pedestals the aluminium joist is the usual choice. Note it is 2900 mm, not 3000 — an easy hundred millimetres to lose when you take off quantities.
Material, Sections and Colour
The same board schedule as our market pages, summarised here so a roof specification can be written from one page.
What the board is made of
60% wood powder, 30% HDPE, 10% additives. On the capped range a co-extruded shield wraps the WPC core, and the shield is the wearing layer — it carries the fade and scratch resistance. A roof terrace is usually the most exposed surface on a building, which is the argument for the capped grade where the budget allows it. The material can be recycled and reused at end of life.
Colours — three ranges
Capped co-extrusion: Teak, Antique, Redwood, IPE, Light-grey, Walnut.
Classic uncapped: Teak, Redwood, Walnut, Coffee, Chocolate, Ebony.
3D embossing: Teak, Red Wood, Walnut, Chocolate, Light Grey, Dark Grey.
Grey is available in the capped range (Antique, Light-grey) and the 3D embossing range (Light Grey, Dark Grey). A roof terrace usually has no shade at all, so where it will be crossed barefoot, specify from the lighter end. Order the whole terrace in one batch — pigment goes into the extrusion, so two production runs of the same colour will not be identical, and on a roof there is nowhere to hide a mismatched board.
Sections, with the dimensions on them
ZSGJ138H23A and ZSGJ138H23C at 138 × 23 mm, ZSGJ138H25 at 138 × 25, ZSGJ140S23 at 140 × 23, ZSGJ140S25 at 140 × 25, ZSGJ145S30 at 145 × 30. The full schedule with metre weights is on the model page.
Certification: What We Hold, and the Exact Scope
A roof terrace usually gets looked at more carefully than a garden deck, so here is the whole picture including the exact scope. Ask and we send the document itself.
| Item | Status | Detail and scope |
|---|---|---|
| CE marking / CPR | Held, scoped | Certificate M.2024.206.C97619, UDEM International Certification (notified body), EU Regulation 305/2011 Annex V Clause 1.5, valid to 11 March 2029. Issued against EN 15534-1:2014+A1:2017 and EN 14041:2018 for WPC decking, brand Bongywood, profiles 138H23A, 140H25B and 138H23C. Ask before assuming it covers a profile outside that list |
| Reaction to fire, EN 13501-1 | Declared through EN 14041 | EN 14041:2018 requires a reaction-to-fire classification to EN 13501-1, and that class sits on the Declaration of Performance rather than on the certificate. Ask for the DoP for the profile you are quoting and you have the class in writing. The declaration covers the decking profile named on it |
| Surface burning, ASTM E84 | Held, scoped | Class A. SGS report GZIN2404000442CM01-1_EN, 25 April 2024, to ASTM E84-23c: flame spread index 10, smoke-developed index 30. Run on a 138 x 23 mm board, white surface exposed, interior wall and ceiling finish category. This is the US method on the US scale; for a European project the Euroclass on the DoP is the reference |
| ISO 9001 | Held | IAS25924Q1473R0M, valid to 29 April 2027 |
| ISO 14001 | Held | IAS25924E0394R0M, valid to 29 April 2027 |
| Slip resistance | Held, scoped | R10 under AS 4586-2013 Appendix D, Intertek report 230625005SHF-001, ramp method at 15 degrees, on profile ZSGJ138H23A. That is the SHOD wet method — the number that applies to a surface walked in shoes. Barefoot wet areas are assessed by a separate method with its own scale, so tell us which one your project has to satisfy |
| REACH | Declaration only | Manufacturer declaration of conformity, issued by us |
Send Us the Levels and We Will Specify the System
A roof terrace enquiry is easier to answer well than a general one, because the constraints are concrete. Give us these and you get a real specification rather than a price list.
What to send
The area in m², the height available under the threshold, the fall across the terrace, where the outlets are, and the exposure. Per-unit price within 24 hours, with the pedestal model and layout named.
And the commercial side
Minimum order is 70 m², which covers most single terraces — a first order does not have to be a container. Samples are free and you cover the courier, credited in full against your first order. We are the plant: roughly 2,000 tonnes a month, about 100 containers, on dies we own and cut ourselves, with a test report travelling with every shipment.
Roof Terrace Decking: Common Questions
The system
Does anything fix into the roof?
No. The pedestal base plate rests flat on the roof surface and the assembly is held by its own weight. Nothing is screwed, bolted or anchored through the waterproofing. That is the reason the system exists, and it is why the four-step installation sequence on this page never involves a fixing into the roof deck. Whoever warrants the waterproofing should still see the drawing before work starts.
What height can the pedestals make up?
From 30 mm to 320 mm across six models: MB-DPAO-A 30–45 mm, B 45–75 mm, C 80–130 mm, D 130–175 mm, E 160–260 mm and F 190–320 mm. Pick by the total you need — the fall of the roof, plus the joist, plus the board, plus any threshold you have to meet. If your figure lands near the top of one model's range, take the next model up so the adjustment is not working at its limit.
How do the pedestals deal with the fall on the roof?
Each one is threaded to its own height, so they are taller at the low end of the fall and shorter at the high end. The roof keeps its fall and keeps draining exactly as designed; the deck reads level underfoot. Nothing is screeded, packed or levelled on the roof itself.
Can the deck be lifted later?
Yes, and it should be designed so that it can. Boards unclip from the joists, joists lift out of the pedestal heads, and pedestals pick up off the membrane. Make the boards over each outlet a liftable section cut to the joist line, and mark on the as-built drawing which panels lift — in five years nobody will remember where the outlet is.
What holds the deck down in wind?
The mass of the assembly and the clips tying boards to joists. Uplift is strongest at edges and corners, so those zones usually want closer pedestal centres and more clips than the field of the deck. Wind loading is set by your own national code against the building's height, location and terrain — give us the exposure and we will give you the fixing specification and the pedestal layout in writing with the quotation.
Load, boards and specification
What is the dead load of the decking?
It depends on the profile, so work it out from the metre weight rather than borrowing a figure. Board self-weight per m² = metre weight × linear metres per m², and the linear metres come from the covered width — the board plus its gap. A 138 mm board with a 5 mm gap covers 143 mm, so about 7.0 linear metres per m². At 2.85 kg/m that is roughly 20 kg/m² of boards. Metre weights across our deck schedule run from 2.00 to 7.27 kg/m, so the answer moves a long way with the section. That figure is boards alone — joists, pedestals, clips, trim and everything standing on the deck are on top of it, and the total imposed load on the roof is a structural question for your engineer. We put the metre weight, the covered width and the pedestal layout in writing on the quotation so the calculation is done from our numbers.
Hollow or solid boards over a roof?
Hollow sections weigh less per metre, which is the usual reason they are chosen over a roof. Solid sections get specified where the traffic justifies them, and they weigh more. On a large terrace that trade-off changes both the dead load and the container quantity, so it is worth settling with us before the quotation rather than after.
What are the installation gaps?
5–7 mm between the sides of adjacent boards, about 5 mm between a capped board and a wall, and 3–5 mm at butt joints where two board ends meet. Butt joints sit on double joists — each board end on its own joist, never shared. On a roof an upstand or parapet counts as a wall for that 5 mm. Those gaps also let water through to the roof below, which is how the drainage is meant to work.
Which joist should be used on pedestals?
The aluminium joist, 40 × 25 × 2900 mm, is the usual choice over pedestals — note it is 2900 mm and not 3000, which is an easy hundred millimetres to lose when you take off quantities. WPC and stainless steel joists also exist. Tell us the layout and we will quote the joist with the boards rather than leaving it to be sourced locally.
What joist spacing do the boards need?
It is per profile and per grade, and we give it in writing with the quotation. Our own catalogue shows the span changing by a third or more between grades for the same section, which is exactly why a single published figure would mislead you. Over pedestals it also drives the pedestal centres, so it is not a number to guess at.
What colour should a roof terrace be?
Usually the lighter end, because a roof terrace typically has no shade at all and a dark board in full sun gets genuinely hot underfoot — that is true of every composite, and of stone and porcelain too. Grey is available in the capped range as Antique and Light-grey, and in the 3D embossing range as Light Grey and Dark Grey. Order the whole terrace in one batch, because pigment goes into the extrusion and two runs of the same colour will not be identical.
Do you supply the pedestals as well as the boards?
Yes — boards, aluminium joists, clips, L-trim and pedestals quoted together as one system. That is the point of sending us the levels: you get a quotation with the pedestal model and layout named rather than a board price and a shopping list.
Send the levels, the area and the threshold height
Pedestals 30–320 mm across six models · Nothing fixes into the roof · Factory direct from Guangdong · CE / CPR M.2024.206.C97619 under EN 15534-1 and EN 14041 · ISO 9001 & 14001 · MOQ 70 m² · Test report with every shipment · Trade buyers only