UK Decking Calculator

Calculate materials, boards, joists and fixings needed for your decking project

Decking Dimensions

Board Specifications

Joist Specifications

Material Requirements

Enter your dimensions and click “Calculate Materials” to see results

How to Plan Your Decking Project

Planning a decking project requires careful consideration of materials, dimensions, and structural requirements. This calculator helps you determine the exact quantities needed for your outdoor space.

Step-by-Step Planning Guide

  1. Measure your space: Accurately measure the length and width of your desired decking area
  2. Choose board specifications: Select appropriate board width and length based on your design preferences
  3. Determine joist spacing: Standard spacing ranges from 300-600mm, with 400mm being most common
  4. Calculate gaps: Allow 5-8mm gaps between boards for drainage and expansion
  5. Plan for waste: Add 10% extra material to account for cuts and potential damage
Top Tip: Always check local building regulations before starting your decking project, especially for raised decks or those near boundaries.

Choosing the Right Materials

Softwood Decking: Most economical option, requires regular treatment but readily available in standard UK sizes.

Hardwood Decking: More durable and naturally weather-resistant, though requires higher initial investment.

Composite Decking: Low maintenance alternative that combines wood fibres with plastic, available in various colours and finishes.

Standard UK Decking Board Sizes

  • Width: 120mm, 125mm, 140mm, 145mm, 150mm, or 200mm
  • Thickness: Typically 32mm or 38mm
  • Length: Available in 2.4m, 3.0m, 3.6m, 4.0m, and 4.8m

Joist Requirements

Joists provide the structural framework for your decking. The spacing depends on the board thickness and expected load:

  • 32mm boards: Maximum 400mm joist centres
  • 38mm boards: Maximum 600mm joist centres
  • Heavy-duty applications: Reduce spacing to 300mm

Frequently Asked Questions

How much extra material should I order?
We recommend ordering 10% extra decking boards to account for cutting waste, potential damage during installation, and future repairs. For joists and fixings, 5% extra is typically sufficient.
What’s the best joist spacing for my decking?
For standard 32mm thick boards, use 400mm centres maximum. For 38mm boards, you can extend to 600mm centres. Closer spacing (300mm) provides additional strength for heavy-duty applications or areas with high foot traffic.
How do I account for board expansion gaps?
Leave 5-8mm gaps between boards to allow for natural expansion and contraction with weather changes. This also improves drainage and ventilation beneath the decking.
Can I install decking directly on the ground?
No, decking should never be installed directly on soil. You need a proper foundation system using concrete pads, adjustable pedestals, or concrete strip foundations to support the joist framework and provide adequate drainage.
What fixings do I need for decking installation?
Use stainless steel or galvanised screws specifically designed for decking. You’ll need approximately 2 screws per board per joist. For composite decking, use the manufacturer’s recommended fixing system.
Do I need planning permission for my decking?
Most ground-level decking projects under 30cm high fall under permitted development rights. However, raised decking, decking near boundaries, or large installations may require planning permission. Always check with your local planning authority.

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