Construction of wooden decking

This page is not aimed at being an exhaustive guide for the design and completion of wooden decking. Its only claim is to provide a brief overview of a number of critical aspects, detailed in the texts of standards (notably, NF DTU 51.4 Outdoor wooden decking and NF B54-040 Outdoor wooden decking boards) and other reference works, from which the majority of texts and drawings below are extracted.

In order to guarantee the best result, we recommend to call on professionals and to only use quality materials.

The artisans and professionals who design and manufacture wooden decking must comply with the requirements of the standards and regulations in force, professional standards as well as with instructions and specifications supplied by the manufacturers of the installed products and materials.

 
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Components of wooden decking

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  • 1. Boards
  • 2. Sleepers. Two options possible: with or without sleepers
  • 3. Joist work: joist in main areas and belt
  • 4. Joist work hardware
  • 5. Joist work anchoring hardware
  • 6. Pedestals
  • 7. Excavation
  • 8. Natural terrain

Joist work

Joist work is made up of rectangular profile wood. The joists play a structural role, as mechanical support and fastening for the wooden decking. The joists are arranged in the direction of the height of the section, i.e., the short side of the section is horizontal and rests on the supports. The peripheral frame is made up of edge joists. During installation, avoid the joist remaining in contact with stagnant water.

If the boards are installed directly on the joists, without backing, we recommend protecting the latter with a bituminous strip with polyester reinforcement.

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Sleepers

Two options are possible: with or without sleepers.
Sleepers are smaller wooden beams laid at regular intervals, perpendicularly to the joists. They form an integral part of the wooden decking and constitute the interface between the primary structure (the joists) and the deck boards.
The peripheral frame is made up of edge sleepers.
Their size is important, notably to guarantee perfect anchoring of board fasteners and their spacing to prevent excessive bending of the deck under a load.
The sleepers are installed widthwise, i.e., the section’s long side rests parallel to the ground. During their alignment, adequate spacing must be respected between supports, notably, depending on the wood species, the size of the boards to be installed and the type of structure (Type 1 or Type 2).

The sleepers must be sealed, screwed or fixed to the support in order to prevent premature bending due to subsidence. We insist on the fact that the species of the sleepers must be, at least of the same density and durability as those of the boards used. Generally speaking, securing a harder wood on a softer wood is to be avoided.
The width of the sleepers must be at least 50 mm for the central part of the board and 70 mm for both ends of the boards aligned end to end, except in case of double sleepers. This solution is recommended, taking care to space the ends of the boards in order to ensure the drainage of the outer boards.

We recommend protecting the sleepers with a bituminous strip with polyester reinforcement.

Deck boards

The most frequently used species for decking boards are included in the tables in the section on “Wood”, for which information has, for the most part, be extracted from the NF B54-040 standard. The deck boards must be proportioned in compliance with the general principles of Eurocode 5 (NF EN 1995-1-1). It is important to respect the maximum width/thickness ratio for the chosen species.

The bow criteria considered include:

- Type 1 (or standard): maximum instantaneous deflection under stress must be less than 5 mm.
- Type 2 (or comfort): maximum instantaneous deflection under stress must be less than 3 mm.

Each anchor point for the boards on the supports must be secured using a fastener, whether it is in the main sections or at the end of boards and regardless of the width of the board.
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Board moisture

Due to its hygroscopic nature, wood is likely to absorb and lose moisture depending on environmental conditions and temperature in which it is installed. Moisture in the boards is the critical aspect to be monitored when using wood. It is one of the keys to the durability and dimensional stability of the installed components. The moisture content of the deck boards must, in any case, be monitored, before their installation with the help of a moisture meter with pins, correctly calibrated. The boards’ moisture lever during installation must not exceed 18%.

It is ideal to install wood whose moisture content is close to the average moisture content on the construction site. In this case, the movement of the wood between dry and wet periods will be limited. Generally speaking, it is not recommended to install humid deck boards during dry periods. The wood’s moisture content will not have had time to balance itself with the environment and too rapid drying will lead to major stress, which may lead to splitting and warping.

In order to maintain superior deck board stability (notably, reducing the effect warping effect), it is important to plan for adequate ventilation (at least 10 cm between ground and sleepers). The use of narrow boards (maximum 120 mm) also considerably reduces the risk of warping and the extent of the swelling and shrinking the boards will be subjected to.

Spacing between boards

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1. spacing range allowed 3 to 12 mm

The succession of seasons, with variations in relative humidity leads to the swelling and shrinking of the wood. Extreme precision is necessary to determine the adequate spacing to be planned between boards during installation (neither too much, nor too little). The spacing to be planned between boards during installation mainly depends on the wood species, moisture content, width and extreme climate conditions on site (temperature and relative humidity in summer and winter determine the wood’s balanced moisture content).

The spacing between boards should never be less than 3 mm and should not exceed 12 mm.

HardWood Clip and SoftWood Clip® fasteners feature an integrated spacer wedge, available in several sizes (3 mm – 5 mm – 7 mm), designed to automatically provide the adequate spacing between boards and control swelling and shrinking of the boards and thus prevent abnormal situations with excessive spacing or joining boards.

Spacing and drying of the wood

There are two types of wood drying: AD (Air-Dried) and KD (Kiln-Dried).

The residual moisture of AD wood typically exceeds 18%. After installation and more particularly during dry or summer periods, AD boards will shrink while losing a large part of their moisture. Spacing between deck boards will then increase. In order to avoid having abnormally wide gaps between boards during dry periods, it is recommended to install the boards with narrower gaps, typically from 3 to 5 mm depending, among others, on the wood species, its moisture content at the time of installation, its dimensions and the temperature and moisture conditions on site (equilibrium moisture content). 3-mm minimum spacing during installation only applies if their moisture content during installation is equal or higher than the site’s maximum equilibrium moisture content (on average, 22% for Belgium) or the saturation point of the wood fibres installed, if less (e.g., 20% for Ipe).

Conversely, after installation, the KD boards, whose residual moisture content is typically less than 18%, will swell and dilate, during humid and winter periods, due to their increased moisture content until the level of maximum equilibrium with the site. In order to avoid having abnormally narrow gaps between boards or touching boards during wet periods, it is recommended to install the boards with wider gaps, typically from 5 to 7mm depending, among others, on the wood species, its moisture content at the time of installation, its dimensions and the temperature and moisture conditions on site (equilibrium moisture content).

The moisture content of the deck boards must, in any case, be monitored, before their installation (see above “Boards’ moisture”).


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Hidden fasteners

Hidden fasteners are valuable for matters of durability of the structure (decrease in the number of water retention points) but also for cosmetic and safety reasons.
It is essential to choose a system that has been subjected to an assessment by a certified independent organisation and that features design and installation specifications. HardWood Clip® is the only hidden fastening system to have been assessed by two certified independent organisations, the FCBA in France and the CTIB in Belgium.


Pre-drilling

Pre-drilling is always recommended, especially for hardwoods.

Il is essential:
  • - for wood whose density is greater than 800 kg/m3 
  • - at end of boards.
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Types of support for wooden decking

Supports can be of various types:
Wooden support   Concrete support  
Decking (boards + sleepers) on linear wooden components10-2-2-1-img08 Decking (boards + sleepers) on concrete slab10-2-2-1-img08
Decking (boards + sleepers) on linear wooden components10-2-2-1-img08 Decking (boards + sleepers) on concrete slab10-2-2-1-img08
Metal support   Polymer pedestal support  
Decking (boards + sleepers) on linear metal components10-2-2-1-img08 Decking (boards + sleepers) on pedestals and concrete slab10-2-2-1-img08
Decking (boards only) on linear metal components10-2-2-1-img08 Decking (boards + sleepers) on pedestals, geotextile and rough stabilised ground10-2-2-1-img08

Examples of installation on several supports

Installation on concrete or tiled slab

The sleepers will be pegged, screwed or embedded on decks and levelled.

2. Cross-section of sleepers on paving (main areas)
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  • 1. Deck board, thickness 3-1-2:”e”.
  • 2. Sleeper, height depending on 3-1-2:”h”.
  • 3. Reinforced concrete slab, 120-mm thick, dosed at 350kg/cement/min. m concrete(welded wire-mesh frame).
  • 4. Sand thickness 50 to 100 mm.
  • 5. Excavation bottom level. Stripping level -200 mm/natural terrain.
  • 6. Damp-proof barrier.
  • 7. Fastening.
  • 8. Calage ponctuel matériau rigide de synthèse d’entraxe “x” according to 3-1-2,thickness 10 mm or 20 mm following sleeper direction compared to paving incline.
  • 9. Incline 1% minimum.

Installation on watertight decking

Installation of sleepers and fasteners on adjustable PVC pedestals (8 pedestals/m2).

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  • 1. Decking boards or slab
  • 2. Sleeper
  • 3. Polymer pedestal
  • 4. Concrete slab or stabilised and levelled ground with load-bearing capacity exceeding 2 bars
  • 5. Geotextile

Installation on soil support

In this type of design, in order to prevent differential subsidence of the decking, all of the sleepers will rest on setting blocks ensuring the overall consistency of the load-bearing capacity compared to the soil. Apply a layer of sand or compressed and levelled gravel. Install mortar pedestals (cobblestones or breeze blocks) as supports for the sleepers (generally, 1 pedestal every 50 cm). Check levels and height. Fix the sleepers onto the pedestals (screws, pegs).

2. Cross-section on concrete pedestals

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  • 1. Concrete made with coarse aggregate, pedestal beds.
  • 2. Bottom of the excavation for each pedestal.
  • 3. Prefab pedestal or poured in situ.
  • 4. Joist (according to 3-1-3).
  • 5. Deck boards
  • 6. Natural terrain.

Additional recommendations

Off-cut rate

Optimise your efficiency with an accurate layout. For random installation, plan between 5% and 10% of extra surface area. For installation with 45° cuts, with curves or with regular joints plan between 15 % and 20 % of extra surface area.

Wood storage

It is recommended to stock wood boards, preferably well strapped and battened between each row, indoors, in a hangar and well-ventilated space or outdoors under a tarpaulin. Direct sun exposure is to be avoided.

Straightening a board using a spring

When the board exhibits major lengthwise warping (spring), mechanical straightening is to be avoided. Once the board straightened, the woods’ internal stresses are such that the screws will only temporarily suppress the warping. It is highly recommended to make a lengthwise incision on the board’s underside, to release any stress. It is also recommended to reduce its length.

Adjustment of screw gun

It is important to adjust the screw gun’s torque and rotation speed to avoid breaking or damaging screws during installation. It is recommended to carry out a prior test on an off-cut.

Concrete support incline

In order to allow for optimal water drainage on the slab surface, the latter must be built with a minimum 1.5% incline with the highest point on the side of the house.

Checklist for the construction of a wooden deck:

Designing and building a quality wooden deck requires compliance with a minimum number of rules:

STANDARDS

  • 1. Respecting regulations and requirements defined in the standards in force, notably DTU 51.4 Outdoor wooden decks, NF B54-040 Wooden outdoor deck boards, NF EN 335-1 and NF EN 335-2 Durability of wood and wood derivatives, NF EN338 Structure wood – Resistance classes , NF B 52-001-1 Visual classification for use in structures of French coniferous and broad-leaved sawn wood.

TERRAIN

  • 2. Preparing the terrain in accordance with the nature of the ground and the chosen design, paying attention to perfect water evacuation (through drainage or seepage).

STRUCTURE

  • 3. Building a rigid, stable and robust structure. Double structure is recommended, particularly in the case of a deck on natural terrain.
  • 4. Levelling the structure.
  • 5. Fixing the sleepers on adapted supports (joists, concrete pedestals, polymer pedestals, etc.). Sleepers placed on pedestals are not sufficient.
  • 6. Isolating sleepers from the ground or concrete support using plastic wedges, rubber buffers or inalterable bituminous membranes (preferably with polyester reinforcement, of the NÖVLEK type®). As a result water drainage will also improve.
  • 7. Respecting sufficient spacing between deck and ground to allow for adequate ventilation of the deck.

SLEEPERS

  • 8. Using sleepers in stable Class 4 wood.
  • 9. Installing an inalterable bituminous strip (preferably with polyester reinforcement, of NÖVLEK type®) on the top side of the sleepers to protect against moisture, improve water drainage, create a watertight joint between sleepers and limit the risk of the boards warping.
  • 10. Respecting a sleeper width of at least 50 mm for the central part of the board and 70 mm for both ends of the boards aligned end to end, except in case of double sleepers. This solution is recommended, taking care to space the ends of the boards in order to ensure the drainage of the edge boards.
  • 11. Respecting a maximum centre-to-centre distance of 40 to 45 cm between sleepers depending on the type of decking, on the wood making it up and the thickness of the boards.

BOARDS

 
  • 12. Using boards of equal width in Class 4 wood, without sapwood and whose species is said to be stable. The moisture content of the boards will not exceed 18% at the time of installation, ideally it will be close to the average moisture content on the construction site. The boards must have a well-kept finish and surface.
  • 13. Dimension the boards by respecting the maximum width/thickness ratio of the species, defined in the NF B54-40 standard. Going for narrower boards (maximum 120 mm) considerably reduces the risk of warping, bending and the degree of swelling and shrinking.

FASTENERS

 
  • 14. Hidden fasteners: Always use a fastening system whose compliance has been assessed by a independent certified organisation (FCBA, CSTB, CSTC, CTIB, UBAtc) and that has been the subject of complete design and installation specifications, such as the HardWood Clip fasteners® that have been assessed by the FCBA and the CTIB/UBAtc and come with specifications. Installation instructions must be scrupulously respected.
  • 15. Visible screws: use superior quality screws in stainless steel A2 or A4, correctly dimensioned, with countersunk or raised head, with double thread or partial thread and Torx SPAX T-Star plus.

INSTALLATION

 
  • 16. Align the boards by ensuring to guarantee constant spacing between them using rigid wedges of equal thickness.
  • 17. Be very precise in determining the adequate spacing to be planned between boards during installation (neither too much nor too little), depending on their species, moisture content (to be measured using a moisture meter with pins), their width and extreme climate conditions on site.Hidden HardWood Clip fasteners® are available in several sizes, thus guaranteeing adequate and constant spacing.
  • 18. Major lengthwise warping of a board (spring): avoid forcing the board by straightening it using a mechanical system. Internal stresses are such that the screws will only contain the warping temporarily. Make a lengthwise incision on the board’s underside, to release any stress.
  • 19. Respect a lateral board overhang from 10 to 15 mm maximum and longitudinal between 20 mm and maximum 3 times the thickness of the board.
  • 20. For raised screws only: a) Perfectly align the screws using a chalk line – b) Pre-drill and carefully countersink the holes in the boards – c) Carefully screw the screws ensuring that the top of all heads are at the same level, i.e., slightly under the level of the top side of the boards.
  • 21. Plan access hatches for sites that have to remain accessible and/or to clean under the decking using a water hose or equivalent.

FINISH

 
  • 22. Hide the deck’s structure by means of side boards, making sure to guarantee minimum ventilation to prevent the containment of moisture.