Avoid these 9 composite decking installation mistakes!

When it comes to installing composite decking, several things can go wrong if you don’t take the right precautions. 

After all, any DIY project is a challenge, and one small misstep when you don’t know what you’re doing often has a domino effect. The result? Poor-quality decking that won’t last as long as you want it to. 

You can have the best timber decking in the world, but your deck will still fail entirely due to one avoidable slip-up. But here at WPS Handrails, we’re determined not to let that happen.

With our list of common composite decking installation mistakes, we’re here to make sure you get the job done right. You’ll be thanking us in the future when you don’t have to spend loads of time and money on decking repairs and replacements!

A quick reminder

You may be thinking, ‘Surely one little mistake won’t cause much harm,’ but we’re here to remind you that it really can. 

We’re not just talking about a mismatched pattern or a slightly wobbly board. What you’re really chancing is serious safety risks from loose boards or an unstable frame, warped boards, poor drainage, and even full tear-downs.

The worst part is that most problems don’t show up right away; they creep up on you months down the line when you realise you didn’t take our word for it! Plus, if you fail to follow the manufacturer's installation guidelines, any future claims and warranties may be denied.

Top 10 composite decking installation mistakes to avoid

So, if you’re determined to go down the DIY route and install your decking yourself, here’s what you’ll need to avoid. 

1. Ignoring joist spacing and subframe compatibility

Joist spacing is one of the most common errors we see during DIY decking installation.

While composite may not warp or split like timber, it will flex more if it’s not properly supported. Wider joist spacing, which can work fine for timber, can cause composite to crack, bounce, or dip.

For this reason, you’ll need to leave a spacing of no more than 300–400 mm between the joists; 350 mm for installations using our round-hollow boards and 400 mm for installations using our solid boards. If you’re planning a diagonal layout, the spacing needs to be even tighter, typically around 250 mm.

As for the frame itself, we recommend using treated C24-grade timber for basic garden decks, and aluminium or galvanised steel subframes for high-traffic environments.

2. Getting the gaps wrong

If you press the boards too tightly together, there will be no room for necessary expansion and contraction, affecting the quality of your deck.

For most composite decking systems, joists should be spaced at no more than a 3–6mm gap for both side-to-side and end-to-end boards. Another common error is ignoring perimeter gaps. This means leaving boards flush against walls, fences, or posts with no room to move. 

To avoid this, leave a 6 to 13mm gap around the perimeter of your deck.

You can read more about the specifics for different circumstances, like temperature conditions, in our blog on the expansion and contraction of composite decking.

3. Reusing old or rotten frames

If you couldn’t already guess, this is a big no-no. 

When installing your deck, you want to make sure that all of the materials and fixings are in great condition. Sure, reusing and recycling are important, but not when it comes to structures that need to handle heavy loads and pressures.

Moisture, rot, and insect damage often develop out of sight, inside the timber itself. Unlike traditional decking boards, composite decking is designed to last 20–25 years or more, and rotted timber can significantly decrease this lifespan.

As a rule of thumb, if your subframe is more than 5–7 years old or was built using basic softwood, replace it ASAP.

4. Poor drainage & ventilation

Insufficient ventilation and drainage are among the most common issues for decking problems down the line. 

If rainwater and condensation have nowhere to go, then you’re kicking yourself in the foot, as you’ll have to replace the boards due to rotting, damp, and mildew. All of which affect the structural integrity of your deck. 

To avoid this, you’ll need to aim for a subtle slope on your deck, around 1–2% (or 10–20 mm per metre), to allow water to run off. You’ll also need to make sure there’s at least 50 mm of space between the underside of your boards and the ground.

Always lay your composite over a raised framework! You can use adjustable pedestals or composite frame supports to lift the structure, creating a clean, breathable cavity underneath.

5. Skipping acclimatisation

This one is fairly simple, but many people forget to do it. 

Before installing your composite decking, remove the packaging and allow the boards to acclimate to the environment for approximately 48 to 72 hours. Doing so will help avoid buckling, bending, and warping.

Store them flat and fully supported, ideally on a level surface with bearers underneath.

6. Using the wrong fixings

We’ve all been there: finding random screws in our house and thinking, ‘They’ll do’ for the job. 

But trust us on this; using the wrong fixings for your decking will lead to disaster. Standard screws will rust quickly, split, and loosen over time, causing movement in your boards. If you don’t want this, then use fixings specifically designed for composite decking.

The two main types are:

• Hidden clip systems: These slot into pre-grooved edges on the board and fix down to the joists.

• Colour-matched surface screws: These are used on solid-edge boards or where clips can’t be used.

7. Skipping double joists at board ends

When two board ends meet, they both require individual support.

However, many DIYers try to fix both to the same joint or support only one side. While it may look fine, it can be the main culprit for movement and shifting, visible flexing when walking on your deck, and loose fixings. 

To avoid this issue, always install a double joist at every butt joint and leave a small gap between two side-by-side joists for sufficient airflow. Secure each board end to its own joist and use compatible clip systems or screws.

8. Laying boards the wrong way up

Yes, this really does happen. You’d be surprised how many people begin installing the boards only to realise they’re upside down, or they just don’t know which way is right in the first place.

The boards must always be installed groove-side down.

The grooved or fluted side is there to help moisture run off and circulate underneath the deck. This prevents water pooling and algae growth, resulting in a long-lasting deck. 

If your decking boards have two textured sides, one will usually be marked or labelled as the “finished face”.

9. Not choosing WPS Handrails for your composite decking

The quality of your boards has the biggest impact on the outcome of your composite deck. 

By choosing us for your composite decking, you’re already setting yourself up to create a beautiful, long-lasting deck! We guarantee a stunning selection of boards from our innovative range, including ECOBOARD, PROCAP, and LUXECAP.

We even offer a decking configurator, so you can calculate the number of boards and fixings you’ll need for your project; no wasted time or money on our watch!