Deck Joist Calculator — Subframe Sizing & Quantities

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. All deck substructure timbers — joists, beams and posts — should be preservative pressure-treated to Use Class 4 (UC4). Since 2020 the TDCA (in line with the WPA and BS 8417) recommends UC4 for the whole substructure regardless of whether the timber is in ground contact, because substructure decay is the single most common cause of deck failure.

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 (45°) board layouts, reduce spacing to about 300mm (12in) centres — the figure manufacturers such as Trex require for angled boards, since the diagonal run increases the effective span between supports.

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.

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.