What size joists should I use for decking?

47 x 150mm (approx 45 x 145mm finished) C16 treated softwood is the standard for most residential decks with clear spans up to about 2.4m at 400mm centres. For longer spans or heavy loads, use 47 x 200mm. For very short spans (under 1.2m) or lightweight use, 47 x 100mm may suffice. Joists in a raised deck are out of ground contact, so Use Class 3 (UC3) pressure-treated timber is the minimum per BS EN 335; use Use Class 4 (UC4) for any timber in ground contact, such as support posts.

What spacing should deck joists be?

400mm centres is the standard for most decking boards. This provides adequate support and prevents board bounce. Composite decking may require 350-400mm centres depending on the manufacturer. For diagonal board layouts, reduce spacing to 350mm.

Do I need bearers under deck joists?

Yes, bearers (also called beams) run perpendicular to the joists and transfer loads to the posts or pads. Bearer spacing is set by how far your joists can safely span between supports, not by a fixed rule: TDCA Table 4 (Code of Practice RD0801) allows a 47 x 195mm C24 deck joist to span about 3.0m at 400mm centres, and a 47 x 170mm C16 joist about 2.7m, under the 3.0 kN/m2 deck design load. This calculator uses a conservative 1200mm default, which suits almost any joist; you can increase it (up to the joist span limit) in the advanced options. Bearers should be the same size or larger than the joists, and sit on concrete pads, pier blocks, or screw piles.

What are joist hangers and do I need them?

Joist hangers are galvanised metal brackets that connect joists to a wall ledger (wall plate) or end beam. Per TDCA Code of Practice RD0801 (section 3.7), joists rest on top of intermediate bearers and are diagonally screwed or strapped to them; they are face-fixed or hung only at the ledger end. This calculator therefore counts one hanger per joist for the ledger connection, not one at every joist/bearer crossing. A free-standing deck with no wall ledger may need no hangers at all. Always use the correct nails (not screws) specified for the hanger.

WoodCalcs

Deck Joist Calculator — Subframe Sizing & Quantities

Calculate joist sizes, spacing, bearer quantities, and hardware for a deck subframe.

Joist Size

Joist Spacing

Deck Length

Direction joists are spaced along

Deck Width

Direction joists span across

Price per Joist (optional)

Enter your supplier price for a cost estimate

How We Calculate This

This calculator determines the number and size of joists needed for your deck subframe.

Joist count formula

Number of joists = ceil(Deck length / Joist spacing) + 1

The +1 accounts for the first joist at the starting edge. Joists span the deck width and are supported by bearers running perpendicular to them. Bearer spacing is governed by how far the chosen joist can safely span (TDCA Code of Practice RD0801, Table 4) — up to about 2.7m for a 47 x 170mm C16 joist or 3.0m for a 47 x 195mm C24 joist at 400mm centres under the 3.0 kN/m² deck design load.

Bearer calculation

Bearers = ceil(Deck width / Bearer spacing) + 1

Bearer spacing defaults to a conservative 1200mm and can be set in the advanced options up to the joist span limit above.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.