How do you lay a wooden deck?

With the fine weather, laying a wooden deck is the dream and the project of many people in France. It is the ideal way to enjoy the sun alone, with family or with friends, offering an outdoor space that is practical, attractive and comfortable.

To be a success, however, the laying of a wooden deck has to follow certain steps and observe certain precautions, at the risk of an unstable or unusable deck. Type of wood, dimensions of the deck, structure, laying joists or boards, and so on: there are many parameters to take into account.

Would you like to know how to build a wooden deck so you can enjoy the summer season? WoodPartners gives you a detailed guide to laying your wooden deck perfectly, whatever the nature of your ground: soil, lawn, sand, concrete and so on:

Which type of wood should you choose for a deck?
Defining the dimensions of your wooden deck
Which load-bearing structure should you choose according to the ground? Soil, concrete, lawn and so on,
Method no. 1: laying a deck on adjustable pedestals for stabilised ground,
Method no. 2: laying a suspended wooden deck (on piles) for sloping ground,
Method no. 3: laying a wooden deck on ground beams or concrete pads for any type of ground,
Method no. 4: laying a deck on stabilised ground,
Sizing and laying the joists of a wooden deck,
Laying the boards of a wooden deck,
Other questions about laying a wooden deck.

Which type of wood should you choose for a deck?

The choice of the type of wood for a deck depends on what you want: density of the wood, durability, appearance, ease of maintenance. It is recommended, however, that you choose class 4 or 5 woods in order to install a deck that is durable and resistant to the elements (moisture, UV, insects, fungi and so on).

Three types of wood can be considered for building a wooden deck:

-European solid woods:Douglas fir, larch, fir, black locust, and many others.

-Exotic woods : Ipe, itauba, padauk, and many others.

-Composite woods : a mixture of wood and resin.

Each type of wood has its advantages, so it is your aesthetic criteria (colours, smooth, grooved or rounded surface) and the performance of the wood that will help you make your choice.

For a deck in intensive use, which will withstand heavy traffic and the weather, you will need to look towards class 4 woods as a minimum. And if your deck is by the sea and in contact with sea water, you will then have to choose a class 5 wood.

Take care, then, to choose class 4 or 5 woods, or a composite wood, which easily withstands the weather for nearly 20 years. After that, it is the price of the wood for the deck and the appearance that will help you decide.

Defining the dimensions of your wooden deck

Before the work begins, take the time to define the dimensions of your wooden deck. It is important to take several variables into account according to your project: where the deck starts and finishes, the arrangement of panels or steps, the orientation of the boards and so on.

Also take the time to calculate the height of your deck if you want it to line up with your patio door. In that case, you will have to take into account the thickness of the boards and of the joists, as well as any pedestals to be buried or stabilising supports.

Which load-bearing structure should you choose according to the ground? Soil, concrete, lawn and so on

Building a deck on bare ground calls for particular attention to the load-bearing structure. It is imperative that the ground be stable, so as to prevent any future damage linked to movement of the ground.

Four solutions are open to you for assembling your deck on bare ground, each with its advantages in terms of ease of installation, durability or adjustability according to your type of ground:

-Adjustable PVC pedestals, a solution adaptable to any type of stable, flat ground (soil, sand, gravel, concrete),

-Suspension or the use of piles, suited to sloping ground of any type (soil, sand, gravel, concrete, clay),

-Ground beams or concrete pads, suited to stabilised as well as unstabilised ground (soil, sand, lawn, gravel),

-Stabilising the ground.

Method no. 1: laying a deck on adjustable pedestals for stabilised ground

This way of installing a wooden deck is simple and quick. The pedestals can adapt to different types of flat, stabilised surface (concrete slabs or stabilising slabs, for example) with a levelling margin of around 1%.

This type of assembly also makes it easier to dismantle the deck on pedestals without leaving traces, and makes it possible to install electrical or water supply structures. Finally, it lets the air circulate, preventing the wood from being damaged by water.

How do you lay a wooden deck on adjustable PVC pedestals?

Laying a wooden deck on adjustable PVC pedestals is extremely simple. On your stabilised ground, install your adjustable PVC pedestals at regular intervals. It is recommended not to exceed a spacing of 60 cm between the pedestals when your joists rest on the thinner part (thickness of about 45 cm), or 1 m when they rest on the thicker part (edge of about 70 cm).

Method no. 2: laying a suspended wooden deck (on piles) for sloping ground

Building a suspended wooden deck for sloping ground is the solution suited to soft or sloping ground. It requires the installation of a wooden wall plate on the façade or the use of metal "U" supports to hold the joists (support beam).

How do you lay a suspended deck?

At the far end of your future suspended wooden deck, plan for a concrete foundation ground beam (a kind of beam in reinforced or prestressed concrete) or various concrete foundation pads to be buried.

These elements will serve to bear the load of the piles of your deck, thanks to the installation of a post base plate in galvanised steel (or anchors) to be screwed or cast into the concrete. These supports hold the posts off the ground, away from the moisture that can travel up by capillary action.

If you want to avoid this installation, you can use ground screws that screw directly deep into the ground and that allow the fixing of an adjustable joist support. These structures are height-adjustable and support wooden decks of up to 900 kg when they are buried to the maximum.

Method no. 3: laying a wooden deck on ground beams or concrete pads for any type of ground

This type of deck laying requires concrete foundation ground beams, structural elements similar to beams whose thickness is between 15 and 20 cm wide by 20 to 30 cm deep. These ground beams will support all the joists of the wooden deck.

How do you lay a wooden deck on ground beams?

The ground beams must be installed at right angles to the joists, in positions dug into the ground at regular intervals. If you do not want to install adjustable pedestals afterwards, take care to keep a slope of 1% (that is 1 cm/m) towards the outside so that the water can drain away.

How do you lay a wooden deck on concrete pads?

The concrete pads have to be individually adjusted for height. To build a wooden deck on pads, dig a position suited to the pad in which you will pour a small volume of concrete. Place the pad, making sure it is at the right height and keeping a slope of 1%.

You can thus line up your pads with a patio door, for example. Note also that these pads are installed at the same time as the joists, so as to be able to adapt to any defects in the wood.

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Method no. 4: laying a deck on stabilised ground

In this type of installation, the ground serves as the support for your deck. If it moves, the whole structure of your deck moves, creating risks in terms of stability and durability. Before building your deck, it is therefore compulsory to stabilise your ground.

How do you stabilise your ground for laying a wooden deck?

To build a wooden deck on stabilised soft ground, start with preparing the ground. Excavating the soil is a compulsory first step. We recommend a minimum of 15 to 25 cm depending on the type of ground and its potential for movement (gravel, sand or rock), up to 35 cm if your ground is particularly soft. Then compact the surface, taking care to create a slope of 2% (2 cm/m).

Even on ground that has already been levelled, we advise you to wait 1 or 2 years before installing a wooden deck, a period during which the soil moves and settles before stabilising.

Then:

-Install a geotextile membrane (which prevents weeds from growing back and water from sitting), covered with a 10 to 15 cm layer of granular fill with a grading of 0/31.5 mm (a mixture of sand and fine gravel available from specialist stores),

-Compact again using a tamper or a plate compactor, keeping a slope of 2%,

-Install screed rails or levelling rails standing 3 cm proud of the granular fill,

-Lay sand (or gravel) following the rails installed beforehand until they are covered. You can then remove them and fill in the spaces left empty,

-Install your concrete slabs or pads using a rubber mallet and a spirit level. You can also create a concrete edging by digging the ground 30 cm wide and 20 cm deep so as to prevent the sand from escaping outwards. You can reinforce the edging with rebar at least 3 cm from the edge to avoid frost damage.

Sizing and laying the joists of a wooden deck

What dimensions for the joists of a deck on pedestals?

The joists, which serve as the base for fixing the boards of a wooden deck, must have a width of at least between 45 and 50 mm and a height of between 40 and 65 mm.

French standards require a joist height at least equal to 1.5 times the thickness of the wooden deck boards and to 6 times the diameter of the screws fixing the wooden boards + 10 mm.

Which materials should you choose for the joists of a deck in wooden boards?

For the joists of a wooden deck, choose class 4 woods such as pine (pressure treated), black locust or exotic woods such as ipe and cumaru, which are more expensive but more durable (service life of about 20 years).

These woods are rot-proof, and therefore resist fresh water. To keep their colour, we advise you to wash these wood species 1 to 2 times a year with soapy water.

What distance between the joists of a deck in wooden boards on pedestals?

The centre spacing of the supports for the wooden boards (the distance between each joist) depends on the type of wood chosen for building the deck (the nature and strength of the wood) and on the dimensions of the decking boards. Allow on average 40 to 50 cm between two joists, with a margin for adjustment.

For example, for hardwood boards of 28*140 mm section, the recommended centre spacing is 87 cm.

Beware: a centre spacing that is too large and unsuitable can be a source of danger because of the distortion of the wood (whose extent is called the "deflection").

How do you fix the joists?

To fix the joists of your wooden deck, two situations are possible:

-Laying the joists of a wooden deck on a concrete support : screw the joists into the concrete (using concrete screws or hammer-in anchors) with PVC shims, so as to avoid direct contact with the concrete and to line the joists up with each other;

-Laying the joists of a wooden deck on stabilised soft ground : fix the joists onto adjustable pedestals, preferably.

Laying the boards of a wooden deck

The stage of laying the wooden deck boards is crucial to the appearance and the durability of your project. You have the choice between two distinct methods: with or without visible screws.

If you opt for visible screws, follow the recommendations of DTU 51.4 by using 2 stainless steel screws at each intersection between boards and joists, placed 15 to 18 mm from the ends of the boards. Note that it is imperative to pre-drill exotic woods so as to avoid splitting them.

We also strongly recommend that you pre-drill softwoods (with a diameter 0.8 times the diameter under the head of a partially threaded screw, or of the thread of the screw for double-threaded screws) so as to make installation easier. Do not hesitate to adjust the torque of your screwdriver and to grease the tips of your screws to avoid breaking the heads of the stainless steel screws.

Finally, use spacers to maintain an even gap between the boards (7 mm for very dry woods down to 3 mm for pressure-treated woods) and the wall (10 to 15 mm).

If you prefer a finish without visible screws, plastic clips can be used to fix the boards from underneath.

Laying a wooden deck – every method for building a deck on bare ground