Decking calculator boards, joists and screws

How the calculator works out a deck

The boards run along the deck's length and the joists run across it, under the boards. So the width decides how many rows of boards you need, and the length decides how many joists sit under them.

Rows of boards are the width divided by one board plus one gap, rounded up. Board metres are rows × length, plus your wastage, divided by the board length and rounded up. Joists are the length divided by the joist spacing, plus one for the far end, with two more joists along the sides to close the frame. Screws are two per board every time it crosses a joist. Supports go under each joist at the spacing you set, plus one at the far end.

Worked example: a 4.0m by 3.0m deck on level ground, with 144mm boards in 3.6m lengths, a 5mm gap, joists at 400mm centres in 3.6m lengths, supports every 1.2m and 10% wastage on the boards.

• Rows of boards: 3,000 ÷ (144 + 5) = 20.1, so 21 rows.
• Boards: 21 × 4.0m = 84m. Plus 10% = 92.4m. 92.4 ÷ 3.6 = 25.7, so 26 boards.
• Joists across the deck: 4,000 ÷ 400 = 10, plus 1 = 11 joists, each 3.0m, cut from 3.6m lengths.
• Side joists: two runs of 4.0m, each needing 2 lengths of 3.6m, so 4. Total 15 joist lengths.
• Screws: each row crosses 11 joists and has one butt joint, which needs its own pair. 2 × 21 × 12 = 504 screws.
• Posts or pads: each 3.0m joist gets one every 1.2m plus one at the end, so 4. 11 × 4 = 44.

Joist spacing and spans

The calculator starts at 400mm centres, the tightest spacing in the Timber Decking and Cladding Association's span table. Its builder's guide to timber decking gives the maximum span for C16 joists on domestic decks, meaning the distance a joist can run between supports.

Keep the distance between your posts or pads within the span for the joist you are using. With 100 × 50mm joists at 400mm centres, that is 1.79m. The 1.2m default comes from Jacksons Fencing's deck guide, which sets posts at 1,200mm centres under 100 × 50mm joists, so it sits well inside the TDCA figure.

The TDCA also says C16 is the minimum strength class, with C24 recommended for domestic decks that will carry heavy loads, and that all substructure timber should be treated to Use Class 4. Its joist advice says that applies whether or not the joists touch the ground.

Board sizes and the gap

Enter the board's real width, not its nominal one. Wickes, for example, lists its 27 × 144mm Pro deck board at 3.6m long, covering 0.52m². A narrower board means more rows, more screws and a few more joist crossings to fix.

The TDCA asks for a gap of 5mm to 8mm between boards, to let water drain and to allow the timber to move with the seasons, plus 5mm where a board end meets a post. The calculator's default is 5mm. Some guides online suggest 3mm, which is tighter than the TDCA's range.

Where two boards meet end to end, the TDCA says the joint must land over a joist with an extra piece of joist or batten fixed alongside it, so both ends have something solid under them. Grooved boards go on a slight fall, laid in the direction of the fall, and need keeping clear of debris or they hold water.

Composite boards work differently. Their makers set their own joist spacing and fixing method, often with hidden clips, so follow the fitting guide for the board and change the spacing in the calculator to match.

How many deck screws, and which

Two screws every time a board crosses a joist. The TDCA's guide puts them at the board's quarter points, 25mm in from each end, and its fixings advice says a deck screw should be at least 2.5 times the board's thickness. For a 27mm board that is 67.5mm, so a 70mm screw.

The calculator gives the exact count. Screws come in boxes, so round up to whole boxes, and keep a few spare for the boards that split or the screw that snaps.

• Use stainless steel, hot-dip galvanised or high quality coated screws. The TDCA says electroplated, brass and uncoated steel fixings should not be used outside.
• Pre-drill near the ends to stop the board splitting.
• Hardwood boards: pre-drill every hole 2mm oversize so the board can move.
• Drive the heads flush, not below the surface, or each hole holds a little puddle of water.

Posts, pads and height

The calculator puts one support under every joist at the spacing you set, plus one at the far end. That suits a low deck where each joist sits on its own posts or adjustable pads. If you are building with beams (bearers) carrying the joists, you will need fewer posts, and the beam sizes need working out separately. The calculator does not size beams.

For posts, Jacksons' guide digs holes at least 300mm deep and 200mm across, sets the posts at least 150mm into the ground in concrete, and brings the concrete to within 50mm of ground level, sloped to shed water. The TDCA's guide says posts, beams and joists must be timber suitable for Use Class 4, and that cut ends must never go into the ground, even if they have been treated.

Height changes the rules. The TDCA's guide calls a deck under 600mm raised and over 600mm high level. For homes, Approved Document K of the Building Regulations calls for guarding where someone could fall more than 600mm: 1,100mm high at an external balcony, with no gap a 100mm sphere can pass through. The TDCA also suggests a 900mm balustrade on lower decks. Ask building control which applies to yours.

Before you buy the timber

Check planning first. In England, the government's permitted development guidance allows garden decking without planning permission only if it is no more than 0.3m high and, with extensions and outbuildings, covers no more than half the garden. The TDCA's planning guidance also flags decks within 20m of a highway, decks that could affect a neighbour's privacy, and decks at listed buildings, in conservation areas or in National Parks.

Get the client to see a sample board before you order. The TDCA's guide makes the same point: grades and surfaces vary, and the board on the website may not be the one that turns up.

Treat every cut end with an end grain preservative, as both the TDCA and Jacksons advise, or you risk the treatment warranty. If you are pricing the deck for a customer, list boards, joists, fixings and labour on the quote template.

• What the calculator does not include: beams, steps, balustrades, fascia boards, weed membrane, gravel, joist tape, hangers or brackets.
• It is for a simple rectangle. Split an L-shaped deck into two rectangles and add them, remembering the joists where they meet.
• It does not design a raised or high-level deck. Over 600mm, or attached to a house, have it designed by someone who can check the loads.

Common questions