How to calculate deck materials: a 20 m² example

Most quoting errors on decks are not in the boards. They are in the substructure: a forgotten row of supports, tape for the joists but not the bearers, or screws counted per square metre instead of per crossing. This guide shows how to calculate deck materials for the frame with one worked example, then turns it into a method a dealer or installer can reuse on every job.

Start with the board, not the frame

Everything below the boards follows from one figure: the maximum joist span in the board maker’s installation instructions. It sets the joist spacing, which sets the number of joists, which sets the supports, the tape and the fixings. Find that figure first. Typical ranges by board type are in deck joist spacing for timber, thermowood and composite boards.

Deck substructure material list for a 20 m² deck

Assumptions (change them to match your project):

• deck 5 m × 4 m on a level concrete slab, 20 m²;
• timber boards 145 mm wide with a 5 mm gap, so each row covers 150 mm;
• joists at 400 mm centres, running 4 m, spread across the 5 m width;
• boards running the full 5 m, so there are no board end joints in the field;
• support spacing along each joist checked against the joist supplier’s span table.

Item	How it is counted	Example quantity
Joists	5000 ÷ 400 = 12.5 → 13 spaces → 14 joists × 4 m	14 pcs, 56 running metres
Supports, single joist layout	3–3.5 per m² (FT pedestals)	about 60–70 pedestals
Supports, double joist layout	about 1 per m² (FT pedestals or ground screws)	about 20, plus bearers
Support pads	one under each pedestal on slabs and membranes	same as pedestal count
Joist tape 50 mm	56 m ÷ 20 m rolls = 2.8	4 rolls (3 leaves no spare)
Board crossings	4000 ÷ 150 = 26.7 → 27 rows × 14 joists	378 crossings
Face screws	2 per board per joist	756 + spare → about 800–830
Hidden edge screws	per system instructions, usually both edges at each joist	about 800–830 with spare
Hidden clips	one per joist in each gap between boards: 26 gaps × 14	364 + first and last boards + spare
Spare	5–10 % on all of the above	round up to full packs

The pedestal figures are planning rates for Fastshifter FT pedestals, taken from how many pedestals per m², which also explains the single and double joist layouts in detail. They are not a structural design.

The method, step by step

Step 1: area and direction. Measure the deck and decide which way the boards run. The joists run at right angles to the boards. In the example, boards run along the 5 m side, so the joists are 4 m long and spread across 5 m.

Step 2: joists. Divide the width across the joists by the joist spacing, round up and add one for the second edge. Multiply by the joist length for running metres. Add a second joist wherever two board ends meet, and extra joists or blocking for borders, picture frames and steps.

Step 3: supports. Choose the layout. On a single layout every joist sits on its own supports; on a double layout the supports carry bearers and the joists cross on top. Apply the rate, then cross-check: joists × supports per joist from the span table. In the example, 60–70 pedestals spread over 14 joists means about 4–5 supports per 4 m joist. On soil, the same logic applies to ground screws: about one per m² on a double layout, with the exact number set by the frame spacing. For screw length by soil and frost depth, see ground screw length.

Step 4: heights. On a slab, measure the highest and lowest points. A fall of a few centimetres usually means two or three FT sizes, for example FT3 40–64 mm and FT4 64–110 mm. Split the count by zone rather than ordering one size for the whole deck.

Step 5: joist tape. Add up joist metres, plus bearer metres on a double layout, and divide by the roll length. The 50 mm butyl tape covers standard joists; use 80 mm over double joists and at board end joints. Allow for folding over the joist ends. Why it is worth it is covered in joist tape: why it matters.

Step 6: board fixings. Count crossings, not square metres: rows of boards × number of joists. Then apply the fixing method:

• Face screws: two per board at every joist. Edge boards, fascia and steps are often face-fixed even on a hidden-fixed deck, so count them separately.
• Hidden edge screws: follow the system instructions; boards are usually fixed on both edges at each joist. Fastshifter invisible screws come in 250 and 600 packs, for example one 600 pack plus one 250 pack for the example deck.
• Clips: only for grooved boards. Each clip sits between two neighbouring boards at a joist, held by one screw, so plan one clip per gap per joist and fix the first and last boards as the board maker specifies.

Treat these as rules of thumb and always follow the board manufacturer’s instructions on fixing pattern and spacing. For choosing the method, see hidden screws vs face screws vs clips; for the steel grade, A2, A4, C4 or SS410.

Step 7: frame connections. Count every joist-to-bearer and rim connection and use the structural fixing the frame design specifies. Do not estimate these per square metre.

Step 8: spares and packs. Add around 5–10 % to fixings, tape and supports, then round each line up to full packs. Doors, thresholds, steps, heavy planters and hot tubs need extra supports beyond the average rate.

Common mistakes on the material list

• Counting screws per m². Joist spacing and board width change the crossing count; always count rows × joists.
• Forgetting the bearers. On a double layout, bearers need tape and connections too.
• One pedestal size for the whole job. Slabs fall towards a drain; split the count by height.
• No spare. A few missing fixings can stop a crew for half a day.

FT pedestals are rated up to 800 kg each, and the C4 invisible screws are tested to 1500 hours of salt spray testing, but the binding limits on any deck remain the board span and the joist span table. For the bigger picture, read why the substructure matters more than the boards.

Quoting a project or building a range for your customers? Send us the deck size and board type at info@fastshifter.com and we will help you put the material list together, or request samples through the distributor form.