The joist: the invisible foundation of a successful deck
A composite deck is what you see. But it's what you don't see that makes all the difference. Silvadec® composite wood decking boards must never rest directly on the ground: they must always be supported by joists, the structural components that ensure ventilation, stability and durability of the entire assembly.
Good news: Silvadec has designed two families of joists adapted to all types of substrates. Whatever the configuration of your project — concrete slab, adjustable pedestals, deck on waterproofing membrane — there is a solution designed to last.
Common rules to follow, regardless of joist type
Before choosing your joist type, certain principles apply in all installation cases. The clearance between the ground and the underside of the boards must be at least 50 mm to ensure adequate ventilation beneath the decking. Joists must have a minimum width of 40 mm and be installed at a maximum spacing of 400 mm centre to centre. It is also prohibited to lay any mesh, felt or geotextile above the joists, as this would obstruct ventilation. Finally, the ends of each board must rest on a joist, with an overhang not exceeding 25 mm.
The Silvadec® composite joist: ideal for installation on concrete slabs
Made from the same composite as the decking boards — 2/3 French wood flour, 1/3 high-density polyethylene — the Silvadec® composite joist is the natural solution for flat concrete slab installations. Rot-proof and maintenance-free, it fits perfectly into the logic of a 100 % composite deck.
Installation conditions to strictly observe
The Silvadec® composite joist is a non-structural element: it must rest along its entire length on a solid and level surface (concrete slab or compacted ground with integrated seating shims). It must not be used on pedestals, stilts or directly on a waterproofing membrane. It must not stand in water. Cutting is only permitted along the length — never through the thickness or the width.
Specifications and product codes
Silvadec® composite joists
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Product code
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Description
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Dimensions (mm)
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Colour
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SILAMB0601
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Brown joist 3000 mm
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50 × 50 × 3000
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Brown
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SILAMB1001
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Brown joist 4000 mm
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50 × 50 × 4000
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Brown
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Available in Anthracite grey and Brown, the composite joist blends discreetly beneath the decking. It is installed perpendicular to the boards, in the direction of the slope to encourage water run-off.
The Silvadec® Réversil aluminium joist: the all-terrain structural solution
Designed for demanding configurations, the Silvadec® Réversil aluminium joist is the technical answer for situations where the composite joist reaches its limits. Installation on adjustable pedestals, shims, waterproofing membranes, in wet conditions or at height: this is the domain of this matt black powder-coated aluminium profile.
A reversible profile with multiple advantages
The name Réversil is no coincidence. This profile can be turned over to allow the butt-joining of two boards on a single joist, thanks to two rails on its underside. In one orientation, it accommodates standard fixing clips for regular installation; reversed, it distributes the butt-joint across two separate rails. A significant saving in material and time on large surfaces.
Another key advantage: the clip rail is slightly deeper than the clip itself, meaning the decking boards rest directly on the joist and not on the metal piece. The result: a remarkably quiet footfall, with no knocking or resonance.
A complete system, designed for the installer
The Réversil joist was designed as a complete ecosystem. It is supplied and sold with a dedicated range of accessories that ensure the planarity and solidity of the structure: folding connectors for length joins, horizontal and vertical brackets for assemblies, flat and notched shims for height adjustment, and rubber pads for ground insulation. Screws and clips are guided by rails and centring lines integrated into the profile, securing and speeding up installation.
Installation rules on pedestals
The spacing between pedestals must not exceed 600 mm for private projects and 400 mm for public access buildings (ERP). The authorised overhang at the end of the joist is limited to 150 mm. Each pedestal must be secured to the joist, ideally using pedestals fitted with fixing brackets. For long runs, joists are butt-jointed using folding connectors, ensuring that each end is supported by a pedestal.
For the installation of a Silvadec deck, always consult our installation guides.
Technical specifications
Silvadec® Réversil aluminium joist — product data
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Reference
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Dimensions (mm)
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Material
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Finish
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Colour
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Weight
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SILAMB2102
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63 × 40 × 3600
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Aluminium
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Powder coating
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Matt black
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3.23 kg
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10-year guarantee. 100 % recyclable through standard aluminium recycling channels. Download installation guide PU39.
Which joist to choose according to your configuration?
Silvadec® joist selection table by substrate type
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Substrate type
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Recommended joist
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Installation guide
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Flat concrete slab, compacted ground with integrated shims
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Silvadec® composite joist 50 × 50 mm
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PU7
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Polymer pedestals, adjustable pedestals, stilts, foundation screws
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Silvadec® Réversil aluminium joist
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PU39
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Waterproofing membrane, flat roof deck, wet environment
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Silvadec® Réversil aluminium joist
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PU39
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Adjustment shims (low height unsuitable for pedestals)
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Silvadec® Réversil aluminium joist with rubber pads and shims
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PU39
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See all decking accessories
For any mixed or atypical configuration (diagonal decking, angles other than 90°, access to substructure required), our sales team and our distribution network are available to advise you.
