The right fastening system for decking boards: Visible screw connections and hidden fasteners compared

The fastening system is part of the technical planning

The fastening of decking boards should not be chosen solely based on the desired appearance. Decking material, dimensions, wood moisture or material behavior, substructure, location, and future usage must be considered together.

For wooden decking boards, both visible screw connections and concealed fastening systems are options, depending on the wood species and construction. With concealed systems in particular, however, it must be verified whether the respective manufacturer expressly approves the wood species, board dimensions, and substructure used. Some wood species are excluded by individual system manufacturers due to their deformation or swelling and shrinkage behavior. For WPC or NFC decking boards, there are no uniform installation specifications that apply across all products. In these cases, the current installation instructions and the system components approved by the manufacturer are decisive. Different formulations and manufacturing processes can lead to different requirements regarding the substructure, fastening, joints, and movement gaps.

Fastening of decking boards

Visible screw connection of wooden decking boards

With visible fastening, the decking boards are screwed to the substructure from above. The screw type, screw dimensions, and installation must be tailored to the wood species used and the substructure (wood or aluminum). Suitable stainless steel screws must be used for wooden decking boards. However, the designation "stainless steel" alone is not sufficient for selection. Wood species, location, and corrosion stress must be taken into account. Especially with woods containing tannins, unsuitable metal fasteners can lead to discoloration.

For particularly hard and heavy hardwoods as well as many modified woods, proper pre-drilling is important. The specifications of the screw manufacturer and, if applicable, the board manufacturer are decisive. A countersink for decking boards allows for pre-drilling while simultaneously creating a uniform countersink for the screw head. The screw-in depth as well as the specified torque or speed must also be observed. Improper processing can damage screws or lead to problems if the wooden boards move later due to changes in moisture.

Concealed fastening systems

With concealed fastening systems, the connection is made, depending on the system, for example at the underside of the decking board or via a side groove. As a result, there are no visible screw heads on the top side. However, the term "concealed" does not necessarily mean that the entire fastening system is invisible – individual components may remain visible through the joints. With some bottom-mounted systems, a structural gap is simultaneously created between the decking board and the substructure. In addition, there are systems whose fastening principle accounts for the material-related movement of the board. These properties, however, should not be universally applied to every concealed fastening system.

Fasteners, decking boards, and the substructure must be considered as a coordinated system. Therefore, only use components that the respective manufacturer has approved for the specific application. An example from our range is the DILA2/22 terrace connector. This fastening system can be used for all wooden decking boards we sell with a material thickness of 24-32 mm.

Fastening of side-grooved decking boards

For side-grooved boards, suitable terrace connectors or clips can be inserted into the groove and connected to the substructure. It is crucial that the connector, groove geometry, board material, and substructure are coordinated with each other and approved by the manufacturer. For wood, the fastening system must account for the material-related swelling and shrinkage movements. Therefore, a given clip is not automatically suitable for a board simply based on its dimensions or visually fitting seat.

This applies especially to WPC/NFC: use only the accessories approved for the respective board system. A WPC terrace clip from our range should therefore not be understood as a universal clip for any WPC profile. The specific system approval is always the deciding factor.

What factors determine the fastening system?

Decking material and material behavior

In the case of wood, factors such as wood species, wood moisture, and swelling, shrinkage, and deformation behavior influence the choice of the fastening system. Especially with concealed systems, the respective wood species must be approved by the system manufacturer. For modified woods and WPC/NFC, specifications for conventional solid wood decking boards should not be adopted without further consideration. Technical literature explicitly points to the special importance of manufacturer information for these materials. Regarding WPC/NFC, general statements on material behavior are not possible due to the various formulations and production processes.

Board thickness, board width, and wood moisture

Dimensions and, in the case of wood, particularly the installation moisture, affect the construction. A fastening system must therefore only be used within the dimensions intended by the manufacturer. The required substructure spacing also depends on board thickness and material. For modified woods and WPC/NFC, these should not be derived from general table values but determined based on manufacturer information.

Profile and groove geometry

A side groove alone does not mean that every terrace clip can be used. Profile and fastening system must match structurally. With rear-mounted systems, the respectively approved board dimensions and installation conditions must also be adhered to.

Substructure

The substructure is also part of the fastening system. Screws and connectors must be suitable for the substructure material used. For an aluminum substructure, for example, self-drilling or self-tapping screws intended for this purpose are required. The specific design depends on the aluminum profile and manufacturer specifications. You can find suitable substructure materials in our collection for terrace substructures.

Location and corrosion stress

The selection of screws should not be based solely on a designation such as A2 or A4. Decisive factors include, among others, the environmental conditions as well as the specific material and product approval.

Especially with woods containing tannins, attention must also be paid to material compatibility. Iron-containing or unsuitable fasteners can react with wood components and cause discoloration. You can find more information on this in our guide on removing gray-blue stains on decking boards.

Subsequent disassembly and replacement

Future accessibility should also be considered during selection. With concealed systems, replacing individual boards can require more effort depending on the structure. The seminar documentation explicitly mentions this as a possible difference compared to visible fastening.

Specific features of different decking boards

Solid wooden decking boards

For solid decking boards, a visible screw connection or a concealed system expressly approved for the wood species, dimensions, and substructure can be used.

In the case of a visible screw connection, the screw position, pre-drilling, countersinking, and screwing must be performed professionally. For hard and heavy hardwoods, special attention must be paid to the specifications for pre-drilling.

Douglas fir and larch boards

Douglas fir and larch can swell and shrink due to moisture changes; furthermore, cracking can occur. The fastening system must take these properties into account. A general recommendation for or against a concealed system cannot be derived from this – the concrete approval of the manufacturer is decisive.

Hardwood boards

For hard and heavy hardwoods, screw fastening in particular places increased demands on tools and pre-drilling. Here, too, it must be verified whether a concealed fastening system has been approved for the respective wood species. This can be limited, especially for woods prone to deformation.

Modified decking boards

Thermally or chemically modified woods exhibit different material properties compared to untreated solid wood. Fastening systems, substructure spacing, and processing must therefore be determined based on the respective manufacturer information. Technical literature also points out that thermally modified wood and WPC/NFC do not currently fall under the scope of the technical rule 02. For these products in particular, rules for conventional solid wood terraces should not be applied universally.

WPC decking boards

For WPC decking boards, the board, substructure, and fastening must be considered as a product-specific system. The installation instructions specify, among other things, the permissible substructure spacing, accessories, and required movement and edge gaps. WPC/NFC profiles change their dimensions under temperature influence, primarily in the longitudinal direction. Therefore, the gaps prescribed by the manufacturer at board ends, components, and longitudinal joints must be taken into account. Blanket values should not be cited across different products.

Do not set joint spacing for decking boards as a blanket value

For wooden decking boards, the required joint depends on, among other things, the wood species, board dimensions, and especially the wood moisture during installation. Due to seasonal moisture changes, the boards swell and shrink, which also changes the joint width. For WPC/NFC, on the other hand, the temperature-related movements specified by the manufacturer and the gaps provided for this purpose must also be taken into account. A uniform joint value for all wood and WPC boards would therefore not be technically sound.

A joint gauge for decking boards helps to evenly apply a previously technically determined dimension during installation. However, it does not determine which joint spacing is required for the respective decking board.

Closing joints after installation

If the technically required joint is to be closed subsequently, a terrace joint sealing tape can be used depending on the terrace system. It is important that the required joint width is first determined based on the boards used and the respective installation specifications. The joint tape does not replace this structurally required planning. You can find suitable terrace joint sealing tapes for various joint widths in our range.

Planning fastening material correctly

A universally valid consumption per square meter cannot be specified for every fastening system. The starting point should always be the consumption value or the manufacturer's planning specifications, as well as the actual number of support and fastening points.

• For complex areas, planning is therefore recommended
• based on the actual board layout and substructure.
• Initial and end areas,
• board joints,
• stairs,
• fascia boards or other structural details
• must be considered separately.

Recommended order for planning

1. Determine decking board including material, profile, and dimensions.
2. Check field of application, location, and future usage.
3. Select suitable substructure and define its required spacing.
4. Coordinate fastening system with the specific board and substructure and check manufacturer approvals.
5. Plan joints, movement gaps, edge connections, and drainage according to material and system.
6. Select suitable screws or system components based on wood species, substructure, and environmental conditions.
7. Determine material requirements based on the actual structure and the current installation instructions.

Common errors in decking board fastening

• Typical errors include selecting a fastening system based solely on appearance,
• the use of unapproved system components,
• a clip that does not fit the groove geometry,
• unsuitable screws for the wood species or substructure,
• lack of pre-drilling for corresponding materials,
• an unsuitable substructure,
• as well as blanket joint spacing.
• For WPC/NFC, it is particularly problematic
• to transfer installation specifications or accessories
• from one product to another.

Conclusion: The fastening must fit the entire terrace

The suitable fastening for decking boards results from the interaction of board material, dimensions, substructure, location, and fastening system. For wood, depending on the product, a visible or concealed fastening may be an option. Concealed fastened boards do not have screw heads on the usable surface; however, parts of the system may remain visible within the joints. For WPC/NFC and modified materials, blanket installation recommendations should be avoided. In these cases, the current product information, installation instructions, and components approved by the manufacturer are decisive.

An overview of terrace connectors, spacers, installation tools, and other accessories for wooden decking boards can be found in the Gunreben online shop. You can find the matching boards in our collection for wooden decking boards.