Decking Calculator

Guide

How the amount is worked out

Boards run along the deck’s length, across the joists. The number of board rows = (deck width + gap) ÷ (board width + gap), rounded up. Each row is as many boards as it takes to span the length. Joists run across the width, one every joist spacing along the length, plus one at the end. Screws = rows × joists × screws per crossing.

Example. A 4 m run covered by 140 mm boards with 5 mm gaps has a 145 mm pitch, 27.6 rows, so 28 boards.

Joist spacing

400 mm centres is a common figure for softwood decking boards laid across the joists, and suppliers differ for thicker boards, so the tool starts at 400 mm. Composite boards often need closer joists. The TDCA Builders’ Guide has span tables for the joists themselves, and your board manufacturer’s guide has the rule for its boards.

Common mistakes

• Forgetting the gap. Boards need a small gap for drainage and movement, so board width alone undercounts rows.
• Ignoring the extra joist at the end.
• Choosing board lengths that leave short offcuts. Joints should land on a joist.

Safety and rules

This is not building or structural advice. Posts, bearers, footings, handrails and steps are outside this calculator. Raised decks and some decks near boundaries may need planning permission or Building Regulations approval. Before you dig, check for buried pipes and cables, for example with the free LSBUD service, and check local rules and any permissions that apply to your property.

Common questions