Can screws be driven directly into a composite decking board?

How is a composite decking board normally fixed?

Most WPC decking systems use concealed fixings. The decking boards have specially shaped side edges that accommodate dedicated clips. The clip sits between two decking boards and is fixed to the joist below. This means that no rows of screw heads are visible on the surface of the finished terrace. This solution is important for more than just its appearance. The clip system helps maintain even gaps between the decking boards, and appropriately designed fixings allow for changes in the material's dimensions. That is why composite decking board installation should be considered as a complete system comprising decking boards, joists, clips, screws and finishing components. 

Why do composite decking boards need room to move?

WPC, or wood-plastic composite, is a material containing wood and polymer components. It behaves differently from a traditional wooden decking board. Changes in length caused by temperature are particularly important. A terrace may be heated by intense sunlight and then cool down in the evening or during rainfall. Temperature differences are even greater over the course of a year. For this reason, the manufacturer specifies, among other things, the required expansion gaps at the ends of decking boards, next to walls and around other fixed elements. If a WPC decking board is rigidly fixed at arbitrary points, its ability to move naturally may be restricted. 

Can you screw a WPC decking board directly to a joist?

You should not assume that, just because a screw can be driven in, this is the correct way to fix the board. If the manufacturer specifies installation using concealed clips, this is the method that should be used. Driving ordinary screws in from above without checking the instructions may cause localised stress, detract from the appearance and, in some cases, damage the surface. What is more, different composite decking boards may have completely different constructions. The market includes solid and hollow profiles, decking boards with different types of coating, and products with different geometries. There is therefore no single direct-screwing method that is suitable for all WPC products. 

When are screws used when installing composite decking?

Screws certainly are used when building a terrace. However, that does not mean they have to go through the surface of every decking board. In a typical system, a screw may be used to fix a clip to a joist. In this case, the fixing remains practically invisible once the next decking board has been laid. Screws may also be used with starter clips, finishing profiles or other system components. Some manufacturers may also specify particular points for fixing a decking board directly. If so, you should use exactly the type of screw, hole preparation method and fixing location given in the instructions. 

Does a composite decking board need to be pre-drilled before screwing?

If the specific system permits direct fixing with screws, creating a pilot hole is often important. However, you should not assume there is a universal hole diameter that suits all decking boards. Pre-drilling may reduce the risk of localised damage of the material and make it easier to drive the screw in under control. Particular care should be taken when working close to the ends and edges of the board. The manufacturer's instructions remain paramount. If they require pre-drilling, skipping this step simply to speed up composite decking installation is not a good idea.

Which screws should you choose for composite decking?

There is no single universal screw model suitable for all WPC structures. You need to consider both the decking board and the joist material. Fixing to a composite joist may require a different solution from a structure made of aluminium profiles. In outdoor conditions, the corrosion resistance of the fixing components is also important. The type of steel, coating or screw material should be selected to suit the system and the environment in which it will be used. That is why it is best to use screws for composite decking boards and fixing clips specified by the manufacturer of the system, rather than choosing random screws available in the workshop. 

Why are fixing clips used so often?

Fixing clips solve several problems at once. Firstly, they allow you to create an attractive surface without visible rows of screws. Secondly, they help maintain even gaps between the decking boards. This is important for proper water drainage and for the system to work as designed. Thirdly, they are designed to work with the decking board profile and joist. As a result, fixing composite decking boards is more consistent than determining each screw position individually. 

A screw should not restrict expansion.

This is one of the most important reasons why randomly screwing WPC directly into place can cause problems. Imagine a long decking board screwed very rigidly at many points. As the temperature rises, the material changes dimensions, but the fixings limit its ability to move. The resulting stresses have to be released elsewhere. That is why both expansion gaps and the correct fixing method are very important in WPC systems. More screws do not automatically mean a more stable deck. Sometimes an overly rigid fixing is precisely the mistake. 

What can happen if a screw is driven in incorrectly? 

The consequences depend on the board's construction, the position of the hole and the installation method. Potential problems include damage to the surface around the screw head, localised deformation, restricted movement of the board or an unattractive finish. With hollow profiles, the position of the hole in relation to the board's internal structure also matters. A screw tightened too much can also press into the surface locally. The principle of “the tighter, the better” definitely should not be applied when installing WPC. 

Can you use ordinary wood screws? 

You should not assume that an ordinary wood screw will be suitable just because a composite decking board contains wood. WPC is a different material from solid wood. What's more, the screw may work not only with the decking board, but also with a fixing clip and a joist made of aluminium or composite. Your choice should therefore be based on the requirements of the complete system. If the manufacturer offers dedicated WPC screws, using them is usually a much safer option than experimenting with random fixing components.

Screws and aluminium joists.

With an aluminium structure, it is particularly important to use fixing components designed to work with a metal profile. A fixing clip may look similar to one used with a WPC joist, but the way it is fixed does not have to be identical. Factors include the geometry of the aluminium profile, the thickness of its wall and the type of screw used. That is why aluminium joists for composite decking boards should be treated as components of a specific structural system. 

What about installation on composite joists?

Compatible fixings should also be used with WPC joists. A composite joist has different characteristics from aluminium, so randomly swapping screws between systems may not be a good solution. In a typical installation, the fixing clip is placed in the side profile of the decking board and then fixed to the joist with the appropriate screw. The next decking board covers a large part of the fixing, leaving the deck surface neat and attractive.

What about the first and last decking boards? 

The edges of a deck often require a different solution from the middle of the surface. Many systems use starter clips, which secure the first decking board without the need to make a visible hole in its top surface. How the last decking board is installed depends on the design of the specific system and how the edge is finished. It is particularly in these places that the temptation often arises to “just secure the board with a screw”. Before doing so, it is worth checking the instructions. The manufacturer may specify a much neater and safer solution.

Can you fix something to a finished composite deck? 

This is a slightly different situation from fixing the decking board itself. Once the deck is complete, the owner may want to install a balustrade, pergola, lamp, planter, awning or another fixture. You should not automatically assume that a composite decking board is a structural component to which heavy fixtures can be safely screwed. For components carrying heavier loads, the fixing often needs to be transferred to the appropriate structure beneath the deck or to a separate foundation. This is particularly important for balustrades and pergolas, which are subject to considerable forces.