How to build a deck

Three variants · Ground-level, elevated and on a slope · Span tables and quantity take-off · Step by step

This guide covers the three most common deck configurations: a ground-level deck, an elevated deck and a deck built on a sloping site. You will find material lists, span tables, step-by-step instructions and a worked example with a full quantity take-off for a 6.0 × 4.0 m deck.

Choose the right variant

Materials

Timber

• Decking boards: 28×120 mm pressure-treated timber. The NTR/AB impregnation class (suitable for outdoor use above ground) is the minimum recommended standard. A minimum thickness of 28 mm is recommended.
• Deck joists: 48×98, 48×148 or 48×198 mm — choose based on span (see table)
• Bearer beams: Double 48×198 mm is standard for most elevated decks
• Posts: 98×98 mm, or two 48×98 mm pieces bolted together
• All structural timber should be strength-graded C24 and impregnated to minimum NTR/AB (suitable for outdoor use above ground) — choose NTR/A for timber in direct contact with soil or concrete

Fixings and hardware

• Decking screws: Approved exterior-grade screws — grade A4 stainless steel in coastal or salt environments
• Structural connections: Structural screws or bolts rated for outdoor use
• Joist hangers, post bases, angle brackets: Must be compatible with treated timber and outdoor exposure
• Two screws per board at each joist. Always pre-drill at board ends and splices to prevent splitting.

Foundations

• Ground-level deck: Paving slabs, concrete pads or adjustable plastic feet on compacted gravel
• Elevated deck: Cast-in-place point foundations (cylindrical, Ø200–300 mm) with post bases, or precast concrete pillars
• Frost depth varies considerably — assess local ground conditions. Free-draining material around foundations reduces frost heave.

Key principles

1 — Correct joist spacing

The spacing between deck joists depends on the board thickness:

• 21 mm boards → 400 mm centres
• 28 mm boards → 600 mm centres (standard)

2 — Board gap

Minimum 3 mm between boards. In practice 5–6 mm. Boards delivered wet will shrink as they dry — if they are very wet, install them at 2–3 mm gaps.

3 — Railing

A railing is required where the fall height exceeds 0.5 m (check local regulations). Minimum railing height is 1.0 m measured from the deck surface. Maximum opening between balusters is 100 mm.

4 — Drainage and ventilation

Good airflow beneath the structure is critical for durability. Joists should be kept as far above ground as practical, and water must be able to drain freely under the deck.

5 — Fixing to the house

The ledger beam must be fixed into the load-bearing structure of the house — never into cladding alone. If in doubt about the wall construction, build a freestanding deck with its own posts close to the wall.

Variant A — Ground-level deck

Construction sequence

1. Mark out the area with pegs and string line. Check diagonals are equal (for a true rectangle).
2. Excavate turf, roots and soft topsoil down to firm ground.
3. Lay weed-suppressing membrane to prevent soil mixing with the gravel over time.
4. Fill and compact gravel or aggregate — approximately 100–200 mm. A stable, free-draining layer is more important than exact depth.
5. Place paving slabs or concrete pads as bearing points. Use a laser level to ensure all bearing points are in the same plane.
6. Install the joist frame at 600 mm centres (for 28 mm boards). Check for level and straightness.
7. Fix decking boards with a 5 mm gap. Two screws per board at each joist. Splices must always land on a joist.
8. Trim edges cleanly and fit fascia boards for a neat finish.

Variant B — Elevated deck

Construction sequence

1. Establish the finished deck height from the door threshold. A maximum step-down of 25 mm is recommended for accessible designs; otherwise aim for at least 50 mm clearance above any potential moisture source.
2. Choose freestanding or ledger-mounted. Freestanding is simpler and safer if you are unsure of the wall construction.
3. Mark out the bearer lines and post positions. Check right angles by measuring diagonals.
4. Excavate and cast foundations. Ensure free-draining material in the base. Confirm the local frost depth and build foundations below it.
5. Set post bases to the exact height and position while the concrete is still workable.
6. Erect the 98×98 posts in the bases. Do not cut to final height yet.
7. Fix the double bearer beams to the posts. Check laser level carefully in both directions.
8. Hang deck joists (600 mm centres) with joist hangers or bearing on top of the beams. Cut posts to the correct height.
9. Install diagonal bracing between posts and bearer beams — especially at the corners.
10. Lay decking boards and fix with screws. Pre-drill at all ends and splices.
11. Erect railing where the fall height exceeds 0.5 m.

Variant C — Deck on a slope

Built in the same way as an elevated deck (Variant B), but requiring greater precision in levelling and more robust bracing.

1. Measure the ground height at all post positions with a laser. Record a height schedule.
2. Choose your strategy: Moderate slope → single deck level with posts of varying height. Steep slope → two deck levels (stiffer structure, better appearance).
3. Cast foundations so that all tops land in exactly the right place. Use a laser — not eye measurement.
4. Erect posts and bearer beams — start from the highest side. Set the beams to an exact level.
5. Cut each post individually to its correct length — do not cut all posts the same.
6. Install deck joists in the same way as Variant B.
7. Install generous diagonal bracing in both directions. A sloped site generates greater lateral forces — never scrimp on bracing.
8. Lay boards and install railing wherever the fall height exceeds 0.5 m.

Span tables (C24 treated timber)

Deck joists — indicative maximum spans

Bearer beams — indicative maximum post spacing

Source: Bergene Holm. Applies to C24 pressure-treated structural timber. Joists hung in joist hangers (rather than bearing on top) should be reduced by a factor of 0.9.

Quantity take-off

Decking boards

Effective module width = board width + gap (e.g. 120 + 5 = 125 mm = 0.125 m)

Number of boards = deck depth / module width

Add 5–10% for cutting waste and grading.

Deck joists

Number of joist lines = ceil(dimension perpendicular to joists / spacing) + 1

Decking screws

Screws = number of boards × number of joist lines × 2

Add 10% reserve.

Concrete for point foundations

For a cylindrical foundation:

Volume (m³) = π × radius² × depth

Example: Ø300 mm (r=0.15 m), depth=0.70 m → π × 0.15² × 0.70 ≈ 0.049 m³ = 49 L per foundation.

Worked example: 6.0 × 4.0 m deck

Decking boards 28×120 mm, 5 mm gap. Boards run lengthways (6.0 m); joists run across (4.0 m span, distributed over 6.0 m).

Number of boards: 4,000 mm (perpendicular to board direction) / (120 + 5) = 32 pcs at 6.0 m = 192 lm

Decking screws: see table below by variant

Common mistakes

• ✗Structural: Too few or shallow foundations — causes sagging, heaving in frost and creaking over time. Footings must go below local frost depth.
• ✗Structural: Undersized or over-spanned joists — a standard 28 mm deck board needs joists at 400–600 mm c/c; wider spacing feels springy underfoot.
• ✗Structural: Incorrect fixing to house wall — never fix the ledger board only into cladding or render. Fix directly into the structural wall or use a freestanding frame.
• ✗Drainage: Poor drainage and no fall away from the building — sub-structure with no slope traps water against the wall and accelerates rot and water ingress.
• ✗Drainage: No air gap between deck boards or between deck and wall cladding — moisture is trapped and decay follows. Keep a minimum 5 mm gap between boards.
• ✗Drainage: Insufficient ventilation clearance under the frame — keep joists as high above ground as practical so the underside can dry out between rain events.
• ✗Fixings: Wrong screws — electroplated steel screws corrode rapidly in treated timber and leave rust stains. Use stainless A2 minimum; A4 (marine grade) in coastal climates.
• ✗Fixings: No pre-drilling at board ends and on hardwood or thermally modified timber — boards split. Always pilot-drill within 50 mm of any end or edge.
• ✗Planning: Too many board splices on the same joist — concentrate all splices at one point and the joist becomes overloaded at that location. Stagger so no more than every third board splices at the same joist.

Tools you will need

Calculate your materials

References

• → Bergene Holm — deck guide and span tables
• → Treteknisk — timber use classes and impregnation guidance
• → AWC — Details for Conventional Wood Frame Construction (deck framing)
• → Local planning authority — building permit requirements and height exemptions for decks in your area