You lay composite decking on a solid, level subframe of joists set at centres of usually no more than 40 cm, with the boards fixed using hidden clips or screwed straight into the joist.
The two things that really matter: leave enough room for expansion (roughly 5 mm between butt ends and 10 to 15 mm all round against walls and fixed structures) and build in a fall of 1 to 2 cm per metre so water runs away. That is 90 percent of the story.
The rest is working neatly. Below you get the whole picture: composite versus timber, which board to choose, the subframe, the step-by-step plan, the mistakes most people make, and roughly what it costs. Important first: always follow the installation instructions from your own manufacturer. Brands differ in their dimensions, and those instructions override every rule of thumb in this article.
Composite or timber: which do you pick?
Bluntly: composite costs more to buy, but you spend years less time and money on it. Timber is cheaper to buy, but it needs maintenance and on average lasts less long.
Composite (also called WPC, short for Wood Plastic Composite) is a mix of wood fibres and plastic. It doesn’t rot, doesn’t splinter, and you never have to oil or stain it.
Timber, and hardwood especially, looks warm and natural and costs less per board. But it greys off, can crack, and moves with the weather. Without maintenance the quality drops off noticeably.
| Point | Composite (WPC) | Hardwood |
|---|---|---|
| Purchase price per board | Higher | Lower |
| Maintenance | Cleaning, 1 to 2 times a year | Cleaning + annual oiling/treating |
| Service life | Roughly 25 to 30 years | Shorter, heavily dependent on maintenance |
| Splinters/rot | No | Possible, if maintenance is neglected |
| Heat in the sun | Gets hot, dark colours especially | Stays cooler |
| Scratches | More prone | Less noticeable |
The downsides of composite, said honestly: on a hot day it feels warmer underfoot than timber, particularly in dark colours, and it can pick up scratches. So don’t talk yourself into “maintenance-free”. Composite is low-maintenance, not maintenance-free.
Which composite board do I need: hollow or solid?
This is the first real decision, and it determines how you lay.
Hollow-core boards have chambers running through the inside. They’re lighter and cheaper, often around 20 to 30 percent cheaper than solid. Perfectly fine for a normal patio deck, a balcony or a roof terrace where weight matters.
Solid boards are one dense board all the way through. They’re heavier, stronger and feel more substantial. The logical choice under heavy loads or lots of furniture, and they sit in the more expensive bracket.
The big difference when laying: hollow boards must be laid to a fall so water doesn’t sit in the chambers. Solid boards may be laid dead level because there are no voids for water to get into. Remember that one, it saves you grief later.
Capped or uncapped
A second distinction is the finish. A “capped” board (also called coated or co-extruded) has a protective shell wrapped around the core. That shell stands up to UV, staining and colour change better than a bare, uncapped board.
Uncapped boards are cheaper, but they usually grey off and fade faster. If you want an even colour for years, capped is the safer bet. Check with the manufacturer what warranty covers colour fastness.
Smooth or grooved
Many boards have two faces: a smooth one and a grooved one. The grooved face gives a bit more grip, the smooth face is easier to keep clean. Both are fine, it’s mostly taste. Pick one face for the whole deck so it looks calm.
What it all sits on: the subframe
A composite deck is only as good as the structure underneath it. This is where most goes wrong, so read this section twice.
The boards sit on joists (also called bearers or the subframe). You can get those joists in aluminium or in composite. Aluminium is stiff, strong and barely moves with temperature, which makes it a pleasant choice. Composite joists expand a bit more and shrink along with the boards. Timber as a subframe works too, but then you’re reintroducing exactly the maintenance-hungry material you wanted to avoid by going composite.
Joist centres
Joist centres means the distance from the middle of one joist to the middle of the next. This is the measurement that decides whether your deck feels solid or flexes underfoot.
Common rule of thumb: 400 mm maximum centre to centre. Under heavier loads or with certain hollow boards you drop back to 300 mm. If you lay the boards on the diagonal (at 45 degrees, for instance), you have to bring the joists closer together, because the board’s span gets longer.
Again: this is a rule of thumb. One board is allowed to span 400 mm, another demands 300 mm. Your manufacturer gives the exact figure, and that’s the one that counts.
Foundation and supports
The joists can’t just sit on the ground. They have to be level (or to a fall) and on a stable base.
Common options:
- Adjustable pedestals under the joists. Handy because you can set the level precisely and it creates space underneath for ventilation and drainage.
- Concrete paving slabs or bearing pads as feet under the joists, at regular spacings.
- An existing, flat surface such as a concrete slab or a paved patio, on which you level the joists.
Lay weed membrane under the structure if you’re building on sand or soil. That stops weeds pushing up between your boards.
Fall for drainage
Water has to go away from your house. So build in a fall of 1 to 2 cm per metre (that’s 1 to 2 percent), running away from the wall towards the garden.
With hollow boards this fall is compulsory, otherwise water sits in the chambers. With solid boards you can work dead level, but a small fall always helps against puddling on the surface.
Ventilation under the deck
This is a silent killer. Air has to be able to circulate under your deck, otherwise it stays damp and the structure deteriorates over time.
So make sure there’s space under the boards and don’t seal the sides completely shut. If you finish the edge with a fascia or trim, leave openings or a gap so air and water can get out. A deck that’s walled in on every side collects moisture.
The biggest mistake: expansion and contraction
If you remember one thing from this article, make it this. Composite expands in heat and contracts in cold. Count on a board in full sun becoming measurably longer than that same board in winter.
Lay the boards tight against each other and tight against the wall, and in warm weather they have nowhere to go. The result: boards press against each other, bow up or lift. This is the classic mistake that ruins decks.
So you leave room everywhere:
- Between butt ends (where two boards meet end to end along their length): roughly 5 mm. With long boards and certain manufacturer specs, sometimes more. The longer the board, the bigger the gap you allow.
- Sideways between boards (the joint running lengthways): usually 5 to 6 mm. If you’re working with clips, the clips set this gap automatically. Handy, because then it’s right by definition.
- All round walls, house walls and fixed structures: 10 to 15 mm clear. That’s for expansion and for ventilation.
A simple trick: use spacer blocks or the supplied clips as a gauge, so every joint is exactly the same. Don’t measure by eye.
The right figure for your board is in the manufacturer’s instructions and depends on the board length and the temperature on the day you lay. If you’re laying on a cold day, keep the butt joints slightly wider, because the board still has expanding to do.
Clips or screws: how do you fix it down?
There are two ways to fix composite boards.
Hidden clips sit between the boards and clip or screw onto the joist. You see no screw heads, the deck looks clean, and the clip immediately sets the correct side gap between boards. For most composite decks this is the standard and the tidy choice.
Screwing straight through the board works with some boards, mainly solid ones. You then see the screw heads. Pilot drilling is recommended with composite so the material doesn’t tear out. You’ll use screws anyway for the starter and end clips and for fixing trims to the joist.
For the first and last board you’ll usually use a starter clip that you screw to the joist. That way the edge is held down without visible screws on top.
Tools you’ll need
- Circular saw or mitre saw with a fine-tooth blade (lots of teeth, 60 or more for instance) for clean cuts without tear-out
- Cordless drill/driver
- Spirit level (1 metre minimum, longer is nicer)
- Tape measure and pencil
- Rubber mallet to tap boards home neatly
- Spacer blocks, or the clips themselves, for the joints
- Carpenter’s square for marking square
- Possibly a plate compactor or a tamper if you’re compacting the ground
Step by step: laying composite decking
Here’s the order. Work calmly and keep checking that everything runs square and straight.
Step 1. Measure up and set out.
Work out the size of your deck. Allow some margin, around 20 cm extra in length and width, so you’ve got room to play with. Run a string line to get the shape crisp.
Step 2. Prepare the ground.
Make sure the ground is firm, level and root-free. Compact loose ground with a plate compactor. Lay weed membrane if you’re building on sand or soil.
Step 3. Set the subframe.
Place your pedestals or slabs and lay the joists on top. Keep to the joist centres (usually 400 mm maximum, or 300 mm under heavier loads or where your manufacturer specifies it). Set everything to a fall of 1 to 2 cm per metre, running away from the house.
Step 4. Check for square and level.
Measure the diagonals, they have to be equal. Lay the spirit level across the joists in both directions. Correcting now is easy, later it isn’t.
Step 5. Lay the first board.
Start on the least visible side or against the house wall, using the starter clips. Keep 10 to 15 mm clear of the wall. Check that the board sits square to the joists.
Step 6. Clip or screw your way along.
Put a clip on every joist, slide the next board up and fix it down. The clips set the joint. Every few boards, check that the joints still run even and that you’re not drifting out of line.
Step 7. Keep the butt joints.
Where boards meet end to end, leave roughly 5 mm (or your manufacturer’s figure). Make sure a butt joint always lands on a joist, so both board ends are supported.
Step 8. Cut the last board to width.
Measure the remaining width, allow for the clearance to the wall, and cut with the fine blade. Fix it with an end clip.
Step 9. Finish the edges.
Use fascia trims, corner trims or skirting boards to tidy up the sides. Leave openings or a gap for ventilation and drainage. Don’t block the airflow.
Step 10. Clean up.
Sweep away sawdust and dirt. Go over the lot checking for loose clips and even joints.
Maintenance: how do you keep it looking good?
The nice thing about composite: the maintenance is light. No oil, no stain.
Clean the deck around 1 to 2 times a year with water, a soft brush and, if you like, a cleaner suitable for composite. That keeps leaves, algae and dirt at bay.
Avoid aggressive cleaning products and go carefully with a pressure washer. Too much pressure from too close can damage the surface. Use it at a distance and on a low setting, or leave it in the shed.
Grease or wine stains are best tackled quickly, before they soak in. Even though capped composite is stain-resistant, cleaning it up fast always works better.
What does laying composite decking cost?
Prices vary widely depending on board choice (hollow or solid, uncapped or capped), brand, and whether you lay it yourself or have it installed. Below are realistic ranges; treat them as direction, not as a quote.
| Situation | Guide price per m² |
|---|---|
| Materials only, laying it yourself (boards + subframe + clips) | Roughly 55 to 100 euros |
| Complete, including professional installation | Roughly 100 to 180 euros |
Solid and capped boards sit at the top of the range, hollow and uncapped boards at the bottom. The subframe, the drainage, the edge finishing and how easy your garden is to get into all push the price around too.
Work the cost out over the whole service life. Composite costs more to buy, but you save years of oil, stain and labour. Over 25 to 30 years the sums often come out better than you’d think standing at the till.
Frequently asked questions
How far can composite span, or put differently, what are the joist centres?
The common rule of thumb is 400 mm maximum centre to centre, and 300 mm under heavier loads or with boards laid on the diagonal. The exact figure differs per board, so always follow your own manufacturer’s instructions.
Why does there have to be a gap between composite boards?
Composite expands in heat and contracts in cold. Without a gap the boards press against each other and bow up. So keep roughly 5 mm between butt ends and 10 to 15 mm all round against walls and fixed structures. With clips, the side joint is set automatically.
Does composite have to be laid to a fall?
Hollow-core boards must be laid to a fall, usually 1 to 2 cm per metre, so water doesn’t sit in the chambers. Solid boards may be laid dead level, although a small fall always helps against puddling.
Can I lay composite boards straight onto paving or concrete?
Not directly. You first build a subframe of joists, levelled on pedestals or bearing pads, so ventilation and drainage can happen under the boards. Laying boards flat on a solid surface traps moisture.
Clips or screws, which is better?
Hidden clips give a clean look with no visible screw heads and set the correct joint straight away. For most decks that’s the tidy standard. Screwing through the board works with some solid boards, but pilot drill to avoid tear-out.
How much maintenance does a composite deck take?
Not much. Cleaning with water and a soft brush, around 1 to 2 times a year. No oil or stain needed. Go carefully with the pressure washer, because too much pressure up close can damage the surface.
Want to draw up your deck or subframe properly from the start, with the right dimensions, the right joist centres and a materials list alongside? Then have a quick look at Fred’s DIY Plans. That way you know exactly what you need up front and avoid the mistakes that get expensive afterwards.