The Complete Decking Guide: Planning, Pricing, and Materials

What You Will Find in This Guide

Welcome to the Lawsons decking guide – your definitive, step-by-step resource for planning, specifying, and installing a professional-grade deck. Whether you are a trade professional looking for compliant specifications on what size joists for decking UK regulations require, or a homeowner asking how to build decking on grass, this guide is designed to be your comprehensive manual.

As the largest independent timber, building materials, and fencing merchant in London and the South East, Lawsons has supported local trades and DIY projects since 1921. We understand that garden decking is more than just wood; it is an investment in your property. From selecting the right decking boards to understanding how to fit decking for maximum longevity, our team is here to help you get it right the first time.

• Timber vs. Composite: The pros, cons, and differences between Softwood, Trex, and Millboard.
• Cost Analysis: A breakdown of pricing and whether decking is cheaper than paving.
• Calculations: Standard board dimensions and how to calculate materials for a 3m x 3m area.
• Ground Preparation: How to build a frame on grass, soil, or uneven ground.
• The Frame: Correct joist spacing (400mm vs. 300mm), C24 timber specs, and squaring your base.
• Installation Guides: Step-by-step instructions for screwing timber and using hidden fixings for composite.
• Maintenance: How to clean, power wash, and replace damaged boards safely.

Before diving into technical installation, it is worth clarifying what is decking in the context of modern landscaping. Traditionally, decking refers to a raised flat surface capable of supporting weight, similar to a floor, but constructed outdoors. It is typically built from timber or composite planks set upon a structural frame.

Unlike a patio, which requires extensive excavation and backfilling, decking can be built over uneven ground, slopes, or existing structures. This versatility makes it the UK’s most popular landscaping solution for creating defined zones for dining, relaxing, or entertainment.

One of the most frequent questions we face at our trade counters is the choice between materials. The market has evolved significantly, and understanding the difference between traditional timber and composite deck boards is vital for your budget and long-term maintenance plans.

Traditional Timber Decking

Timber remains the classic choice. High-quality softwood decking boards sourced by Lawsons are pressure-treated (often referred to as "tanalised") to protect against rot, fungus, and insect attack.

• Aesthetics: Timber offers a natural, warm look that you pay prefer.
• Workability: It is easy to cut, sand, and modify. If you scratch a timber board, it can often be sanded out.
• Heat: Wood tends to stay cooler in direct sunlight compared to darker plastics.

What is Composite Decking?

So what is composite decking... it is a modern alternative designed to minimize maintenance. These boards are manufactured using a blend of recycled wood fibres and plastic polymers. The result is a board that looks like wood but behaves like a durable plastic. 

High-quality options are often "capped," meaning they feature a protective outer shell that resists staining and fading. This leads to common questions about durability, specifically: does composite decking warp? Generally, unlike timber which twists due to moisture absorption, high-quality composite decking does not warp from water. However, it is subject to thermal expansion—it grows and shrinks as the temperature changes. This means understanding how to fit composite decking with the correct expansion gaps is critical to preventing bowing (we will cover this in the installation section).

What is Trex?

When researching composite deck boards, you will inevitably encounter the name Trex. So, what is Trex? Trex is a leading decking brand and the inventor of timber-alternative decking. It is a capped composite known for its high-performance shell which resists fading, staining, and scratching. If you are looking for where can I buy Trex or where can I buy Trex near me, Lawsons is a specialist stockist. We hold a wide range of colours and fixing systems in stock.

What is Millboard?

Another premium option is Millboard. Unlike standard composites, Millboard is a resin-mineral composite molded from real oak timbers. It contains no wood fibre, meaning it is non-porous and highly resistant to algae. If you are wondering where to buy Millboard decking, our branches can supply a range of grain patterns and colours.

Budget is often the deciding factor in landscaping. A very common query we receive is: is decking cheaper than paving?

The answer depends entirely on your site conditions and material choice.

• Softwood Decking: This is generally the most cost-effective option. Decking boards made from softwood are cheaper to buy than stone, and the installation on a timber frame is faster than laying a sub-base and mortar bed for paving. If you are building on a slope, decking is significantly cheaper than building retaining walls and backfilling for a patio.
• Composite Decking: If you choose premium brands, the upfront cost can be higher than mid-range paving. However, when asking how much does decking cost, you must factor in the "lifetime cost." Timber requires annual cleaning, staining, and oiling. Composite requires only soap and water. Over 10 years, the cost gap narrows significantly.
• Labour: Decking is a "dry fix" solution. It doesn't require waiting for mortar to dry, meaning trades can finish the job faster, saving you labour costs.

For a precise figure, we recommend measuring your area and visiting a Lawsons branch. We can calculate exactly how much for your specific square footage.

Ordering the right amount of material is essential to avoid waste. To do this, you need to know the standard dimensions.

Dimensions and Sizing

• How wide is a decking board? The most common width for a standard timber board is roughly 120mm to 145mm. However, how wide is decking boards in the composite market can vary, with wide-board options becoming popular. Always check the "coverage width" (the board width plus the required expansion gap) when calculating.
• How thick are decking boards? Standard structural decking is usually between 28mm and 32mm thick. This thickness is required to span the standard 400mm joist spacing without bouncing.
• What length do decking boards come in? To minimise joinery, boards are available in long lengths. Common stock lengths include 3.0m, 3.6m, 4.2m, and 4.8m. We recommend measuring your deck's longest run and buying boards that fit that length to avoid butt joints.

Calculating Materials: How Many Decking Boards for 3m x 3m?

Let’s walk through a practical example using a popular search query: how many decking boards for 3m x 3m?

1. Area: 3m x 3m = 9 square metres.
2. Board Coverage: Assume you are using a standard 120mm wide timber board. You must leave a 5mm gap for drainage. Total coverage width per board = 125mm (0.125m).
3. Rows Needed: Divide the width of the deck (3.0m) by the coverage of the board (0.125m).
   • 3.0 ÷ 0.125 = 24 rows.
4. Total Linear Metres: You need 24 boards at 3m length.
   • 24 x 3m = 72 linear metres.
5. Wastage: detailed cuts and mistakes happen. We always recommend adding 10% for wastage.
   • Total to order = approx. 80 linear metres.

Whether you are looking for where to buy decking boards for a small DIY patch or a large commercial project, our staff can perform this calculation for you in seconds.

The longevity of your garden decking relies entirely on the frame underneath. If the frame rots, the deck fails.

Ground Preparation: How to Build Decking on Grass

A frequent DIY mistake is laying timber directly onto soil. You must separate the timber from the ground. Here is the correct method for how to build decking on grass:

• Clear the Site: Mark out your area and dig out the turf to a depth of roughly 100mm.
• Weed Prevention: Lay a heavy-duty weed control fabric. This stops vegetation growing up through your boards.
• Drainage: Cover the membrane with pea shingle or gravel. This prevents water pooling under the frame.
• Support: Do not place the frame on the gravel. You must use concrete pads, patio slabs, or specialist adjustable pedestals to support the timber joists.

For those asking how to build ground level deck frames, the principle is the same: airflow is King. Ensure there is a gap around the perimeter and underneath the joists to allow air to circulate. Damp, stagnant air causes rot.

Structural Timber: What Timber for Decking Frame?

Never use standard studwork timber for a deck frame; it will rot within a few years.

• Timber Grade: C24 Graded Structural Timber. C24 is stronger and has fewer knots than C16 or ungraded wood.
• Treatment: Ensure the timber is pressure-treated (UC3 or UC4 graded) for outdoor use.
• Joist Sizing: The industry standard for domestic decks is 47mm x 100mm (4x2) or 47mm x 150mm (6x2). The larger 150mm joist allows for longer spans and a more rigid feel underfoot.

Post Selection

If you are building a raised deck, you will need posts to lift the frame.

• What posts to use for decking? We recommend 100mm x 100mm (4x4) pressure-treated posts. These provide ample surface area to bolt your bearers to.
• How to fit decking posts: Dig a hole approximately 600mm deep (or one-third of the post height). Insert the post, ensuring it is plumb (vertical), and backfill with Postcrete—a rapid-setting concrete mix available at Lawsons.

Once your posts are set and your ground is prepared, it is time to construct the frame. This is the skeleton of your project; if this isn't square and level, your decking boards will never look right.

Attaching to the House (The Wall Plate)

If your deck adjoins the house, you will typically secure a "ledger board" (wall plate) to the masonry. This ensures the deck is anchored firmly.

Fixing to Masonry: Standard screws often fail in old brickwork. This is where many customers ask how to use resin fixings. Resin (chemical) anchors provide a stress-free, incredibly strong bond.

• Drill the hole into the masonry and brush out the dust.
• Inject the resin into the hole.
• Insert a threaded stud and let it cure (set hard).
• Bolt your wall plate onto these studs.
  • This method prevents the brick from cracking and handles heavy loads, making it ideal for the main support of your deck.

Squaring the Frame

A common DIY error is building a parallelogram instead of a rectangle. To ensure your corners are perfect 90-degree angles, use the 3-4-5 Triangle Method:

1. Measure 3 units along one side (e.g., the wall plate).
2. Measure 4 units along the adjacent joist.
3. Adjust the joist until the diagonal line between these two points measures exactly 5 units.

Joist Spacing: The Critical Detail

When planning how to build a deck frame, spacing is everything. The distance between your joists (centres) depends on your material choice.

• Timber Decking: Standard spacing is 400mm centres.
• Composite Decking: Composite is heavier but more flexible (plastic can sag if unsupported). Most manufacturers, including Trex, specify 300mm to 350mm centres for standard layouts. Always check the installation guide.

With a solid frame in place, you are ready to lay the surface. If you have chosen traditional wood, here is your step-by-step guide on how to install decking boards for a professional finish.

Seal the Cuts

This is the most skipped step, and the reason many decks rot prematurely. Whenever you cut a pressure-treated board, you expose the untreated core. You must apply an end-grain preservative to every cut.

The First Board

Start at the outer edge or the house wall (depending on your design preference). Fix this board perfectly straight using a string line. Do not rely on the house wall being straight—it rarely is!

Gapping and Fixings

Wood is a natural material; it breathes.

• Gapping: You must leave a gap of 5mm to 8mm between boards to allow for expansion when wet and to let water drain through. If you butt them tight, the boards will buckle when it rains.
• Screwing: Use high-quality exterior decking screws (green or stainless steel). Use two screws at every joist crossing to prevent cupping. Pre-drill your holes 2mm from the edge to stop the wood splitting.

Staggering Joints

If your deck is wider than your board length (e.g., wider than 4.8m), you will have joins. Never line all your joins up on a single joist—it creates a weak point and looks messy. Stagger the joints across the deck in a random or brick-bond pattern. Always use a double joist where two board ends meet so that both ends have full support.

Troubleshooting: Can I Stain Wood Filler?

Mistakes happen. Perhaps a knot falls out, or you mis-drill a hole. A common question is: can I stain wood filler? The answer is yes, but with caution. You must purchase a specific "stainable wood filler." Standard fillers often repel stain or absorb it at a different rate to the timber, leaving a noticeable patch. For the best finish, try to use offcuts to plug holes, or accept that timber is rustic and small imperfections add character.

Installing composite deck boards requires a different mindset. Plastic-based boards expand and contract with temperature changes, not moisture.

Hidden Fixings: How to Install Grooved Decking

Modern composite boards often come in two profiles: solid edge (for borders) and grooved edge (for the main deck). If you are asking how to install grooved decking, the process is faster and neater than timber:

1. Starter Clip: Screw a starter clip onto the joist at the edge of the deck. Slide the first board into it.
2. Universal Clip: Insert a hidden fastener clip into the groove of the board at every joist intersection. Screw the clip down halfway.
3. Next Board: Slide the next board onto the clips. The clip automatically sets the consistent gap (usually 6mm) between boards.
4. Tighten: Fully tighten the screws. Result: A flawless, screw-free surface that is safe for bare feet.

Managing Expansion

This is the most critical part of how to lay composite decking. You must leave expansion gaps at the ends of the boards (butt joints) and where the deck meets a wall.

• Example: On a hot summer day, a 4m composite board can expand by several millimetres. If it is wedged tight against a wall, it will bow upwards.
• Rule of Thumb: Check the manufacturer’s "gapping matrix" table, which tells you the exact gap size based on the air temperature during installation.

How to Install Composite Decking on the Ground

We touched on this in ground preparation, but it bears repeating: do not lay composite directly on the ground. Even though it doesn't rot like wood, it requires ventilation. Without airflow, moisture builds up underneath, which can invalidate your warranty and cause the frame (if timber) to rot beneath the composite.

You have built it. Now, how do you keep it looking new?

How to Clean Wood Decking

Algae and moss are the enemies of timber. They hold moisture and make the surface dangerously slippery.

• Routine: Sweep the deck regularly to remove wet leaves.
• Annual Wash: Use a dedicated decking cleaner and a stiff broom to scrub the grooves.
• Treatment: Once dry, apply a high-quality decking oil or stain. This replaces the natural oils lost to the sun and creates a barrier against rain.

Can You Power Wash Trex Decking?

Power washers are powerful tools that can cause damage if used incorrectly.

• Timber: We generally advise against aggressive power washing on softwood, as it can strip the fibres and make the surface "furry" and splintered.
• Composite: Users frequently ask can you power wash Trex decking? The answer is usually yes, but with strict caveats. Use a pressure washer set below 3100 psi and use a fan attachment (never a pinpoint jet). Keep the nozzle at least 8 inches from the surface.
• Best Practice: Often, warm soapy water and a soft-bristle brush are safer and just as effective for cleaning composite.

How to Replace a Decking Board

If a board gets damaged or stained beyond repair, you may need to swap it out.

• Timber: Simply unscrew the damaged board. If the screws are rusted/stripped, you may need to carefully pry the board up or cut it out. Check the joist underneath is sound, cut a new board to length, and screw it down.
• Composite: If you used hidden fixings, how to replace a decking board is slightly more technical. You may need to unscrew the clips on the adjacent boards to release the tension, allowing you to slide the damaged board out and click a new one in.

Building a deck is one of the most rewarding home improvement projects you can undertake. It adds value to your property, extends your living space, and provides a durable area for family enjoyment.

Whether you are calculating how many decking boards for 3m x 3m, debating is decking cheaper than paving, or looking for the best composite decking UK suppliers, Lawsons is your partner in the process.

Why Shop at Lawsons?

• Huge Stocks: From structural C24 timber to the quality Trex and Millboard ranges.
• Expert Advice: Our branch teams can guide you on any questions you may have.
• Fast Delivery: We deliver bulky items directly to your site or driveway across London and the South East.

Ready to build? Explore our full range of Decking Supplies online, or visit your local branch to see the products in person. Bring your measurements, and let us help you build a deck that lasts.