Decking expertise

Rules & tips for perfect decking

Your decking project: Actually quite simple when you know how. If you’ve already built decking, or you’re a professional who builds decking for others, you will probably already be familiar with most of the basic technical tips and tricks for a perfect result.

Nevertheless, it’s always a good idea to refresh your memory occasionally. You could also use these expert tips as a checklist or as the proverbial knots in your handkerchief! Have you thought of everything and not missed anything? By sticking to this list you will 


At a glance

-          Never fit wood to wood − use SPAX Air

-          Prevent damp from accumulating − use SPAX Pads

-          Check that the intended height of your decking is feasible

-          Use stainless steel screws with suitable corrosion resistance

-          Pre-drill boards and the base frame to the full screw depth

-          Use two screws for each attachment point

-          Fit two braces under longitudinal joints

-          Adapt the board spacing in line with the wood’s moisture content at the time of installation

-          Lay boards with flutes inclined in the water runoff direction

-          Use wood with the same resistance for both base frame and boards

-          Reinforce the base frame and embed if necessary

Good planning for long-lived decking

Keep your eyes peeled when buying boards – and not only then. If you want your decking to have a long life, always take a second look, not just when choosing your wood, but also when considering the location, installation, and care and maintenance. To a certain extent, the durability and service life of decking are achieved through a combination of technology and talent.

Durability. A class society with many levels

The first clue to the resistance of untreated wood to the effects of fungi, bacterial rot and other micro-organisms is provided by the five durability classes according to DIN EN 350. This classification tells us whether boards are made of a type of wood with higher or lower resistance. However, these grades refer only to the heartwood, not the sapwood. Sapwood is the living, as yet unhardened outer part of the trunk. In all types of wood, the sapwood has the lowest durability.

Go slow for the best results

It isn’t just bulk density and specific weight that govern the durability of a type of wood, but also the sum of various factors such as region of provenance and wood constituents. Rule of thumb: Slow-growing wood mostly has better durability than fast-growing wood.

Constant dripping hollows stone … and rapidly ages wood

Logically enough, a disposition to biological decay is not the only factor influencing the life expectancy of your decking. Service life depends to a great degree on where and to what extent the decking is exposed to the forces of nature. Rain, icy cold, scorching hot sun and ground humidity can place this natural material under stress and accelerate its ageing process. Most stress factors can be minimised or eliminated through structural measures, however.

SPAX Air. Inclined fluting. Height relative to the lawn and border

Probably the most successful anti-ageing strategy for decking is to plan your construction cleverly and with foresight. Decking exposed to extreme weather, in particular, should not be 100% horizontal, but installed at a slight inclination with board flutes running in the direction of water runoff. The use of spacers such as the SPAX Air is also recommended. Another bonus is the higher base frame relative to your flowerbed and lawn.


Reduce moisture sources with ventilation from below

On the other hand, wet leaves and all kinds of soil and dirt are not at all welcome on decking. And neither are permanently damp flowerpots and plant containers, under which moisture tends to gather. Sources of damp such as these must be removed to prevent the onset of soft rot. Flowerpots and containers must be provided with good ventilation from beneath using suitable accessories. Parasol bases should be banned from the decking at wet and cold times of year. Natural wood really doesn’t like continuous damp!

Be smart – choose the right spot

If it’s at all possible to position the decking so that it isn’t exposed to the prevailing elements, or erected in a particularly damp or shady spot, that will also be beneficial. Then, your decking will age beautifully and thank you by lasting a very long time!

Reliably stop damp accumulating – here’s how!

Don’t give damp a chance to accumulate! The best way is to keep water at bay before it can penetrate the wood. Controlled runoff is also important. If you stick to this basic rule when building your base frame, you will effectively keep water and moisture in check.

Keep your distance

The contact surfaces of the wood merit special attention here. To ensure the base frame is not in direct contact with damp earth or stones, it is mounted on evenly spaced concrete platforms. Other kinds of support are possible, but only if they don’t jeopardise stability!

It’s better with rubber

A suitable base of non-absorbent material is installed between the wooden base frame and the concrete slab. SPAX recommends SPAX Pads of robust recycled rubber. These rubber pads permanently protect the wood above from penetrating moisture. At the same time, they help to compensate for minor unevenness.  The pads enable ideal water drainage.  Wood rot literally has the rug pulled from under its feet!


No more than 50 mm wide

Long-term observations show that the side of longitudinal base frame joists facing the boards must not be wider than 50 mm. With larger cross sections, such as 60 x 100 mm, the 60 mm wide edge facing the boards should be tapered to reduce the contact surface to 50 mm.

Live longer with SPAX Tape

Before board installation, you should seal the longitudinal joists of the base frame with a moisture barrier of self-adhesive SPAX Tape. This adhesive tape is UV-resistant and can be removed without problem. For the less durable Douglas fir or larch, this protective tape is virtually indispensable, and it’s also recommended for hardwoods. This is a simple but effective step for preventing the risk of damp at the contact points between the base frame and boards, and therefore extending its service life.



Don’t do things by halves when building the base frame

To ensure that your decking has sufficient load-bearing properties, you must observe a minimum spacing between the joists of the base frame. This spacing is 50 cm for deck boards 25 mm thick and 40–45 cm for boards 21 mm thick.

For boards made of wood with a spiral grain, such as cumaru or bongossi, not maintaining the minimum distance can easily lead to unwanted warpage and, with a bit of bad luck, trip hazards.

For NFC/WPC deck boards and modified wood, please pay attention to the manufacturer’s instructions.

Paving stones as the foundation for success

As mentioned above, the base frame is always mounted on load-distributing pedestals, never directly in the soil or gravel layer. Concrete paving slabs positioned on the gravel bed are a tried and tested solution here.

Upstanding members

Square timber approximately 42 x 29 mm thick is ideal for the base frame. This is mounted side-on. Don’t forget to use SPAX Pads for moisture protection between the square timber and the paving slab!

Boards and base frame: Use wood with the same durability!

With hardwoods such as bangkirai and massaranduba, the base frame should be made of the same species of wood, due to its specific torsion and shrinkage characteristics.  Material shortage? If the right type of wood is not available, a hardwood with similar durability may be used as an alternative.

This rule of thumb also applies to decking with boards of Douglas fir, larch and other less hard woods. The type of wood used for the base frame depends on the wood of the deck boards – it must be identical or at least as similar as possible!

Structural measures to protect the base frame

So, you’re flirting with the idea of deck boards from a less resistant type of wood, such as Douglas fir or larch? In this case, you can harness hidden reserves of durability by “tuning” the base frame. Your retrofit kit for achieving a higher durability class: the SPAX Air spacer.

Upgrade the durability class with SPAX Air

The SPAX Air spacer reduces the accumulation of damp between deck boards and the base frame. This structural upgrade greatly improves durability and bumps the classification up from durability class 3.2 to class 3.1.

When durability is optimised, this also enables larch and Douglas fir to be used in balconies and other elevated structures, for which FR02 of the BDZ1 requires a minimum durability class of 3.1.


Expert board installation for great results

Once decking installation is complete, there’s no going back. The right preparation is paramount.

Before laying your deck boards, cut off both ends at right angles, then taper or round them.

Remove paraffin, protect head ends

The paraffin sealant applied to the ends of boards as splice protection by many manufacturers in the country of origin should always be removed. Otherwise, the paraffin may liquefy when exposed to sunlight, leading to unwanted discoloration. If necessary, treat board ends with a suitable end-grain sealant or a special wax end sealer.

Calculate the right laying span

Before drawing your installation diagram, check the moisture content of your deck boards. Otherwise, you won’t be able to guarantee the correct board spacing. An average moisture content of approximately 16–18% is recommended for ideal deck board installation. If boards are drier than this, they need to be more widely spaced.

So you don’t have a wood moisture meter? In this case, randomly select five boards and measure their width. The mean value – i.e. the sum total of your measurements divided by 5 – is then the actual dimension for your installation plan.

Desired gap: 5 millimetres

Proceed as follows with your installation diagram: From the delivery information (sales receipt/delivery slip), establish the specified or nominal dimension, i.e. 145 mm, for example. If the desired board spacing is 5 mm, the laying span is 150 mm in total.

If the actual and nominal dimensions differ, change the actual dimension to the specified laying span by adapting the board spacing – 150 mm in the example calculation.

In other words, for an average measured size of, say, 139 mm, the board spacing must be increased to 11 mm to achieve the laying span of 150 mm: 139 + 11 = 150.

SPAX Spacers make the right board spacing simple

For faster work and optimum control, use the SPAX Spacer. This enables consistent and uniform board spacing throughout.

To ensure the last board has a neat end, keep measuring the overall width of the already laid boards and adapt the board spacing where necessary.


On the straight and narrow

Bent or warped boards ruin the uniformly linear appearance – and that is the alpha and omega of perfect decking! With SPAX Kaiman pro, you no longer need to automatically throw away crooked boards: SPAX Kaiman pro effectively straightens most crooked boards, enabling you to achieve relatively uniform spacing.


Shrinkage is inevitable

Wood is a hygroscopic material, which means it naturally takes on water from its environment and gives it off again. The extent of this depends on an array of factors – particularly the surrounding humidity. The more moisture the wood absorbs, the more it increases in volume. When its moisture content decreases, so does its volume. Wood swelling and shrinkage take place predominantly across the grain and generally account for between 5% and 10% of the material width and thickness.

Better to do the sums twice than the entire installation

If you don’t take the difference between the actual and specified dimensions of boards into account at the time of installation and factor this into your calculations, you may have to completely dismantle and re-lay your decking. No screws in the world could make up for the huge shear forces caused by hygroscopic changes in the wood’s volume! In the worst case, the resulting deformation of the screws will make disassembly and re-assembly truly difficult.

Maintain a healthy distance

Leave a gap of at least 10 mm from the house wall and other neighbouring structures. 15 or 20 mm is ideal. This makes it easier to paint and clean rendered and wooden façades, for example. At the same time, this distance ensures good ventilation below the deck surface.

Take care with the distance from façades

If the structure is an exterior wall, the minimum distance increases to 300 mm. An additional layer of gravel at least 150 mm wide is recommended to better protect the façade from splash water. For new buildings and modernisations, these benchmarks serve as a binding standard for carpentry, in the form of Regulation 01 and DIN 68800-2.


Screwing deck boards onto the base frame

crewing deck boards is a chapter in its own right. You need not only the right stainless steel, but also the right screw-in depth, screw diameter, pre-drilling and staggering. Do you wish your screws to be visible or concealed? So many questions, so many answers! However you decide to install and secure your decking – SPAX always has the right made-to-measure, technically perfect screw for you.

A little bit of screw expertise

The steel has it – stainless steel is a must for decking. The type of wood dictates what kind of stainless steel is most suitable. Screw experts distinguish between martensitic and austenitic steel. Their special features are specified in the DIN EN ISO 3506 standard. Why be so fussy?

Woods containing tannin need A2 stainless steel

What matters here is the specific corrosion resistance to aggressive chemicals and weather. Such chemicals can also include natural tannic acids, which occur in oak, bangkirai and garapa, for instance.

Deck boards with visible screws

A deck board of normal thickness in decking with average usage requires screws with a diameter of 5 mm. Multiply this by at least 6 to calculate the screw-in depth in the base frame, in other words 30 mm in this case. For thicker boards and higher stress, e.g. due to larger numbers of people using the decking, this figure increases to a diameter of 6 mm and a screw-in depth of 36 mm.

Popular screw lengths

Based on experience in decking installation, the following screw lengths are recommended:

Boards up to 21 mm thick                                                  =         screw length 60 mm

Boards from 22 to 28 mm thick                                          =         screw length 70 mm

Boards up to 28 mm thick or with additional spacer          =         screw length 80 mm


Pre-drilled for easier screw fastening

Hard or heavy woods from deciduous trees, in particular, always require pre-drilling. If you skip pre-drilling, gaps and cracks may form. This pre-drilling rule also applies to thermowoods, which have similar material properties to hardwoods.

Ideal for pre-drilling: the SPAX step drill 4

Pre-drilling down to the depth of the full screw length is also recommended for the base frame. The SPAX step drill 4 is a handy practical tool for this job. The special drill bit is available in different lengths. The SPAX step drill 4 makes sure the desired drilling depth is reached without splitting or cracking the wood. The risk of splitting is of course especially high at the ends of the boards.

Even if no mishaps occur during installation, boards may still crack later as they dry in the sun, which is why pre-drilling is always best. Pre-drilling may take a bit of time, but it helps to protect the wood and means you’re sure to avoid unwanted surprises.


Never insert screws too close to the board end!

To guarantee optimum stability, maintain a distance of 50 mm minimum and 150 mm maximum between the last attachment point and the board end. A brace screwed under the board ends brings large overhangs – and the material behaviour of wood with a spiral grain, such as cumaru – under control. This brace prevents the overhanging ends from getting out of kilter, twisting and creating trip hazards.

Two are better than one

Deck boards more than 80 mm wide must be secured to the base frame with two adjacent screws.

The distance from the board edges at the sides should be 20 mm minimum and 30 mm maximum. For a board 145 mm wide, the ideal distance between the two screws is 100 mm. This distance minimises the risk of deck boards curling. The curling effect – the bulging of the board edges at the sides – is not just visually unattractive, people can also easily trip over the raised edges.

Spoiled for choice

An enormous variety of screws are available for installing deck boards. We already discussed the material width of stainless steel in the screw expertise section. However, it isn’t just the quality of the stainless steel that influences the resulting quality of your decking – so does the decking screw’s technical design.




Who would want their decking to advertise every step with creaking or rattling? Our SPAX solution to this problem is a combination of a screw section at the front and a locating thread underneath the head. The front screw section draws the board powerfully onto the base frame. The locating thread takes the strain off the screw head on the one hand, and holds the board in position on the other.

When everything is securely fixed in its place despite summer heat and a shrinking wood volume, nothing can move or be noisy. SPAX turns the volume down: creaking and rattling are greatly reduced.


Countersinking is the answer

With visible screws, for reasons of appearance we recommend the SPAX decking screw with a small head.  Insert screws so that they are at least flush with the surface. Better still, countersink them to a depth of 1 to 1.5 mm.

For surface-treated boards, the countersink should not exceed 1 mm. Otherwise, the maximum depth is 2 mm. To achieve these precise depths, please use the SPAX Drive Stop for electric screwdrivers.

Precise, not any old how: SPAX Drive Stop

Fine decking is not the same as a Euro pallet! The SPAX Drive Stop lets you set the depth of the countersink precisely, so that your results satisfy the most exacting aesthetic demands for uniformity and precision.


Procedure for special cases

There are exceptions to every rule and a possible resolution to every tricky situation. Thanks to our research and development and our close collaboration with woodworkers and industry, SPAX has the right screw and the right method for most special cases.

Aluminium – a special case

Aluminium is chiefly used if a wooden base frame is impossible because the height of the decking is too low. There are mostly architectural reasons for this.

The SPAX aluminium screw is ideal for the requirements of aluminium base frames. It features a sturdy steel double-start thread and a special drill point for material up to 4 mm thick. There is no need to pre-drill the aluminium, despite the resulting strong retention force. However, this retention force comes up against its physical limits where deck boards with a strong spiral grain are concerned. Consequently, SPAX aluminium screws are not approved for cumaru deck boards, for example.


Turn necessity into a virtue

As well as deck boards laid with a visible join, traders also offer boards with a groove. Once installed, they have a different appearance and they are also laid differently. The screws may be inserted in the surface or, alternatively, in the groove. Both options come with advantages and disadvantages.

Screws in the surface

Inserting screws in the surface is more demanding because it requires a great deal of precision. Minor errors are punished immediately by an unsatisfactory appearance. Ultimately, you need to insert all screws exactly and cleanly to a uniform depth and align them with equal precision. However, this can provide an attractive look if combined with the required degree of accuracy.

Screws in the groove

When inserting screws in the groove, the screw heads are less obvious and screw heads that project slightly have less impact, both visually and mechanically.

That’s why screwing in the groove is the state of the art for jetties, bridges and similar structures. With these structures, it’s also a good idea to think ahead about future care and maintenance. To prevent complaints, the corrosion resistance grade of stainless steel screws must be compatible with the substances used for care and maintenance.

For construction projects of this kind, SPAX has developed the special SPAX boardwalk screw. This screw is designed for boards 40 mm or more thick.

The SPAX boardwalk screw copes very well with both fluted and smooth deck boards. Fluted and smooth surfaces must be worked with extra precision and care must be taken to avoid cracking during drilling and screwing.

Line them up

For demanding walkway and jetty building projects, perfectly aligned screws are an absolute must. Perfect alignment means that the maximum deviation from the resulting straight line of all points does not exceed plus / minus 5 mm over a length of 2 m. There are visual and structural reasons for this strict alignment rule.

Screws used in projects that have to conform to building regulations, such as jetties, must be suitable and approved for both coniferous and deciduous wood in accordance with DIN EN 1995-1.

Under cover

Concealed board fastening is as secretive as a hidden investigation: it should always go unnoticed!

Concealed screws and the use of special fastening systems produce an especially attractive appearance. The decision regarding fastening should be made in consideration of the type of wood and boards.

System solutions

Does the concealed fastening use a system solution? If it does, only install modules from this system, not from others. With system solutions, you must also follow the manufacturer’s installation instructions rather than any other building tips.

This conformity requirement also covers the approval of types of wood and other modifications. This is because systems can only work in their system. If you don’t comply with the system requirements, you will lose all your warranty entitlements.

Two birds with one stone

SPAX Stick pro is the concealed – and therefore almost invisible – fastening tool in SPAX quality. The SPAX Stick pro is made of high-tech plastic and can be used on base frames of both wood and aluminium, depending on the screws making the connection.

An unbeatable advantage: The SPAX Stick pro combines concealed fastening with protection of the wood. This protection is achieved by inserting an extra gap between the deck boards and the base frame. Since water runs off faster and ventilation is improved, drying is accelerated. This dramatically increases the service life and therefore the economic efficiency of the decking.

Since this plastic connection tool is not yet standardised, you need to check whether building permission or comparable usability certification is required.


Carpentry Regulation 02; Balconies and Decking. Publisher: Holzbau DEUTSCHLAND – BUND DEUTSCHER ZIMMERMEISTER (Association of German Master Carpenters)