How many decking boards do I need?

The number depends on your deck area and board width. For standard 145mm wide boards with 5mm gaps on a 4m x 3m deck, you need approximately 20 boards at 3.6m length. Use our calculator for an exact figure based on your deck dimensions and chosen board width. Always add 10-15% for cutting waste.

What joist spacing should I use for decking?

For standard softwood decking boards (32mm thick), joists should be spaced at 400mm centres (the distance between the centre of one joist and the centre of the next). For composite decking, follow the manufacturer's recommendation — typically 300-400mm centres. Diagonal board layouts require closer joist spacing of 300mm.

Do I need Building Regulations approval for decking?

Under UK Permitted Development rights, decking does not require planning permission if the deck platform is no more than 300mm above the ground, does not cover more than 50% of the garden area, and is not forward of the principal elevation. Decking over 300mm high may require Building Regulations approval. Always check with your local authority, especially in conservation areas.

Should I choose timber or composite decking?

Softwood timber decking costs £3-8 per board and lasts 10-15 years with annual treatment. Composite decking costs £15-40 per board but lasts 25+ years with minimal maintenance — no staining, no rot, no splinters. Over 20 years, composite is often cheaper despite the higher upfront cost. Hardwood (such as balau) offers a middle ground at £10-20 per board with 20+ year lifespan.

Decking Calculator — Boards, Joists & Fixings Needed

Complete Guide to Building a Deck in the UK

Garden decking has become one of the most popular outdoor improvements in the UK, creating versatile living and entertaining spaces. Whether you are building a simple ground-level platform or an elevated deck with balustrades, understanding materials, regulations and construction methods ensures a safe, long-lasting result.

Timber vs Composite Decking

The choice between timber and composite decking is the most important decision for your project. Each has significant advantages and trade-offs:

Feature	Softwood (Treated Pine)	Hardwood (Balau/Ipe)	Composite
Cost per board (3.6m)	£3-£8	£10-£25	£15-£40
Cost per m² (boards only)	£12-£25	£30-£60	£40-£100
Lifespan	10-15 years	20-30 years	25-50 years
Maintenance	Annual treatment required	Annual oiling recommended	Minimal — occasional wash
Slip resistance	Poor when wet (add strips)	Good	Very good
Splinters	Yes	Rare	None
Environmental	FSC-certified available	Check sourcing carefully	Recycled materials common

Softwood decking is the budget-friendly option and the most popular choice in the UK. Pressure-treated pine boards are widely available from Wickes, B&Q and timber merchants. They must be treated annually with a decking oil or stain to prevent rot, greying and algae growth. Without treatment, softwood decking deteriorates rapidly in UK weather.

Composite decking is made from a mixture of recycled wood fibres and plastic. Leading UK brands include Trex, Millboard, TimberTech and Cladco. While more expensive upfront, the complete absence of ongoing maintenance and much longer lifespan make composite cheaper over the deck's lifetime. Most composite boards come with 25-year warranties.

Joist Spacing and Structural Requirements

The substructure (joists and bearers) is critical to a safe, long-lasting deck. UK best practice for joist spacing:

Standard softwood (32mm thick): 400mm centres maximum
Heavy-duty softwood (38mm thick): 450mm centres
Composite decking: Follow manufacturer specs — typically 300-400mm centres
Diagonal board layout: Reduce spacing to 300mm centres
45-degree herringbone pattern: 300mm centres, additional noggings between joists

Joist timber grade: Use C16 or C24 structural grade treated timber for joists. The minimum recommended size is 47mm x 150mm for spans up to 1.8m. For longer spans, use 47mm x 200mm or add mid-span bearers to reduce the effective span.

Building Regulations for Decking

Under Permitted Development rights, garden decking does not require planning permission provided all of these conditions are met:

The deck platform is no more than 300mm above the ground
Together with other extensions and outbuildings, the decking does not cover more than 50% of the garden area
The decking is not forward of the principal elevation (front of the house)
The property is not a listed building, in a conservation area, AONB, National Park or World Heritage Site

If your deck exceeds 300mm in height, Building Regulations may apply, requiring:

Balustrades at least 1100mm high for decks more than 600mm above ground
Balustrade gaps no wider than 100mm (to prevent children falling through)
Structural calculations for the supporting framework
Fire safety considerations if the deck is attached to or near the house

Essential Fixings and Accessories

Beyond boards and joists, you need several other materials to complete your deck:

Decking screws: Use stainless steel or coated decking screws (minimum 60mm for 32mm boards). Budget approximately 20-25 screws per square metre (2 screws per board per joist crossing).
Joist hangers: For connecting joists to ledger boards or bearers. Use galvanised steel hangers rated for the joist size.
Pad stones / pedestals: Concrete pads (300x300x50mm) or adjustable plastic pedestals placed under each joist support point to keep the timber off the ground.
Weed membrane: Lay heavy-duty landscape fabric under the deck to prevent weed growth while maintaining drainage and airflow.
DPC strip: A damp-proof course strip should be placed between the ledger board and the house wall to protect the brickwork.

Cost Breakdown for a Typical UK Deck

For a standard 4m x 3m (12m²) ground-level deck:

Item	Softwood	Composite
Decking boards	£120-£200	£450-£900
Joists (47x150mm treated)	£80-£120	£80-£120
Screws and fixings	£20-£35	£40-£60
Pad stones / pedestals	£20-£40	£20-£40
Weed membrane	£10-£20	£10-£20
Total (materials only)	£250-£415	£600-£1,140
Professional installation	£600-£1,200	£800-£1,500

Ventilation is critical: Ensure at least 150mm airflow gap between the ground and the underside of the deck boards. Poor ventilation traps moisture and dramatically reduces the lifespan of timber components. Never box in a deck completely — maintain open sides or install ventilation grilles.

Decking Calculator — Boards, Joists & Fixings Needed