How to Calculate Decking Materials and Costs

Getting Your Decking Quantities Right

Building a deck is one of those jobs that looks simple until you're halfway through and realise you're six boards short and the merchant shuts in twenty minutes. Whether you're quoting for a customer or doing your own garden, getting the materials list right from the start saves time, money, and aggravation.

Step 1: Measure Your Deck Area

Start with the finished size of your deck:

• Length — the longest side
• Width — the shortest side

If the deck is L-shaped or wraps around a corner, split it into rectangles and calculate each section separately. Always measure to the outside edges of where the frame will sit.

Allow for Overhang

Most decks have the boards overhanging the outer joist by 20–30mm. Factor this in when you're working out board lengths, but base your joist frame on the actual structural dimensions.

Step 2: Work Out Your Deck Boards

Deck boards typically come in two widths:

| Board Type | Width (face) | Thickness | Common Lengths | |---|---|---|---| | Standard softwood | 120mm (inc. gaps) | 28–32mm | 2.4m, 3.0m, 3.6m, 4.2m, 4.8m | | Composite | 140–150mm (inc. gaps) | 22–25mm | 2.2m, 3.6m, 4.0m |

To calculate the number of boards:

Number of boards = Deck width ÷ Board width (including gap)

For standard 120mm softwood boards with a 5mm gap between each:

A 3m wide deck: 3000mm ÷ 125mm = 24 boards

Then choose your board length based on the deck length. If your deck is 4m long and boards come in 4.2m lengths, you're sorted. If it's 5m long, you'll need to butt-join boards on a joist — always stagger the joints so they don't all land on the same joist.

Add 10% for Waste

Timber is a natural product. You'll get the odd bowed board, a split end, or a knot right where you need to cut. Add 10% to your board count as standard.

Pop your deck dimensions into the Decking Calculator and it'll give you a full materials list in seconds — boards, joists, noggins, posts, and fixings.

Step 3: Calculate Your Joists

Joists run perpendicular to the deck boards and carry the load. Standard spacing is 400mm centres for softwood decking and 350mm centres for composite (composite boards are thinner and flex more).

| Deck Boards Run | Joists Run | |---|---| | Across the width | Along the length | | Along the length | Across the width |

Number of joists = (Deck length ÷ Joist spacing) + 1

For a 4m deck with joists at 400mm centres:

(4000 ÷ 400) + 1 = 11 joists

Use 47 × 150mm C16 treated timber (graded to BS EN 338) for joists spanning up to 2.4m. For longer spans, go up to 47 × 200mm or add a mid-span bearer.

Step 4: Noggins and Bracing

Noggins run between the joists to stop them twisting and to stiffen the frame. You need a row of noggins every 1.2m along the joist span, plus at both ends.

Number of noggins per row = Number of joists - 1

For 11 joists with noggins at 1.2m intervals on a 2.4m span:

2 rows × 10 noggins = 20 noggins

Cut these from the same timber as your joists.

Step 5: Posts and Bearers

If your deck is raised off the ground, you'll need posts to support the bearers that the joists sit on. Post spacing depends on the load, but a general rule for domestic decks:

• Posts at 1.2–1.5m centres along each bearer
• Bearers (doubled-up joists or 100 × 100mm posts) running under the joists perpendicular to them

Each post needs a concrete pad or post anchor. For pads, figure 300 × 300 × 300mm of concrete per post — the Concrete Calculator can help with the volume.

Low-Level Decks

If your deck is within 150mm of the ground, you can often get away with joists sitting on concrete pads without posts. Lay the joists on DPM over concrete blocks or adjustable pedestals.

Step 6: Fixings

You'll need:

• Deck screws — 2 screws per board per joist. For 24 boards across 11 joists, that's 24 × 11 × 2 = 528 screws. Buy a box of 200 at a time.
• Coach bolts or structural screws for joist-to-bearer connections
• Joist hangers if you're hanging joists off a ledger board against a wall
• Post anchors or bolt-down brackets for each post

Stainless steel or coated screws are essential. Standard steel screws will rust and stain the timber within a year. Look for screws specifically designed for treated timber — they resist the corrosive chemicals in the treatment.

Timber Choices and Costs (2026 Prices)

| Material | Approximate Cost | Notes | |---|---|---| | Pressure-treated softwood boards (3.6m) | £8–£14 each | Most common, needs annual treatment | | Hardwood (Balau, 3.6m) | £25–£40 each | Long-lasting, minimal maintenance | | Composite boards (3.6m) | £15–£35 each | No painting, 25-year warranty typical | | Joists (47 × 150mm, 3.6m) | £6–£10 each | Always pressure-treated | | Deck screws (box of 200) | £12–£20 | Stainless or coated only |

Rough Cost Per m² (Materials Only)

| Deck Type | Cost per m² | |---|---| | Softwood | £40–£65 | | Hardwood | £80–£120 | | Composite | £60–£100 |

Labour adds another £30–£50 per m² depending on complexity, height, and access.

Building Regulations

Most domestic decks don't need planning permission under permitted development, provided:

• The deck (including railings) is no more than 300mm above ground level
• It doesn't cover more than 50% of the garden
• It's not forward of the principal elevation

If your deck is higher than 300mm, you'll likely need planning permission. Any deck over 600mm high also needs Building Regulations approval for the balustrade (minimum 1100mm high with gaps no wider than 100mm).

Common Mistakes

• Skipping the sub-base. Even a low-level deck needs weed membrane and gravel underneath. Without it, vegetation grows up through the boards within a season.
• Using untreated timber. Joists in contact with damp ground will rot in 2–3 years without pressure treatment.
• Boards too tight. Leave a 5–8mm gap between boards for drainage and expansion. Wet timber swells — if your boards are tight in summer, they'll buckle in winter.
• No ventilation underneath. Air needs to circulate under the deck or the timber won't dry out. Keep at least 50mm clearance and leave gaps at the edges.
• Forgetting the ledger flashing. If the deck attaches to the house wall, fit lead flashing above the ledger board. Water running down the wall and pooling behind the ledger is the number one cause of rot and damp penetration.

Quick Summary

1. Measure the finished deck area (length and width)
2. Calculate boards: width ÷ 125mm for softwood, ÷ 145mm for composite
3. Calculate joists: length ÷ 400mm centres + 1
4. Add noggins, posts, bearers, and fixings
5. Add 10% waste to all timber quantities
6. Budget £40–£120 per m² for materials depending on timber type

Need the full breakdown fast? The Decking Calculator works out every item on your materials list — boards, joists, noggins, posts, fixings, and even concrete for post foundations.